JP2015193868A - Method for manufacturing thick walled high strength seamless steel pipe for linepipe excellent in sulfide stress corrosion cracking resistance - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high strength seamless steel pipe excellent in sulfide stress corrosion cracking resistance.SOLUTION: A solid round slab piece having a composition containing, by mass%, C of 0.03 to 0.15%, Si of 0.02 to 0.5%, Mn of 0.7 to 2.5%, P of 0.020% or less, S of 0.003% or less, Al of 0.01 to 0.08%, N of 0.005% or less, and Ti of 0.005 to 0.05% with N and Ti satisfying N≤Ti×14/48≤N+10 and the balance Fe with inevitable impurities is heated with a heating temperature of less than 1300°C and applied to piercing rolling, rolling drawing with rolling reduction of 40% or more in a temperature range of 950°C or less in which rolling is stopped in the temperature range of Artransformation point to (the Artransformation point+70°C), or further sizing and rolling, then applied to cooling to 300°C or less with an average cooling rate of 20°C/s or more in the temperature range of 800 to 300°C, and a tempering treatment at less than the Actransformation point.

Description

  The present invention relates to a method for producing a thick-walled, high-strength seamless steel pipe suitable for use in line pipes for transporting crude oil, natural gas, etc., and is particularly resistant to sulfide stress corrosion cracking (SSC resistance) in a sour environment containing hydrogen sulfide. ) Improvement. Here, “high strength” refers to a case where the strength is X65 to X80, that is, the 0.5% proof stress is 450 MPa or more and 700 MPa or less.

  In recent years, from the viewpoint of soaring crude oil prices and the depletion of petroleum resources expected in the near future, the so-called sour environment including deep oil fields and hydrogen sulfide that have not been excluded in the past The development of oil fields and gas fields in corrosive environments has become active. For this reason, the steel pipes used in line pipes for transporting crude oil and gas mined in oil and gas fields under such circumstances are made of materials that have both high strength and excellent corrosion resistance (sour resistance). It is required to have.

In response to such a requirement, for example, Patent Document 1 and Patent Document 2 include, in mass%, C: 0.03-0.11%, Si: 0.05-0.5%, Mn: 0.8-1.0%, P: 0.025% or less. , S: 0.003% or less, Ti: 0.002-0.017%, Al: 0.001-0.10%, Cr: 0.05-0.5%, Mo: 0.02-0.3%, V: 0.02-0.20%, Ca: 0.0005-0.005%, N : After rolling a steel slab having a composition containing 0.008% or less and O: 0.004% or less to a seamless steel pipe by hot rolling, immediately after soaking, the quenching start temperature is set to (Ar ₃ points + 50 ° C) to 1100 ° C. A method for producing a high-strength seamless steel pipe excellent in hydrogen-induced cracking resistance with a yield stress of 483 MPa or more, which is cooled at a cooling rate of ° C / s or more and then tempered at _one point of 550 to Ac is described. The seamless steel pipes obtained by the techniques described in Patent Literatures 1 and 2 are bainite and / or martensite and have a structure in which ferrite precipitates at the grain boundaries.

  Patent Document 3 describes a method for producing a thick seamless steel pipe for line pipe. In the technique described in Patent Document 3, C: 0.03-0.08%, Si: 0.25% or less, Mn: 0.3-2.5%, Al: 0.001-0.10%, Cr: 0.02-1.0%, Ni: 0.02-1.0% , Mo: 0.02 to 1.2%, Ti: 0.004 to 0.010%, N: 0.002 to 0.008%, and a total of one or more of Ca, Mg, and REM, 0.0002 to 0.005%, V: 0 to 0.08 %, Nb: 0-0.05%, Cu: 0-1.0%, P in the impurity is 0.05% or less, S is 0.005% or less, solidified into a billet with a round cross section by continuous casting And a step of cooling the billet to room temperature with an average cooling rate from 1400 to 1000 ° C. being 6 ° C./min or more, and 1150 to 1280 ° C. with an average heating rate from 550 to 900 ° C. being 15 ° C./min or less. A process of producing a seamless steel pipe by piercing and rolling, and a process of forcibly cooling to an average cooling rate of 800 to 500 ° C. immediately after pipe making to 8 ° C./s or higher and continuously to 100 ° C. or lower; , 50 A tempering step at a temperature in the range of 0 to 690 ° C. is sequentially performed to obtain a thick-walled seamless steel pipe for line pipe having high strength and good toughness. In the technique described in Patent Document 3, instead of a round billet formed by continuous casting, a square bloom or slab may be formed by continuous casting and then rounded by forging or rolling.

JP 2004-176172 A JP 2004-143593 A JP 2006-274350 A

However, the various factors affecting SSC resistance are extremely complex, and in the high strength steel pipes exceeding 65 ksi, the conditions for ensuring stable SSC resistance are not clear, and In the techniques described in Patent Documents 1 to 3, there is a problem that excellent SSC resistance cannot be stably secured while combining high strength and high toughness.
In addition, in the techniques described in Patent Documents 1 to 3, since the direct quenching process is performed, the characteristic variation of each part of the steel pipe due to the temperature variation at each position of the pipe that occurs during hot rolling becomes large, and the high strength and high There is a problem that it is difficult to stably ensure excellent SSC resistance while combining toughness. In the technique described in Patent Document 3, a reheating quenching process may be used instead of the direct quenching process. However, the reheating and quenching process has a problem that the process becomes complicated.

The present invention solves such problems of the prior art, and is suitable for use in line pipes. It has a high yield strength of over 450 MPa and a high toughness with a fracture surface transition temperature vTrs of −70 ° C. or less in a Charpy impact test. The object of the present invention is to provide a method for producing a thick-walled, high-strength seamless steel pipe that is excellent in sulfide stress corrosion cracking resistance (SSC resistance) in a sour environment.
Here, “thick” refers to a case where the thickness is 12.5 mm or more. In addition, “Excellent resistance to sulfide stress corrosion cracking” means in a 0.5% acetic acid + 5.0% saline solution (liquid temperature: 24 ° C) saturated with H ₂ S, in accordance with NACE TM0177 Method A. A constant load test is carried out at a load stress of 85% of the yield strength and the load time is 720 hours, and no cracking occurs.

  In order to achieve the above-mentioned object, the present inventors diligently studied various factors affecting the strength, toughness and resistance to sulfide stress corrosion cracking (SSC resistance) of a seamless steel pipe. As a result, in order to make seamless steel pipes for line pipes that have the desired high strength and toughness while maintaining excellent resistance to sulfide stress corrosion cracking, the Mn is adjusted to an appropriate range to achieve hardenability. In addition, the amount of N is reduced, and the proper amount of Ti is added only to fix N to prevent a decrease in toughness, and if necessary, an appropriate amount of Cr, Mo, Cu, V In addition, after making the composition containing Nb, further drilling by low-temperature heating, low-temperature rolling and accelerated cooling that satisfies the predetermined conditions, refine the structure, and further tempering I found out that this is important.

The present invention has been completed based on such findings and further studies. That is, the gist of the present invention is as follows.
(1) The solid round slab is heated, pierced and rolled into a hollow shell, and then the hollow shell is stretched and rolled to form a seamless steel pipe. In mass%, C: 0.03-0.15%, Si: 0.02-0.5%, Mn: 0.7-2.5%, P: 0.020% or less, S: 0.003% or less, Al: 0.01-0.08%, N: 0.005% or less , Ti: 0.005-0.05%, N and Ti are the following formula (1)
N ≤ Ti x 14/48 ≤ N + 10 (1)
(N, Ti: Content of each element (mass ppm))
And a solid round cast slab having a composition comprising the balance Fe and inevitable impurities, the heating is heating to a heating temperature of less than 1300 ° C, and the stretching rolling is 950 ° C or less The following formula (2) in the temperature range: Reduction ratio (%) = (tube cross-sectional area before rolling−tube cross-sectional area after rolling) / (tube cross-sectional area before rolling) × 100 (2) The rolling is finished in a temperature range where the rolling reduction is 40% or more and not less than the Ar ₃ transformation point (Ar ₃ transformation point + 70 ° C.), and the average in the temperature range of 800 to 300 ° C. after the end of the stretching rolling. Cooling at a cooling rate of 20 ° C / s or higher is performed to 300 ° C or lower, and then tempering is performed at a temperature lower than the Ac ₁ transformation point, providing excellent resistance to sulfide stress corrosion cracking and toughness. A method for manufacturing a thick-walled, high-strength seamless steel pipe for line pipes, characterized in that it is a seamless steel pipe.

(2) After the solid round slab is heated and subjected to piercing and rolling to form a hollow shell, the hollow shell is subjected to stretching rolling and subsequent constant diameter rolling to obtain a seamless steel pipe. The solid round cast slab is in mass%, C: 0.03-0.15%, Si: 0.02-0.5%, Mn: 0.7-2.5%, P: 0.020% or less, S: 0.003% or less, Al: 0.01-0.08 %, N: 0.005% or less, Ti: 0.005-0.05%, and N and Ti in the following formula (1)
N ≤ Ti x 14/48 ≤ N + 10 (1)
(N, Ti: Content of each element (mass ppm))
And a solid round cast slab having a composition comprising the balance Fe and inevitable impurities, the heating is heating to a heating temperature of less than 1300 ° C, and the stretching rolling is 950 ° C or less The following formula (2) in the temperature range: rolling reduction (%) = (tube cross-sectional area before rolling−tube cross-sectional area after rolling) / (tube cross-sectional area before rolling) × 100 (2) The rolling with a rolling reduction of 40% or more, and the constant diameter rolling is rolling that ends rolling at a temperature range of not less than the Ar ₃ transformation point (Ar ₃ transformation point + 70 ° C.) and after completion of the constant diameter rolling, 800 Cooling at an average cooling rate of 20 ° C / s or higher in the temperature range of ~ 300 ° C is performed to 300 ° C or lower, and then tempering is performed at a temperature below the Ac ₁ transformation point to prevent sulfide stress corrosion. Manufacture of thick-walled, high-strength seamless steel pipes for line pipes, characterized by being seamless steel pipes with excellent cracking and toughness Method.

(3) In (1) or (2), in addition to the above composition, in addition to mass, Cu: 0.3% or less, Ni: 0.3% or less, Mo: 0.3% or less, Cr: 0.5% or less, V: 0.05 % Or less, Nb: One or more selected from 0.05% or less. A method for producing a thick high-strength seamless steel pipe for line pipes.
(4) In any one of (1) to (3), in addition to the above-mentioned composition, further containing, by mass%, Ca: 0.002% or less, a thick-walled high-strength seamless steel pipe for line pipes Production method.

  According to the present invention, without adding a large amount of alloy elements, yield strength: high strength exceeding 450 MPa, and high toughness with a fracture surface transition temperature vTrs of −70 ° C. or less in the Charpy impact test, Furthermore, low-alloy high-strength seamless steel pipes for oil wells having excellent sulfide stress corrosion cracking resistance in severe corrosive environments containing hydrogen sulfide can be easily manufactured, and there are remarkable industrial effects.

In the present invention, a solid round slab is used as a starting material, the solid round slab is heated and subjected to piercing and rolling to form a hollow material, and then the hollow material is stretched and rolled to obtain a seamless steel pipe.
First, the reason for limiting the composition of the solid round cast slab, which is the starting material, will be described. Hereinafter, unless otherwise specified, mass% is simply expressed as%.
C: 0.03-0.15%
C is an element that contributes to an increase in steel pipe strength through solid solution strengthening and hardenability improvement. In order to obtain such an effect and to secure a desired high strength, a content of 0.03% or more is required. On the other hand, if the content is more than 0.15%, the hardness of the weld heat affected zone (HAZ) becomes too high, and the SSC resistance of the weld zone decreases. For this reason, C was limited to the range of 0.03-0.15%. In addition, Preferably it is 0.06 to 0.12%. For usage in which the winding and rewinding are repeatedly applied a plurality of times, such as reel purge, it is more preferably 0.06 to 0.11%.
The preferred range is a composition range excluding the subperitectic region where the volume expansion increases and the production decreases, and this range cannot be accurately displayed when the component is not clear, but generally C: 0.10 ~ The area is around 0.12%.

Si: 0.02-0.5%
Si is an element that acts as a deoxidizer and has an effect of increasing steel pipe strength by solid solution strengthening. In order to obtain such an effect, a content of 0.02% or more exceeding the impurity level is required. On the other hand, if the content exceeds 0.5%, the toughness of the welded part and the base metal part is lowered. For this reason, Si was limited to the range of 0.02 to 0.5%.

Mn: 0.7-2.5%
Mn is an element having an action of increasing the steel pipe strength through improvement of hardenability. Further, Mn has an action of binding to S and fixing S as MnS to prevent grain boundary embrittlement due to S. In order to obtain such an effect and ensure a desired high strength, a content of 0.7% or more is required. On the other hand, if the content exceeds 2.5%, the hardness of the circumferential weld becomes too high exceeding 250 HV, and sour resistance (SSC resistance) decreases. For this reason, Mn was limited to the range of 0.7 to 2.5%. In addition, Preferably it is 0.7 to 1.5%.

P: 0.020% or less
P tends to segregate at grain boundaries and cause toughness reduction, grain boundary embrittlement cracking, etc., and is desirably reduced as much as possible in the present invention, but is acceptable up to 0.020%. Therefore, P is limited to 0.020% or less. In addition, since excessive reduction leads to an increase in steelmaking cost, it is desirable to make 0.003% or more industrially.

S: 0.003% or less
S is mostly present as sulfide inclusions in steel, and has ductility, toughness and corrosion resistance, hydrogen embrittlement crack resistance (HIC resistance), and sulfide stress corrosion crack resistance (SSC resistance). In order to reduce, it is desirable to reduce as much as possible. However, since seamless steel pipes are rolled in the circumferential direction and longitudinal direction by piercing and rolling, MnS is not stretched extremely long in the rolling direction, and has extremely high HIC resistance and SSC resistance. The decrease is small and there is no need to make an extreme reduction of S. If it is about 0.003% or less, it is acceptable. For this reason, S was limited to 0.003% or less.

Al: 0.01-0.08%
Al is an element that acts as a deoxidizer, and in order to obtain such an effect, it is necessary to contain 0.01% or more. On the other hand, if the content exceeds 0.08%, oxide inclusions are likely to remain in a cluster shape, and the toughness decreases. Oxide inclusions also cause surface defects. For this reason, Al was limited to the range of 0.01 to 0.08%. In addition, Preferably it is 0.05% or less.

N: 0.005% or less
N combines with Ti to produce TiN, and when its amount increases, it tends to reduce toughness. For this reason, in the present invention, N is reduced as much as possible. However, an extreme reduction leads to an increase in refining costs. For this reason, N was limited to 0.005% or less.
Ti: 0.005-0.05%
Ti is contained only for fixing N. Ti other than that in which N was fixed and TiN was formed was adjusted according to the amount of N so as not to remain. Corresponding to the usual minimum amount of N, Ti should be contained 0.005% or more. On the other hand, if the content exceeds 0.05%, the amount of TiN increases, the size increases, and Ti carbide, sulfide, carbon sulfide, etc. are formed, and the toughness is reduced. For this reason, Ti was limited to 0.005 to 0.05%. In addition, Preferably it is 0.0175% or less.

In the present invention, N and Ti are within the above range, and the following formula (1)
N ≤ Ti x 14/48 ≤ N + 10 (1)
(N, Ti: Content of each element (mass ppm))
The content is adjusted so as to satisfy. The median of equation (1) corresponds to the amount of Ti when TiN is formed. In the present invention, the Ti content is adjusted so as to be in a range of N to (N + 10) mass ppm corresponding to the N content. If (Ti × 14/48) is less than N, there will be solid solution N, which combines with nitride-forming elements such as Al, or forms carbonitrides during tempering, resulting in steel pipe toughness. This is a factor that causes a decline.

On the other hand, if (Ti × 14/48) increases beyond (N + 10), there will be the remaining Ti forming TiN, forming Ti sulfides and carbon sulfides, greatly reducing toughness The fear increases. For this reason, it was decided to adjust Ti and N so as to satisfy the expression (1).
The above components are basic components. In the present invention, in addition to the basic composition, Cu: 0.3% or less, Ni: 0.3% or less, Mo: 0.3% or less, Cr: 0.5% or less , V: 0.05% or less, Nb: 0.05% or less, and / or Ca: 0.002% or less.

Cu, Ni, Mo, Cr, V, and Nb are all elements that contribute to an increase in the strength of the steel pipe, and can be selected as needed and contained in one or more.
Cu: 0.3% or less, Ni: 0.3% or less, Mo: 0.3% or less, Cr: 0.5% or less, V: 0.05% or less, Nb: 0.05% or less
Cu is an element that increases the strength of the steel pipe through solid solution strengthening and further hardenability improvement, and can be contained if necessary. In order to acquire such an effect, it is desirable to contain 0.01% or more, but when it contains exceeding 0.3%, toughness will fall and surface flaws will occur frequently. For this reason, when it contains, it is preferable to limit Cu to 0.3% or less.

  Ni is an element that increases the strength of the steel pipe through solid solution strengthening and further improving the hardenability, and can be contained if necessary. In order to obtain such an effect, it is desirable to contain 0.01% or more, but if it exceeds 0.3%, the strength becomes too high and the SSC resistance in a sour environment is lowered. For this reason, when it contains, it is preferable to limit Ni to 0.3% or less. In addition, when 0.05% or more of Cu is contained, it is more preferable to contain Ni by 0.5 × Cu amount or more. Thereby, the generation | occurrence | production of the surface flaw and surface defect resulting from Cu can be prevented.

Mo is an element that increases the strength of the steel pipe through the improvement of hardenability, and can be contained if necessary. In order to obtain such an effect, it is desirable to contain 0.001% or more. However, if it exceeds 0.3%, the strength becomes too high, and the SSC resistance in a sour environment decreases. For this reason, when it contains, it is preferable to limit Mo to 0.3% or less.
Cr is an element that increases the strength of the steel pipe through the improvement of hardenability, and can be contained if necessary. In order to obtain such an effect, it is desirable to contain 0.01% or more, but if it exceeds 0.5%, the strength becomes too high and SSC resistance (sour resistance) in a sour environment, especially welded parts. The sour resistance is reduced. For this reason, when contained, Cr is preferably limited to 0.5% or less.

  V is an element that increases the strength of the steel pipe through an improvement in hardenability and an increase in resistance to temper softening, and can be contained if necessary. In order to acquire such an effect, it is desirable to contain 0.002% or more. However, if it exceeds 0.05%, coarse VN and V (CN) are formed, and the possibility of lowering toughness is increased. For this reason, when it contains, it is preferable to limit V to 0.05% or less.

  Nb is an element that increases the strength of the steel pipe through precipitation strengthening, and can be contained as necessary. Nb also contributes to refinement of austenite grains and improves sour resistance. In order to obtain such an effect, the content is preferably 0.005% or more. However, if the content exceeds 0.05%, the resistance to sulfide stress corrosion cracking and the resistance to hydrogen-induced cracking may be reduced. For this reason, when it contains, it is preferable to limit Nb to 0.05% or less.

Ca: 0.002% or less
Ca has the effect of controlling the form of inclusions, which is the inclusion of sulfide inclusions and oxide inclusions in the form of granular inclusions. It is an element having an effect of improving resistance to sulfide stress corrosion cracking and hydrogen-induced cracking, and can be contained as required. Such effects become prominent when the content is 0.001% or more. However, when the content exceeds 0.002%, nonmetallic inclusions increase, and on the contrary, ductility, toughness, resistance to sulfide stress corrosion cracking, and resistance to hydrogen-induced cracking are increased. Sexuality decreases. For this reason, when it contains, it is preferable to limit Ca to 0.002% or less of range. In addition, the nozzle clogging at the time of continuous casting can be suppressed by containing Ca in molten steel. In addition, when a round slab is not used, Ca may not be added.

The balance other than the components described above consists of Fe and inevitable impurities. As an inevitable impurity, O: 0.006% or less is acceptable.
The seamless steel pipe of the present invention has the above-described composition and further has a structure mainly composed of a bainite phase. The “bainite phase” herein includes a bainitic ferrite phase and an acicular ferrite phase in addition to the bainite phase. In the present invention, the martensite phase is included in the bainite phase because it is difficult to distinguish the bainite phase from the martensite phase in the optical microscope observation of the structure revealed by the nital liquid corrosion. The term “mainly” as used herein refers to a case where the phase is 50% or more in area ratio. Examples of the second phase other than the bainite phase include a ferrite phase having an area ratio of 10% or less.

In the present invention, a solid round slab having the above composition is used as a starting material. The manufacturing method of the solid round slab as a starting material is not particularly limited, and any conventional method can be applied. It is preferable that the molten steel having the above composition is melted by a conventional melting method such as a converter, an electric furnace, a vacuum melting furnace or the like, and is made into a solid round slab of a predetermined size by a conventional continuous casting method.
Next, the solid round cast slab as a starting material is heated and subjected to piercing and rolling to form a hollow shell.

  The heating temperature in the heating is preferably 1150 ° C. or more and less than 1300 ° C. When the heating temperature is 1300 ° C. or higher, the crystal grains become coarse, the scale loss becomes great, and the yield decreases, which is economically disadvantageous. When the heating temperature is less than 1100 ° C., the hot deformation resistance becomes high, the load (rolling load) applied to the rolling mill becomes too high, and rolling becomes difficult. Therefore, in the present invention, the heating temperature of the solid round slab is limited to less than 1300 ° C., preferably 1100 ° C. or more. In addition, Preferably it is 1200 degreeC or more.

The piercing and rolling method is not particularly limited, and the Mannesmann piercing method can be applied regardless of the roll type.
The obtained hollow shell is used as it is without additional heating, or is heated to less than 1100 ° C., preferably 1000 ° C. or more, subjected to stretching rolling or further constant diameter rolling, To do. The stretching rolling is preferably rolling using a usual Mannesmann-Elongator plug mill, Mannesmann-Mandrel mill, or further an Assel mill.

Stretch rolling is performed by the following formula (1) in the temperature range of 950 ° C. or lower: Reduction ratio (%) = (tube cross-sectional area before rolling−tube cross-sectional area after rolling) / (tube cross-sectional area before rolling) × 100. (1)
The rolling is completed in a temperature range in which the rolling reduction (cross-sectional reduction rate) defined by is 40% or more and not less than the Ar ₃ transformation point (Ar ₃ transformation point + 70 ° C.).

In addition, when performing constant-diameter rolling subsequent to stretching rolling, there is no particular problem even if the rolling end temperature of stretching rolling deviates from the above temperature range. If the rolling end temperature of the drawing rolling is less than the Ar ₃ transformation point, the rolling end temperature of the constant diameter rolling cannot be adjusted within a predetermined temperature range when the constant diameter rolling is subsequently performed.
If the rolling reduction (cross-sectional reduction rate) in the temperature range of 950 ° C or lower in drawing rolling is less than 40%, the amount of rolling in the non-recrystallization temperature range is insufficient, and the microstructure cannot be refined, and the desired high strength High toughness cannot be ensured. For this reason, the rolling reduction in the temperature range of 950 ° C. or less in drawing and rolling is limited to 40% or more. It should be noted that the rolling reduction in the temperature range of 950 ° C. or less in the drawing rolling is preferably 75% or less from the viewpoint of rolling load and tube shape during rolling. In addition, Preferably it is 65% or less.

Further, if the rolling end temperature in the drawing rolling is less than the Ar ₃ transformation point, the finally obtained steel pipe structure cannot be a desired structure. Further, when the steel plate temperature is lower than the Ar ₃ transformation point, there is a concern that the precipitation of ferrite starts from austenite and the distribution of processing strain becomes non-uniform. On the other hand, when the rolling end temperature of the stretch rolling exceeds (Ar ₃ transformation point + 70 ° C.), the dislocation (working strain) introduced by the working recovers and the desired structure cannot be refined. Therefore, rolling end temperature in the elongation rolling is limited to a temperature range of Ar ₃ transformation point or higher (Ar ₃ transformation point + 70 ° C.) or less. Incidentally, preferably in the range of (Ar ₃ transformation point + 30 ℃) ~Ar ₃ transformation point.

In the present invention, after the drawing and rolling, constant diameter rolling may be performed in order to further adjust the outer diameter of the pipe to a predetermined dimension. The constant diameter rolling is performed to adjust the outer diameter of the pipe to a predetermined dimension using a rolling machine such as a reducer or a sizing mill. In that case, the rolling conditions of constant径圧extension, as can satisfy the cooling conditions controlled cooling in the subsequent step, the temperature range of the rolling end temperature than the Ar ₃ transformation point (Ar ₃ transformation point + 70 ° C.) or less. If the rolling end temperature of the constant diameter rolling is less than the Ar ₃ transformation point, the precipitation of ferrite starts from austenite and the processing strain distribution becomes non-uniform. On the other hand, when the rolling end temperature of constant diameter rolling exceeds (Ar ₃ transformation point + 70 ° C.), the dislocation introduced by the processing disappears due to recovery or the like, and the desired structure refinement cannot be achieved. Therefore, rolling end temperature in the constant-radius rolling is limited to a temperature range of Ar ₃ transformation point or higher (Ar ₃ transformation point + 70 ° C.) or less. Incidentally, preferably in the range of (Ar ₃ transformation point + 30 ℃) ~Ar ₃ transformation point.

Control cooling is performed after the constant diameter rolling when the constant diameter rolling is performed after completion of the stretching rolling or following the stretching rolling. The controlled cooling is cooling in which the average cooling rate in the temperature range of 800 to 300 ° C. is 20 ° C./s or more, and the cooling stop temperature is 300 ° C. or less. Thereby, it can be set as the structure | tissue which has a bainite phase as a main body.
By setting the average cooling rate in the temperature range of 800 to 300 ° C. to 20 ° C./s or more, the obtained structure becomes a structure mainly composed of a fine bainite phase, and has desired high strength and high toughness. it can. When the average cooling rate is less than 20 ° C./s, the resulting structure becomes a structure containing a coarse ferrite phase and pearlite, and both strength and toughness are lowered. On the other hand, the upper limit of the average cooling rate is not particularly limited, but is preferably about 100 ° C./s or less from the viewpoint of the steel pipe shape. The cooling rate obtained is determined by the thickness of the steel pipe depending on the cooling capacity of the cooling device to be used. However, in the cooling rate range of 100 ° C / s or more, a large change is observed in the obtained structure. Absent. For this reason, the average cooling rate in the temperature range of 800 to 300 ° C. is limited to 20 ° C./s or more in the cooling after the drawing and rolling. In addition, Preferably it is 40 degrees C / s or more.

Further, when the cooling stop temperature of the controlled cooling exceeds 300 ° C. and becomes a high temperature, the structure becomes coarse and desired high strength and high toughness cannot be ensured. For this reason, the cooling stop temperature of the controlled cooling is limited to 300 ° C. or less.
A tempering process is further performed after controlled cooling.
The tempering process is a process of heating to a temperature below the Ac ₁ transformation point and cooling, preferably cooling at a cooling rate equal to or higher than air cooling. In the present invention, the tempering treatment is performed in order to reduce excessive dislocations and stabilize the structure, and to combine desired high strength and further excellent resistance to sulfide stress corrosion cracking. If the heating temperature in the tempering treatment is at least the Ac ₁ transformation point, the α → γ transformation occurs in part, and further transformation occurs by subsequent cooling, resulting in a decrease in strength and toughness. For this reason, the heating temperature in the tempering treatment was limited to a temperature below the Ac ₁ transformation point. In addition, Preferably it is 600-680 degreeC. In addition, the tempering treatment is preferably a treatment in which, after holding for 10 minutes or more within the above temperature range, cooling at a cooling rate of preferably air cooling or more, preferably to room temperature. If the holding time at the tempering temperature is less than 5 minutes, the desired structure cannot be made uniform. In addition, Preferably, it is 30 minutes or less.

  Hereinafter, the present invention will be further described based on examples.

  Molten steel having the composition shown in Table 1 was melted in a converter and made into a solid round slab (diameter: 210 mmφ) by a continuous casting method. The solid round cast slab, which is the starting material, was heated to the temperature shown in Table 2 and pierced and rolled using a Mannesmann piercing and rolling machine to form a hollow shell. Next, the obtained hollow shell was stretched and rolled with a mandrel mill under the conditions shown in Table 2, and further subjected to constant diameter rolling with a sizing mill. After the rolling, controlled cooling was performed under the conditions shown in Table 2. Table 2 A seamless steel pipe having the dimensions shown in FIG. Subsequently, tempering treatment under the conditions shown in Table 2 was performed.

Test materials were collected from the obtained seamless steel pipes and subjected to a structure observation test, a tensile test, a Charpy impact test, and a corrosion test. The test method was as follows.
(1) Microstructure observation test A specimen for microstructural observation was collected from the obtained steel pipe, and the cross section (C cross section) perpendicular to the longitudinal direction of the pipe was polished and corroded (corrosion liquid: nital liquid). : 100 times) and a scanning electron microscope (magnification: 1000 times), the tissue was observed, imaged, and the type of tissue and its fraction were measured using an image analyzer.

(2) Tensile test Tensile test pieces (parallel part 6mmφ x GL25mm) are collected from the obtained steel pipe in accordance with the standard of JIS Z 2241 so that the tube axis direction is the tensile direction, and the tensile test is performed. Yield strength YS and tensile strength TS were obtained.
(3) Charpy impact test Charpy impact test piece (2mmV notch test piece) is taken from the obtained steel pipe so that the direction perpendicular to the pipe axis direction is the test piece longitudinal direction, and conforms to the provisions of JIS Z 2242 Then, a Charpy impact test was carried out to determine the fracture surface transition temperature vTrs (° C.).

(4) Corrosion test Ten corrosion test specimens were collected from the obtained steel pipe, and H ₂ S saturated 0.5% acetic acid + 5.0% saline solution (liquid temperature: 24) in accordance with NACE TM0177 Method A regulations. )), And after 720 hours of loading with 90% of the yield strength YS, observe the presence or absence of cracks in the specimen and evaluate the resistance to sulfide stress corrosion cracking. did. For the observation of cracks, a projector with a magnification of 10 times was used. The sulfide stress corrosion cracking resistance was evaluated by the crack generation rate (= (number of test pieces in which cracks occurred) / (total number of test pieces) × 100 (%)).

  The obtained results are shown in Table 3.

  All of the examples of the present invention have desired high strength (yield strength YS: 450 MPa or more) and desired high toughness (vTrs: −70 ° C. or less), and the crack occurrence rate in the corrosion test is 0%. It is a high-strength seamless steel pipe that combines excellent resistance to sulfide stress corrosion cracking. On the other hand, the comparative example outside the scope of the present invention does not have desired high strength, high toughness, and desired excellent sulfide stress corrosion cracking resistance.

Claims (4)

After the solid round slab is heated and subjected to piercing and rolling to form a hollow shell, the hollow shell is stretch-rolled to obtain a seamless steel pipe.
The solid round cast slab, in mass%,
C: 0.03-0.15%, Si: 0.02-0.5%,
Mn: 0.7 to 2.5%, P: 0.020% or less,
S: 0.003% or less, Al: 0.01 to 0.08%,
N: 0.005% or less, Ti: 0.005-0.05%
And N and Ti are contained so as to satisfy the following formula (1), and a solid round slab having a composition consisting of the balance Fe and inevitable impurities,
The heating is heating to a heating temperature of less than 1300 ° C,
The drawing and rolling is performed at a temperature range of 950 ° C. or lower and a rolling reduction defined by the following formula (2) of 40% or higher and an Ar ₃ transformation point or higher (Ar ₃ transformation point + 70 ° C.) or lower. And finish the rolling and
After the drawing and rolling, the cooling at an average cooling rate in the temperature range of 800 to 300 ° C. is 20 ° C./s or more is performed to 300 ° C. or less, and thereafter
Tempering at a temperature below the Ac ₁ transformation point,
A method for producing a thick-walled, high-strength seamless steel pipe for line pipes, characterized in that the steel pipe is excellent in resistance to sulfide stress corrosion cracking and excellent in toughness.
Record
N ≤ Ti x 14/48 ≤ N + 10 (1)
Where N, Ti: Content of each element (mass ppm)
Reduction ratio (%) = (tube cross-sectional area before rolling−tube cross-sectional area after rolling) / (tube cross-sectional area before rolling) × 100 (2) The solid round slab is heated and subjected to piercing and rolling to form a hollow shell, and then the hollow shell is subjected to stretching rolling and subsequent constant diameter rolling to obtain a seamless steel pipe.
The solid round cast slab, in mass%,
C: 0.03-0.15%, Si: 0.02-0.5%,
Mn: 0.7 to 2.5%, P: 0.020% or less,
S: 0.003% or less, Al: 0.01 to 0.08%,
N: 0.005% or less, Ti: 0.005-0.05%
And N and Ti are contained so as to satisfy the following formula (1), and a solid round slab having a composition consisting of the balance Fe and inevitable impurities,
The heating is heating to a heating temperature of less than 1300 ° C,
The stretch rolling is a rolling in which the rolling reduction defined by the following formula (2) in a temperature range of 950 ° C. or lower is 40% or more,
The constant-diameter rolling is rolling that terminates rolling in a temperature range of not less than the Ar ₃ transformation point (Ar ₃ transformation point + 70 ° C.),
After completion of the constant diameter rolling, an average cooling rate in the temperature range of 800 to 300 ° C is 20 ° C / s or more, and cooling is performed to 300 ° C or less.
Tempering at a temperature below the Ac ₁ transformation point,
A method for producing a thick-walled, high-strength seamless steel pipe for line pipes, characterized in that the steel pipe is excellent in resistance to sulfide stress corrosion cracking and excellent in toughness.
Record
N ≤ Ti x 14/48 ≤ N + 10 (1)
(N, Ti: Content of each element (mass ppm))
Reduction ratio (%) = (tube cross-sectional area before rolling−tube cross-sectional area after rolling) / (tube cross-sectional area before rolling) × 100 (2)   In addition to the above composition, it is further selected by mass% from Cu: 0.3% or less, Ni: 0.3% or less, Mo: 0.3% or less, Cr: 0.5% or less, V: 0.05% or less, Nb: 0.05% or less. The manufacturing method of the thick-walled high-strength seamless steel pipe for line pipes of Claim 1 or 2 characterized by including 1 type, or 2 or more types.   The method for producing a thick high-strength seamless steel pipe for a line pipe according to any one of claims 1 to 3, further comprising Ca: 0.002% or less by mass% in addition to the composition.