JPH06172857A - Production of seamless steel tube with low yield ratio and high toughness - Google Patents

Abstract

PURPOSE:To obtain a seamless steel tube having low yield ratio and high toughness by specially controlling steel components and hot rolling conditions, respectively. CONSTITUTION:A billet of a steel which has a composition consisting of, by weight, 0.03-0.20% C, 0.01-2.5% Si, 0.15-2.5% Mn, <=0.020% P, <=0.010% S, 0.005-0.1% each of Al, Ti, and Nb, <=0.01% N, and the balance Fe and containing, if necessary, prescribed amounts of Cr, Mo, Ni, V, B and/or Ca, Co, and Cu is used. This billet is heated up to >=1100 deg.C and hot-pierced, and the resulting hollow tube stock is cooled, before the cross helical rolling in the final stage, down to a temp. between the Ar point and 900 deg.C and subjected to forming with a reduction ratio of wall thickness cross section of 20-70% by means of the final cross helical rolling mill right behind the above. Further, after continuous hot rolling for shape straightening is done, the hollow tube stock whose temp. is dropped to a temp. between the Ar3 point and 900 deg.C is heated up to 900-950 deg.C, hot-finish-rolled at >=(Ar3 point +50 deg.C), and air-cooled at a rate of <=(10 deg.C/s).

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. For this reason, the seamless steel pipe used as the transportation equipment for the products can withstand the use of high-pressure operation in cold regions, and thus has a low yield ratio, high toughness (-60 ° C guarantee), and high strength (x52 or more).
A property that combines both is required. To give a low yield ratio to a high strength material, 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, the workability during line pipe laying, which requires preheating before on-site welding, 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 The present invention is based on the results of a large number of experiments and examinations without the above-mentioned treatment, and by controlling the steel composition and hot rolling conditions, a low yield ratio and high toughness are obtained. A method for manufacturing a seamless steel pipe is provided.

[0004]

That is, the gist of the present invention is that C: 0.03 to 0.20% as weight% and Si: 0.01 to
2.5%, Mn: 0.15 to 2.5%, P:
0.020% or less, S: 0.010% or less,
Al: 0.005-0.1%, Ti: 0.005-
0.1%, Nb: 0.005 to 0.1%, N:
0.01% or less, if necessary, Cr: 0.1 to 1.5%, Mo: 0.05 to
0.5%, Ni: 0.1 to 2.0%, V:
0.01-0.1%, B: 0.0003-0.0033
%, Rare earth elements: 0.001 to 0.05%, Ca: 0.
001 to 0.02%, Co: 0.05 to 0.5%,
Cu: 0.1 to 0.5% of a steel slab containing 0.1 or 0.5% or more and the balance substantially consisting of Fe is heated to 1100 ° C. or higher, and then a hot hollow pipe is finally subjected to hot piercing and rolling. Ar ₃ points to 900 in front of the inclined rolling mill
° C. until cooled, the final slope subjected to 20% to 70% of the molding in the thickness of the reduction of the rolling machine immediately, further Ar ₁ point to 900 ° C. After the continuous hot rolling for shape correction The hollow rough tube which has been lowered to the temperature is
After heating to 50 ° C., a finishing temperature is Ar ₃ point + 50 ° C. or more, hot finish rolling is performed, and a cooling process at a cooling rate of 10 ° C./s or less is performed, followed by cooling treatment.

[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 which easily causes cracks during processing due to segregation of grain boundaries, and S is MnS.
A system inclusion was formed and stretched by hot continuous rolling, and the contents thereof as components harmful to low temperature toughness were set to 0.010% or less and 0.010% or less, respectively.

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 B hardenability 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 a gamma grain when crystal grain growth is suppressed during tilt rolling and, after continuous rolling, the temperature of the raw tube lowered to 900 ° C. to Ar ₁ point is heated to 900 to 950 ° C. higher than the temperature. Is an important element that suppresses abnormal coarsening. If the amount is too small, the effect will not be obtained, and if the amount is too large, the effect will be saturated, and since it is very expensive, 0.005-
It was set to 0.1%.

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

In the case of the steel having the above-mentioned composition, to further increase the strength of the steel, the components such as Cr, Mo, Ni, V and B are selectively added as required. Cr, Mo, Ni, V are
It is added to enhance the strength. If the amount is too small, the effect will not be obtained, and if the amount is too large, the effect will be saturated, and since it is very expensive, 0.01 to 1.5%, respectively.
0.05-0.5%, 0.1-2.0%, 0.01-
It was set 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 0003 to 0.003%.

Further, in view of the use environment of the seamless steel pipe in recent years, the present invention relates to the SS resistance of the steel having the above-mentioned composition.
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 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 for limiting the hot seamless rolling conditions as described above will be explained. The 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 agglomerating and agglomerating 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 piercing and rolling at a temperature of 1100 ° C. or higher,
The temperature was 1100 ° C. or higher.

The hollow shell that has been subjected to piercing and rolling is cooled to a temperature of Ar ₃ point to 900 ° C. before the final stage of the inclined rolling mill and immediately rough-processed to an outer diameter and wall thickness close to the final shape of the steel tube. Performs inclined rolling. Inclined rolling mills (elongator mills, etc.)
Unlike other rolling mills (mandrel mill, plug mill, etc.) used for rolling seamless steel pipe and rolling mills for steel plates, the component of shear strain is very large. 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, the austenite structure undergoes large deformation even if the work is performed with a small cross-sectional area reduction rate, and a fine ferrite structure is generated during subsequent ferrite transformation.

The photograph of FIG. 1 shows the metallographic structure of the material processed by the elongator mill and the plate rolling machine at the same cross-sectional area reduction rate. It can be seen that the deformation of the austenite structure of the elongator mill is larger than that of 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 method of controlling the temperature of the hollow shell may be either cooling by cooling or forced cooling. Further, if the rolling reduction is small, a fine ferrite structure is not formed, so the lower limit was made 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 tilt rolling is completed, the hollow rough tube is further continuously rolled to correct the shape, and the rough tube lowered to a temperature of Ar ₁ point to 900 ° C. is 900 to 9 higher than the temperature.
Reheat to 50 ° 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.

[0020]

EXAMPLES Next, examples of the present invention will be described. Table 1
Is the yield ratio of the steel pipe that is directly quenched and tempered by hot seamless rolling the steel slab produced by melting in a converter and continuous casting.
It exhibits toughness and SSC resistance. SSC resistance is NAC
Σth according to the constant load method according to ETM01-77
(Threshold Pressure) 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.

[0021]

[Table 1]

[0022]

[Table 2]

[0023]

The steel pipe produced by the method of the present invention as described above is excellent in low temperature toughness and SSC resistance since 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) shows the metallographic structure (unrecrystallized structure) of the material after the final stage of tilt rolling and (b) after plate rolling.
0x magnification micrograph.

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[Procedure amendment]

[Submission date] September 3, 1993

[Procedure Amendment 1]

[Document name to be amended] Statement

[Correction target item name] 0020

[Correction method] Change

[Correction content]

[0020]

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, 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 to ℃ or more, the hollow shell subjected to hot piercing and rolling is cooled to Ar ₃ point to 900 ° C. in front of the final tilt rolling mill, and immediately after that, in the final tilt rolling mill, the wall thickness cross-section reduction rate is 20 to 70%. After performing the forming process of _{No. 1} and further performing hot continuous rolling for shape correction, the hollow crude tube that has dropped to a temperature of Ar ₁ point to 900 ° C. is heated to 900 to 950 ° C. higher than that temperature, and then finished. A method of producing a seamless steel pipe with a low yield ratio and high toughness, which comprises performing hot finish rolling at a temperature of Ar ₃ points + 50 ° C. or higher and performing cooling treatment at a cooling rate of 10 ° C./s or lower. 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 Further, 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 Steel pieces containing 0.0003 to 0.0033% of 1 type or 2 types or more, and the balance being substantially Fe 1100
After heating to ℃ or more, the hollow shell subjected to hot piercing and rolling is cooled to Ar ₃ point to 900 ° C. in front of the final tilt rolling mill, and immediately after that, in the final tilt rolling mill, the wall thickness cross-section reduction rate is 20 to 70%. After performing the forming process of _{No. 1} and further performing hot continuous rolling for shape correction, the hollow crude tube that has dropped to a temperature of Ar ₁ point to 900 ° C. is heated to 900 to 950 ° C. higher than that temperature, and then finished. A method of producing a seamless steel pipe with a low yield ratio and high toughness, which comprises performing hot finish rolling at a temperature of Ar ₃ points + 50 ° C. or higher and performing cooling treatment at a cooling rate of 10 ° C./s or lower. 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%, one kind Alternatively, a steel slab containing two or more kinds and the balance being substantially Fe is 1100
After heating to ℃ or more, the hollow shell subjected to hot piercing and rolling is cooled to Ar ₃ point to 900 ° C. in front of the final tilt rolling mill, and immediately after that, in the final tilt rolling mill, the wall thickness cross-section reduction rate is 20 to 70%. After performing the forming process of _{No. 1} and further performing hot continuous rolling for shape correction, after heating the hollow coarse tube that has dropped to a temperature of Ar ₁ point to 900 ° C. to 900 to 950 ° C. higher than the temperature, the finishing temperature is A method for producing a seamless steel pipe with a low yield ratio and high toughness, which comprises performing hot finish rolling of Ar ₃ points + 50 ° C. or more and performing cooling treatment at a cooling rate of 10 ° C./s or less. 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 Further, 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 0.0003 to 0.0033% of 1 type or 2 types or more, Rare earth elements: 0.001 to 0.05%, Ca: 0.001 to 0.02%, Co: 0.05 to 0.5 %, Cu: 0.1 to 0.5% of a steel slab containing 1 type or 2 types or more and the balance substantially consisting of Fe 1100
After heating to ℃ or more, the hollow shell subjected to hot piercing and rolling is cooled to Ar ₃ point to 900 ° C. in front of the final tilt rolling mill, and immediately after that, in the final tilt rolling mill, the wall thickness cross-section reduction rate is 20 to 70%. After performing the forming process of _{No. 1} and further performing hot continuous rolling for shape correction, after heating the hollow coarse tube that has dropped to a temperature of Ar ₁ point to 900 ° C. to 900 to 950 ° C. higher than the temperature, the finishing temperature is A method for producing a seamless steel pipe with a low yield ratio and high toughness, which comprises performing hot finish rolling of Ar ₃ points + 50 ° C. or more and performing cooling treatment at a cooling rate of 10 ° C./s or less.