JP2009052106A - High strength seamless steel pipe for machine structure having excellent toughness and method for producing the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a high strength seamless steel pipe for a machine structure suitable for a structural member such as a cylinder, a bush and a boom and a machine member such as a shaft, and particularly having excellent toughness. <P>SOLUTION: The high strength seamless steel pipe for a machine structure is provided which has a composition comprising suitable amounts of C, Mn, Ti, Nb and O, restricted contents of Si, P, S, Al and N, one or more kinds selected from among B, Ni, Cr and Cu, and satisfying Al/Ti≤0.75, and the balance Fe with inevitable impurities, and which has metallic structure composed of a self-tempered martensite single structure or a mixed structure of self-tempered martensite and lower bainite, and has an effective crystal grain size of ≤10 μm. A method for producing the above seamless steel pipe is also provided by which the steel pipe is subjected to seamless rolling, and thereafter, its cooling speed is controlled. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a seamless steel pipe suitable for mechanical structural members, particularly structural members such as cylinders, bushes, and booms, and mechanical members such as shafts, and a method of manufacturing the same.

  Conventionally, many mechanical parts used in automobiles and industrial machines are given predetermined mechanical properties by tempering heat treatment after forging and cutting steel bars into a predetermined shape. Also, in recent years, in order to reduce the manufacturing cost of parts, there is an increasing number of cases where hollow shaped parts are manufactured using steel pipes having the mechanical properties required for the parts to shorten the forging process and omit the heat treatment process. It is coming.

  However, in general, a steel pipe is more expensive than a steel bar, and particularly a seamless steel pipe has a high manufacturing cost. Therefore, even if the steel pipe is used as a material for a hollow shape part, the effect of reducing the manufacturing cost may not be sufficient. Accordingly, the provision of inexpensive steel pipes with reduced manufacturing costs has been studied, and so-called non-tempered steel pipes for machine parts and structural steel pipes have been proposed in which tempering heat treatment after hot pipe making is omitted. (For example, Patent Documents 1 to 7).

  The steel pipes proposed in Patent Documents 1 to 6 all have a high C content, and are intended to obtain a predetermined strength by improving the hardenability and precipitation strengthening ability by adding carbonitride-forming elements. . For this reason, there is a problem in that the alloy cost is increased, and preheating for preventing the occurrence of cracks during welding is required, which impairs productivity.

  Further, the method proposed in Patent Document 7 attempts to refine the metal structure and improve the strength by rolling at a hot rolling temperature of 600 to 750 ° C., which is a considerably low temperature. However, although low temperature rolling is a common technique for thick plate rolling, it is actually applied because rolling and rolling of seamless steel pipes tend to cause wrinkles and seizures due to contact with tools. The range is greatly limited.

Furthermore, techniques for improving the strength by cooling the seamless steel pipe after hot working have been proposed (for example, Patent Documents 8 and 9). The method proposed in Patent Document 8 is to cool from the outer surface at a temperature of 10 to 60 ° C./s from the temperature of Ar ₃ point or higher to 500 to 400 ° C. after the final finish rolling, and then air cool. Moreover, the method of patent document 9 performs direct quenching or accelerated cooling with hot rolling, and then tempering. However, since these are oil well pipes and it is not necessary to consider weldability, they contain 0.1% or more of carbon.

  On the other hand, steel pipes used for cylinders and bushes among machine structural steel pipes are often required to have high strength, high toughness, and weldability, and generally the carbon content is limited to 0.1% or less. It is preferable to do. On the other hand, the steel pipes of Patent Documents 1 to 9 all have a carbon content of 0.1% or more, and the accelerated cooling of Patent Documents 8 and 9 is intended to control the structure for ensuring the strength. There is no description regarding toughness and weldability, and tempering is performed after accelerated cooling.

  Therefore, some of the present inventors have proposed a steel pipe that achieves both strength and low-temperature toughness by performing accelerated cooling immediately after seamless rolling at a carbon content of 0.1% or less (for example, patent documents) 10). However, this method is not particularly intended to improve toughness at an extremely low temperature such as −60 ° C.

JP-A-5-202447 Japanese Patent Laid-Open No. 10-130783 Japanese Patent Laid-Open No. 10-204571 JP-A-10-324946 Japanese Patent Laid-Open No. 11-36017 JP 2004-292857 A JP 2001-247931 A Japanese Patent No. 3503211 JP 7-41856 A Japanese Patent Application No. 2006-87723

  The present invention has been made in view of the current situation as described above, and is particularly suitable for structural members such as cylinders, bushes, and booms and mechanical members such as shafts. The present invention also provides a method for producing a high-strength seamless steel pipe for machine structure at low cost without tempering.

  The present inventors have optimized the addition amount of Al and Ti to make use of intragranular transformation, and furthermore, by accelerated cooling after seamless rolling, the fineness capable of achieving both high strength and high toughness over the entire plate thickness direction. It has been found that a steel pipe having a good metal structure can be obtained. This invention is made | formed based on such knowledge, The place made into the summary is as follows.

(1) By mass%, C: 0.030 to 0.100%, Mn: 0.80 to 3.00%, Ti: 0.005 to 0.045%, Nb: 0.003 to 0.040% , O: 0.0010 to 0.0050%, Si: 0.50% or less, P: 0.020% or less, S: 0.008% or less, Al: 0.010% or less, N: 0 .0080% or less,
Al / Ti ≦ 0.75
Further, B: 0.0030% or less, Ni: 1.50% or less, Cr: 1.50% or less, Cu: 1.00% or less, Mo: 0.50% or less It contains more than seeds, the balance consists of Fe and inevitable impurities, the metal structure is a self-tempered martensite single structure or a mixed structure of self-tempered martensite and lower bainite, and the orientation difference is 15 ° or more A high-strength seamless steel pipe for machine structures excellent in toughness, characterized in that an average diameter of a region surrounded by large-angle grain boundaries is 10 μm or less.

(2) In the above (1), the average particle diameter of the iron-based carbide in the metal structure is 400 nm or less, and the density of the iron-based carbide is 2.0 × 10 ⁵ pieces / mm ² or more. High-strength seamless steel pipe for machine structures with excellent toughness described.

(3) Hot piercing and rolling are performed on a steel piece having the chemical component described in (1) above, and the steel piece is reheated as it is or without cooling to a temperature range of less than 650 ° C., and a temperature of 850 to 1050 ° C. After holding in the zone for 1 to 30 minutes, from the temperature range of 750 ° C. or higher, while rotating the steel pipe in the circumferential direction, the temperature range from 450 to 550 ° C. at a cooling rate of 2 to 30 ° C./s from the outer surface of the steel pipe The secondary cooling rate V [° C./s] that is within the range of 10 to 50 ° C./s and higher than the cooling rate of the primary cooling is continuously within the range obtained by the following (formula 1). A method for producing a high-strength seamless steel pipe for machine structures excellent in toughness, characterized by performing secondary cooling to a secondary cooling stop temperature T [° C].
150 ≦ T ≦ 821.34 × V ^−0.3112 (Formula 1)

(4) After holding for 1 to 30 minutes in the temperature range of 850 to 1050 ° C. described in (3) above, the tube is formed by a stretching process, and the steel pipe is rotated in the circumferential direction from the temperature range of 750 ° C. or higher. The manufacturing method of the high-strength seamless steel pipe for machine structures excellent in toughness as described in (3) above, wherein primary cooling and secondary cooling are performed.

(5) The mechanical structure excellent in toughness according to the above (3) or (4), wherein the length of the steel pipe after the seamless rolling or after the stretching process and before accelerated cooling is at least 5 times the outer diameter For manufacturing high-strength seamless steel pipes.

  According to the present invention, a high-strength seamless steel pipe for machine structure and a high-strength seamless steel pipe for machine structure, particularly excellent in toughness, suitable for machine structural members, particularly structural members such as cylinders, bushes, and booms, and mechanical members such as shafts. Can be provided at low cost, and the industrial contribution is extremely remarkable.

  A steel pipe having a high strength that can be applied to a mechanical structure needs to have a metal structure composed of a low-temperature transformation structure. The low temperature transformation structure is martensite and lower bainite. Moreover, in order to ensure toughness, it is necessary to make a martensite into the tempered martensite which precipitated the cementite. Furthermore, from the viewpoint of production cost, it is advantageous to use self-tempered martensite by cooling after rolling, rather than performing tempering after quenching.

  The inventors of the present invention have made a prototype steel sheet having a metal structure composed of a self-tempered martensite and a lower bainite composite structure after cooling in a laboratory after hot rolling and cooling. And the toughness was investigated. As a result, it was found that when the tensile strength was 900 MPa or more, brittle fracture occurred when the Charpy impact test was performed at -60 ° C.

  In addition, a detailed investigation of the fracture surface morphology and crystal orientation difference of the sample after the test was conducted to refine the crystal grain size, especially the average diameter of the region surrounded by large-angle grain boundaries with an orientation difference of 15 ° or more (effective The knowledge that the occurrence of brittle fracture can be prevented by making the crystal grain size finer). That is, it was found that if the effective crystal grain size is 10 μm or less, the temperature at which the fracture mode transitions from ductile fracture to brittle fracture (referred to as transition temperature) decreases, and brittle fracture does not occur at −60 ° C.

  In general, refinement of the crystal grain size is effective for improving the low temperature toughness of steel. When producing a steel pipe, once it is cooled to room temperature after seamless rolling, it undergoes processing, and the metal structure produced by the low temperature transformation structure is reheated. Therefore, if the reheating temperature is made 900 ° C. or lower, the old austenite Can be refined. However, from the viewpoint of production cost, after seamless rolling, accelerated cooling, when producing a steel pipe having a self-tempered martensite single structure or a metal structure composed of self-tempered martensite and lower bainite, the high temperature before rolling It is difficult to refine austenite due to the effect of heating.

  In view of this, the present inventors examined the use of intragranular transformation of prior austenite in order to refine the effective crystal grain size of the steel pipe that had been subjected to accelerated cooling after seamless rolling. Intragranular transformation is a phenomenon in which ferrite transformation occurs with precipitates present in crystal grains as production nuclei. Since the oxide that effectively acts as a nucleus for the formation of intragranular transformation is Ti oxide, it is considered that intragranular transformation is promoted by optimizing the addition amount of Al as a deoxidizing element and the addition amount of Ti. The following tests were conducted.

  In mass%, C: 0.030 to 0.100%, Mn: 0.80 to 3.00%, Ti: 0.005 to 0.045%, Nb: 0.003 to 0.040%, O: 0.0010 to 0.0050%, Si: 0.50% or less, P: 0.020% or less, S: 0.008% or less, Al: 0.010% or less, N: 0.0080% One of the following, further limited to B: 0.0030% or less, Ni: 1.50% or less, Cr: 1.50% or less, Cu: 1.00% or less, Mo: 0.50% or less A steel containing two or more, the balance being Fe and inevitable impurities, and changing Al / Ti was melted and cast to obtain a steel slab.

  The steel piece was made into a steel plate by rolling assuming seamless rolling, and then held in a temperature range of 850 to 1050 ° C. for 1 to 30 minutes, and accelerated cooling was performed from a temperature range of 750 ° C. or higher. The metallographic structure of the steel sheet was observed with an optical microscope and a scanning electron microscope (referred to as SEM), and was confirmed to be a single structure of self-tempered martensite or a structure composed of self-tempered martensite and lower bainite.

The effective crystal grain size of the resulting steel sheet electron backscattering spectroscopy was measured by (E lectron B ack S cattering P attern, called. EBSP). A crystal orientation map was created from the measurement data obtained by EBSP, and the average grain size of a region surrounded by a large-angle grain boundary having a crystal orientation difference of 15 ° or more was measured.

  The results are shown in FIG. The horizontal axis in FIG. 1 is the ratio Al / Ti between the Al content and the Ti content, and the vertical axis is the effective crystal grain size. As can be seen from FIG. 1, when Al / Ti is 0.75 or less, the effective crystal grain size is 10 μm or less. Since the refinement of the effective crystal grain size is due to the promotion of intragranular transformation of prior austenite, when Al / Ti is made 0.75 or less, the oxide produced in the grain mainly becomes Ti oxide, It is thought to act as a nucleus for intragranular transformation.

  Next, a method for obtaining an optimum metal structure capable of achieving both strength and toughness was investigated. In order to promote intragranular transformation during cooling after hot rolling, and to make the metal structure self-tempered martensite alone or a composite structure of self-tempered martensite and lower bainite, two-stage acceleration It is necessary to perform cooling. This is because primary cooling at a high temperature suppresses the formation of ferrite, and secondary cooling at a lower temperature generates self-tempered martensite and lower bainite.

  In secondary cooling, the stop temperature of accelerated cooling, in particular, the relationship between the cooling rate and the stop temperature is extremely important. This is to control the precipitation behavior of iron-based carbide (also referred to as cementite) that affects the improvement of the balance between the strength and toughness of the steel pipe (also referred to as strength-toughness balance). In addition, the formation of iron-based carbides greatly affects toughness. This is because if the generation of iron-based carbide is insufficient, the strength is too high and the toughness decreases, and if the iron-based carbide is coarsened, it becomes a starting point of fracture and decreases the toughness. On the other hand, if the secondary cooling stop temperature is too high, upper bainite is produced and the toughness is lowered.

  Therefore, the present inventors examined the relationship between secondary cooling conditions and low temperature toughness. Steel having a chemical component having an effective crystal grain size of 10 μm or less in the above examination was melted and cast to produce a steel slab. The obtained steel slab is a cylindrical billet, subjected to hot piercing and rolling (seamless rolling), kept in the temperature range of 850 to 1050 ° C. for 1 to 30 minutes, and then 750 ° C. or higher. From the temperature range, two-stage accelerated cooling was performed in which the steel pipe was rotated in the circumferential direction and mist cooling was performed from the outer surface of the steel pipe. First, primary cooling was performed at a cooling rate of 2 to 30 ° C./s up to a temperature range of 450 to 550 ° C., and secondary cooling was performed following the primary cooling.

  The lower limit of the secondary cooling rate V [° C./s] was 10 ° C./s, the upper limit was 50 ° C./s, and the cooling rate was higher than the cooling rate of the primary cooling. Further, the secondary cooling stop temperature T [° C.] was changed, and the relationship between the secondary cooling rate V [° C./s] and the low temperature toughness was examined. The low temperature toughness was evaluated based on the presence or absence of a brittle fracture surface by collecting a V-notch test piece of JIS Z 2242 with the circumferential direction as the longitudinal direction from the steel pipe, performing a Charpy impact test at -60 ° C.

The results are shown in FIG. The horizontal axis in FIG. 2 is the secondary cooling rate V [° C./s], the vertical axis is the secondary cooling stop temperature T [° C.], and the broken line is a curve of T = 821.34 × V ^−0.3112 . In FIG. 2, ◯ and ● indicate the presence or absence of a brittle fracture surface at −60 ° C., ◯ means that no brittle fracture surface is seen, and ● means that a brittle fracture surface is seen.

As shown in FIG. 2, when the secondary cooling stop temperature is less than 150 ° C., a brittle fracture surface is observed at −60 ° C. This is thought to be because the production of iron carbide was insufficient and the strength increased. Further, a brittle fracture surface is observed at −60 ° C. even ^above the broken line, that is, ^above the curve of T = 821.34 × V ^−0.3112 . This is probably because the secondary cooling temperature is too high, and cementite is coarsened, resulting in upper bainite and reduced toughness.

From the above examination, in order to obtain good low temperature toughness, the secondary cooling rate V [° C./s] and the secondary cooling stop temperature T [° C.]
150 ≦ T ≦ 821.34 × V ^−0.3112 (Formula 1)
It was found necessary to satisfy the relationship.

  Hereinafter, the present invention will be described in detail.

  First, the reason why the chemical composition of the steel pipe is limited in the present invention will be described. Note that “%” shown below means “% by mass” unless otherwise specified.

  C: C is an extremely effective element for improving the strength. To obtain the target strength, 0.030% or more must be added. On the other hand, if it contains more than 0.100% C, the low temperature toughness is lowered, and cracking during welding becomes a problem. Therefore, C is limited to 0.030 to 0.100%.

  Si: Si is a deoxidizing element, but when added in excess, upper bainite is generated, and the upper limit is limited to 0.50% in order to impair the low temperature toughness. In addition, when performing deoxidation using Al etc., since it is not necessarily required to contain Si, a minimum is not specifically limited. However, since Si is an element contributing to improvement in strength, it is preferable to add 0.10% or more.

  Mn: Mn is an element that promotes the formation of a low-temperature transformation structure, and is effective for improving the balance between strength and low-temperature toughness. In order to obtain this effect, it is necessary to add 0.80% or more of Mn. However, if the amount of Mn is more than 3.00%, the low temperature toughness may be impaired, so 3.00% was made the upper limit.

  P and S: P and S are impurities, and if contained excessively, the toughness is lowered and the weldability may be lowered. Therefore, the upper limits were made 0.020% and 0.008%, respectively. In order to ensure toughness, the content of P and S is preferably less, and more preferably 0.015% or less and 0.005% or less, respectively. In addition, from a viewpoint of manufacturing cost, the minimum with preferable content of P and S is 0.001%.

  Al: Al is an important element in the present invention, and the upper limit is limited to 0.010% in order to secure an oxygen concentration in the molten steel and generate a Ti oxide suitable for intragranular transformation. The smaller the content of Al, the better, and there is no need to intentionally add Al. However, even when Si is added as a deoxidizing element due to the influence of a refractory during melting, Al may inevitably be contained.

  Ti: Ti is an element necessary for generating a Ti oxide that is the starting point of intragranular transformation, and is the most important element in the present invention. In order to obtain a sufficient Ti oxide, it is necessary to contain 0.005% or more of Ti. On the other hand, if the addition amount of Ti exceeds 0.045%, the toughness decreases due to precipitation of coarse Ti oxide, so the upper limit was made 0.045%. In order to reduce the effective crystal grain size, it is preferable to add Ti by 0.010% or more. Moreover, in order to prevent coarsening of TiN and improve low temperature toughness, it is preferable to make the upper limit of Ti amount 0.035% or less.

  Nb: Nb is an element that produces precipitates such as carbides and nitrides, suppresses recrystallization of austenite during rolling, refines the structure, increases hardenability, and is effective for strengthening steel It is. To obtain this effect, it is necessary to add 0.003% or more of Nb. On the other hand, if the Nb content is more than 0.040%, coarse Nb precipitates may be generated and the toughness may deteriorate, so the upper limit was made 0.040%.

  N: N is an impurity, and if it exceeds 0.0080%, coarse TiN is generated and the toughness is lowered, so the upper limit was limited to 0.0080%. In order to produce fine nitrides such as TiN and NbN to refine the metal structure, it is preferable to add 0.0010% or more of N.

  O (oxygen): O needs to be 0.0010% or more in order to generate Ti oxides that are the nuclei of intragranular transformation. On the other hand, if the amount of O exceeds 0.0050%, a coarse oxide is generated and the low temperature toughness is impaired, so the upper limit was made 0.0050%.

  Further, one or more of B, Ni, Cr, Cu, and Mo may be added. These are elements that enhance the hardenability and contribute to the toughening of the steel of the present invention.

  B: When B is more than 0.0030%, precipitates such as BN are generated, and the hardenability may be lowered. Therefore, the upper limit is preferably made 0.0030%. In addition, the minimum with the preferable addition amount of B is 0.0003% or more.

  Ni: If Ni exceeds 1.50%, it segregates and the structure becomes non-uniform, and the toughness may deteriorate, so the upper limit is preferably made 1.50%. Ni is an element that improves the strength without deteriorating the low-temperature toughness, and it is preferable to add 0.10% or more.

  If Cr, Cu, Mo: Cr exceeds 1.50%, the toughness may deteriorate due to the formation of Cr precipitates, so the upper limit is preferably made 1.50%. When Cu and Mo are added in amounts exceeding 1.00% and 0.50%, respectively, weldability may be impaired. In order to improve the strength, Cu, Cr and Mo are preferably added in an amount of 0.10% or more, 0.10% or more and 0.050% or more, respectively.

  Next, the metal structure will be described. When the steel pipe is manufactured by accelerated cooling from the outer surface like the steel pipe of the present invention, the cooling rate of the inner surface of the steel pipe is slower than that of the outer surface. Therefore, ferrite and upper bainite are easily generated on the inner surface of the steel pipe. In particular, when the plate thickness is thick, the steel pipe may be deformed if accelerated cooling is performed from the outer surface so as to increase the cooling rate of the inner surface.

  Therefore, if the cooling rate is controlled to such an extent that the steel pipe does not deform, bainite transformation may occur on the inner surface side. On the other hand, if the component is within an appropriate range as in the steel pipe of the present invention, lower bainite is generated, and a balance between strength and toughness can be ensured. However, it is necessary to add a large amount of Mo to make the entire surface in the plate thickness direction lower bainite in the steel pipe for machine structure, which may impair the economy. Therefore, it is preferable to use a self-tempered martensite single structure or a composite structure of self-tempered martensite and lower bainite rather than a single structure of lower bainite.

  In the present invention, the lower bainite is defined as a structure in which lath-shaped ferrite is generated during accelerated cooling and fine carbides are precipitated in one direction in the lath. In the present invention, the self-tempered martensite is a structure in which the austenite phase has undergone martensitic transformation during accelerated cooling, and cementite is precipitated in the lath by cooling after the accelerated cooling is stopped. The structure obtained by the normal tempering treatment is tempered martensite, but the cementite of self-tempered martensite is very fine compared to this, and the average value is 400 nm or less.

  Self-tempered martensite and lower bainite are common in that there is no coarse cementite at the grain boundaries and there are fine carbides in the matrix. Self-tempered martensite and lower bainite are structures obtained by accelerated cooling, and are metal structures that can improve the strength-toughness balance without tempering. From the above, the metal structure of the steel pipe of the present invention is a self-tempered martensite single structure or a composite structure of self-tempered martensite and lower bainite.

  Self-tempered martensite and lower bainite can be distinguished by observing them at 2000 to 50000 times using SEM. The self-tempered martensite and the lower bainite are both in a lath-like form and can be classified by the cementite precipitation form in the lath. Self-tempered martensite has multiple cementite major axis directions, and lower bainite has one major axis direction of cementite. The upper bainite forms acicular cementite or martensite and austenite (Martesite Austenite Constituent, MA) at the lath boundary, and the structure differs greatly from the lower bainite. Ferrite differs in that it is not a lath like bainite but a lump. Since pearlite precipitates plate-like cementite at grain boundaries, it is clearly different from lower bainite that precipitates within grains.

  In addition, the average grain size (effective crystal grain size) of the region surrounded by the large-angle grain boundaries having an orientation difference of 15 ° or more affects the propagation of cracks when fracture occurs. Since the toughness at −60 ° C. decreases when the effective crystal grain size exceeds 10 μm, the effective crystal grain size is preferably set to 10 μm or less from the viewpoint of strength-toughness balance. In addition, although the strength-toughness balance is excellent as the effective crystal grain size is small, about 3 μm is the lower limit in the current production equipment.

  In addition, if the effective crystal grain size is 10 μm or less, it can be considered that intragranular transformation using oxide as a production nucleus occurs. In order to promote the formation of intragranular transformation, it is necessary to control the amount of Al, Ti and O added and to disperse the Ti oxide. Furthermore, it is important to enhance the hardenability of the steel in order to promote intragranular transformation. This is because the hardenability of the steel is lowered and ferrite is generated from the grain boundaries during cooling. Since the ferrite transformation from the grain boundary competes with the intragranular transformation, when the ferrite transformation from the grain boundary occurs, the intragranular transformation is suppressed.

In the metal structure of the present invention, the average particle size of cementite is preferably 400 nm or less. This is because the low temperature toughness is remarkably improved by setting the average particle diameter of cementite to 400 nm or less. Moreover, if the density of cementite is 2.0 × 10 ⁵ pieces / mm ² or more, coarse cementite is hardly generated, and an extremely good strength-toughness balance is obtained.

  Cementite can be observed with a transmission electron microscope (referred to as TEM) using an extracted replica, and the size and density are measured at a magnification of about 10,000 times. The average particle size of cementite is preferably as small as possible, but cementite finer than 30 nm is difficult to observe. Therefore, in this invention, the average particle diameter and density of a cementite whose particle size is 30 nm or more are measured. The upper limit of the density of cementite is not particularly limited, but is necessarily determined by the amount of C added and the average particle size.

  Next, a manufacturing method will be described.

  The steel pipe of the present invention is a seamless steel pipe that is manufactured by punching and rolling a steel slab, and may undergo a drawing process after seamless rolling. After seamless rolling, in order to recrystallize the processed structure, it is kept for 1 to 30 minutes in the temperature range of 850 to 1050 ° C., or when the finish rolling temperature is low or cooled to less than 850 ° C., 850 to 1050 It is necessary to reheat to a temperature range of ° C. and hold for 1 to 30 minutes. The steel pipe is cooled to less than 850 ° C. after rolling, for example, when it is transported to a cooling device for performing accelerated cooling, and particularly when accelerated cooling is performed after the stretching step.

  The lower limit of the holding temperature after seamless rolling is required to be 850 ° C. or higher in order to recrystallize the processed structure. On the other hand, if the holding temperature exceeds 1050 ° C., the crystal grain size becomes coarse, and it is difficult to make the effective crystal grain size 10 μm or less even if intragranular transformation is utilized. Further, the lower limit of the holding time needs to be 1 minute or longer in order to recrystallize the processed structure. In addition, what is necessary is just to pass 1 to 30 minutes for the holding time within the temperature range of 850-1050 degreeC. For example, it may be held at a constant temperature using a heating means such as a heating furnace, or may be gradually cooled using a heat insulating cover using a heat insulating material or the like. However, if the holding time is long, productivity is impaired, so the upper limit is 30 minutes.

  When the steel pipe is cooled and reheated, when it is cooled until the temperature of the steel pipe is less than 650 ° C., partial transformation occurs, and after reheating, locally coarse crystal grains are formed by abnormal grain growth. May occur. In addition, precipitates are generated during cooling, and the solid solution amount of the element that enhances the hardenability is reduced, thereby reducing the hardenability. In particular, when B is contained, the decrease in hardenability due to the formation of B precipitates becomes significant. As a result, transformation from grain boundaries competing with intragranular transformation is promoted, making it difficult to refine the effective crystal grain size. Therefore, it is necessary to reheat without cooling to below 650 ° C. in order to suppress abnormal grain growth, further to ensure sufficient hardenability, to suppress transformation from grain boundaries, and to promote intragranular transformation. There is.

  In addition, when the steel pipe is cooled to 650 ° C. or more and less than 850 ° C., when the reheating temperature is less than 850 ° C. or when the holding time is less than 1 minute, the precipitate does not re-dissolve and is quenched. May decrease. As a result, upper bainite may be generated, or ferrite transformation from the grain boundaries may occur, and intragranular transformation may be suppressed.

  You may perform the extending | stretching process for adjusting the shape of a steel pipe after reheating. In addition, the process of reducing the outer diameter without changing the length of the steel pipe is also included in the drawing process. In the drawing process, the change in the plate thickness is small and the upper limit is about 15% in terms of the rolling reduction, so that the toughness is not lowered due to the strain introduced.

  Moreover, in this invention, since accelerated cooling is performed from the outer surface of a steel pipe, it is preferable to make the length of a steel pipe 5 times or more of an outer diameter. This is because when the length of the steel pipe is less than 5 times the outer diameter, when performing accelerated cooling from the outer surface, a coolant such as water wraps around the inner surface and the cooling becomes uneven and the steel pipe may bend. It is. In order to ensure uniform and accelerated cooling, it is more preferable that the length of the steel pipe is 10 times or more the outer diameter.

  After the seamless rolling, it is held at a temperature range of 850 to 1050 ° C. for 1 to 30 minutes, or further subjected to accelerated cooling through a stretching step. In the present invention, the accelerated cooling is limited to the method of cooling only from the outer surface while rotating the steel pipe in the circumferential direction. Thereby, it can cool uniformly over the circumferential direction and a longitudinal direction. On the other hand, if the steel pipe is not rotated, there is a problem that the lower surface of the steel pipe is excessively cooled, and when cooling from the inner surface side, water is accumulated on the lower surface and the cooling rate is not uniform. The cooling method can be arbitrarily selected, for example, a method in which water is directly applied to the outer surface of the steel pipe, a method in which water is applied in the tangential direction of the outer periphery of the steel pipe, or mist cooling.

  In the present invention, accelerated cooling is performed in two stages. Primary cooling promotes intragranular transformation. Subsequently, in the secondary cooling, the metal structure is a self-tempered martensite single structure or a composite structure of self-tempered martensite and lower bainite.

  Since primary cooling promotes intragranular transformation, it is important to suppress ferrite transformation from grain boundaries. Therefore, the temperature at which primary cooling is started is set to 750 ° C. or higher, and the metal structure at the start of accelerated cooling is set to an austenite single phase. Intragranular transformation occurs remarkably in a temperature range from less than 750 ° C. at which ferrite transformation occurs to a range of 450 to 550 ° C., so the lower limit of primary cooling is set to a temperature range of 450 to 550 ° C. When primary cooling is stopped at over 550 ° C. and secondary cooling is started, generation of intragranular transformation becomes insufficient. On the other hand, when primary cooling is performed to less than 450 ° C., upper bainite is likely to be generated, and it becomes difficult to obtain a self-tempered martensite single structure or a composite structure of self-tempered martensite and lower bainite. When the cooling rate of the primary cooling is less than 2 ° C./s, ferrite is generated, whereas when it exceeds 30 ° C./s, intragranular transformation is difficult to occur.

  Subsequent to the primary cooling, secondary cooling is performed at a cooling rate faster than the primary cooling, and the metal structure is made into a self-tempered martensite single structure or a composite structure of self-tempered martensite and lower bainite. When the cooling rate of the secondary cooling is less than 10 ° C./s, upper bainite is generated, and when it exceeds 50 ° C./s, the steel pipe is deformed. Therefore, the secondary cooling rate is limited to 10 to 50 ° C./s.

As described above, the secondary cooling stop temperature is extremely important in the present invention, and the secondary cooling rate V [° C./s] and the secondary cooling stop temperature T [° C.]
150 ≦ T ≦ 821.34 × V ^−0.3112 (Formula 1)
Is satisfied. When the secondary cooling is stopped and air-cooled, fine iron carbide precipitates in the metal structure, and a self-tempered martensite single structure or a composite structure of self-tempered martensite and lower bainite can be obtained. Will improve.

  Steels having chemical components shown in Table 1 were melted, and steel pieces having a diameter of 170 mm were cast by a converter-continuous casting process. These steel slabs are heated to 1240 ° C., pierced and rolled by the Mannesmann-plug mill method, accelerated and cooled as they are under the conditions shown in Table 2, or reheated and reduced in diameter in the stretching process, and then accelerated. Cooling was performed. Accelerated cooling was performed by water cooling from the outer surface side of the steel pipe using a ring-shaped cooling device.

  The steel pipe size was either an outer diameter of 126 mm and a wall thickness of 12.2 mm, an outer diameter of 138 mm, a wall thickness of 16.4 mm, an outer diameter of 146 mm, and a wall thickness of 20.6 mm. The length of the steel pipe is 6.5 m. The shape of the steel pipe is fixed at 3 points to the flat plate every 0.5mm from the end face on one side of the 6.5m long steel pipe, and the bending amount is judged by the floating height from the flat face on the opposite end face. did. A steel sheet having a floating amount of 10 mm or less was evaluated as a pass (◯), and a sample with a lift of 10 mm or more was rejected (x).

  A sample was taken from the vicinity of the center in the longitudinal direction and the thickness direction of the manufactured steel pipe, and the Vickers hardness was measured with a test force of 98.07 N in accordance with JIS Z 2244. SEM and optical microscope are used for the metal structure. In SEM observation, the magnification is increased up to 50000 times. went. The intragranular bainite is a bainite that is generated radially with a Ti oxide as a center, and has a petal shape.

Next, using EBSP mounted on the SEM, crystal orientation measurement is performed to identify a grain boundary having an orientation difference of 15 ° or more, and the average value is obtained by setting the equivalent circle radius of the region covered by the grain boundary as the grain size. Asked. Further, extracted replicas were prepared, and the average value of the equivalent circle radius and the number of unit areas (mm ² ) of cementite were determined from 10 structure photographs enlarged 10000 times to 50000 times using TEM.

The tensile test was performed according to JIS Z2241 using a JIS Z 2201 No. 11 tensile test piece, and the yield strength and tensile strength were measured. The toughness was evaluated by performing a Charpy impact test at −60 ° C. using a JIS Z 2242 V-notch full-size test piece and measuring the absorbed energy (vE- ₆₀ ).

  Weldability was evaluated by checking the presence or absence of cracks by welding the ends of two steel pipes while butting them together. The welding wire had a strength of 780 MPa class, and a steel pipe joint was produced by carbon dioxide gas welding. After welding, after a lapse of 24 hours or more, a visual inspection was performed to inspect for cracks, and those without cracks were determined to be acceptable (O), and those with cracks were determined to be unacceptable (X).

  The results are shown in Table 3. The underline in Table 3 means outside the scope of the present invention or outside the preferred range. As shown in Table 3, the No. Nos. 1 to 13 have an appropriate metal structure including intragranular transformation and strength necessary as a steel pipe for machine structure, and are excellent in toughness at -60 ° C. On the other hand, production No. 14 to 25 are comparative examples, and the component composition and production conditions are outside the scope of the present invention.

  No. No. 14 is an example in which the amount of Al is high, and Ti oxide that becomes the starting point of intragranular transformation is hardly generated, so intragranular transformation is not observed, and the toughness at −60 ° C. is lowered. No. No. 15 is an example in which the amount of C is low and the secondary cooling stop temperature is high, so that the upper bainite structure is generated and the toughness is lowered. Furthermore, no. No. 15 has a deteriorated shape because the cooling rate of the secondary cooling is too fast.

  No. In No. 16, the amount of P is excessive, the amount of Mn is small and the hardenability is insufficient, and the cooling rate of primary cooling is too fast, so the intragranular transformation becomes insufficient and the toughness is reduced. Furthermore, no. No. 16 also has poor weldability due to the amount of P. No. No. 17 is an example in which the upper bainite is generated because the Si amount is excessive, and the effective crystal grain size is coarsened and the toughness is decreased because the Ti amount is small. No. No. 18 contains impurities S and N excessively, resulting in poor toughness and weldability, and Al and O amounts are also excessive, resulting in insufficient intragranular transformation. . No. No. 19 is an example in which the Al amount and the Nb amount are excessive, and the toughness and weldability are lowered.

  No. 20 to 25 are comparative examples in which the component composition is within the scope of the present invention and the production conditions are outside the scope of the present invention. No. No. 20, because the reheating temperature was too high and the holding time was too long, the crystal grains became coarse, the hardenability was lowered, the intragranular transformation was not remarkable, and the upper bainite was formed, and the toughness was low. It is an example.

  No. No. 21, reheating time is too short, hardenability is reduced, intragranular transformation is not remarkable, secondary cooling stop temperature is too high, upper bainite is generated, strength and toughness are reduced. It is an example. No. No. 22 is an example in which, after rolling, the steel was cooled to less than 650 ° C., so that the hardenability was lowered, the intragranular transformation was not remarkable, and a mixed structure of tempered martensite and upper bainite was formed. Furthermore, no. No. 22 has a deteriorated shape because the cooling rate of the secondary cooling is too fast.

  No. No. 23, the reheating temperature is too low, the hardenability is lowered, the primary cooling start temperature is low, the cooling rate of the primary cooling is too slow, ferrite is generated, the intragranular transformation is not remarkable, and the toughness is This is an example of a decline.

  No. No. 24 is an example in which the cooling rate of primary cooling and secondary cooling is too high, and the stop temperature of secondary cooling is too high, and no intragranular transformation occurs, and mixing of tempered martensite and coarse bainite having coarse cementite It becomes a structure, toughness is reduced, and the shape is also deteriorated. No. No. 25 is an example in which the upper bainite was generated and the toughness was lowered because the secondary cooling stop temperature was too high.

It is a figure which shows the relationship between Al / Ti and an effective crystal grain size. It is a figure which shows the relationship between a secondary cooling rate, a secondary cooling stop temperature, and the presence or absence of a brittle fracture surface.

Claims (5)

% By mass
C: 0.030 to 0.100%,
Mn: 0.80 to 3.00%,
Ti: 0.005 to 0.045%,
Nb: 0.003-0.040%,
O: 0.0010 to 0.0050%
Containing
Si: 0.50% or less,
P: 0.020% or less,
S: 0.008% or less,
Al: 0.010% or less,
N: limited to 0.0080% or less,
Al / Ti ≦ 0.75
Satisfied,
B: 0.0030% or less,
Ni: 1.50% or less,
Cr: 1.50% or less,
Cu: 1.00% or less,
Mo: Contains one or more of 0.50% or less, the balance is Fe and inevitable impurities, and the metal structure is a self-tempered martensite single structure or a mixture of self-tempered martensite and lower bainite. A high-strength seamless steel pipe for machine structures excellent in toughness, characterized in that the average diameter of a region surrounded by large grain boundaries having a misorientation of 15 ° or more is 10 μm or less. ^2. The toughness according to claim 1, wherein the average grain size of the iron-based carbide in the metal structure is 400 nm or less, and the density of the iron-based carbide is 2.0 × 10 ⁵ pieces / mm ² or more. High strength seamless steel pipe for mechanical structure. The steel slab having the chemical component according to claim 1 is subjected to hot piercing and rolling, reheated as it is or without cooling to a temperature range of less than 650 ° C, and 1 to 850 to 1050 ° C. After holding for 30 minutes, primary cooling is performed from the outer surface of the steel pipe to a temperature range of 450 to 550 ° C. at a cooling rate of 2 to 30 ° C./s while rotating the steel pipe in the circumferential direction from a temperature range of 750 ° C. or higher. The secondary cooling is stopped within the range obtained by the following (formula 1) at the secondary cooling rate V [° C./s] that is within the range of 10 to 50 ° C./s and faster than the cooling rate of the primary cooling. A method for producing a high-strength seamless steel pipe for machine structure excellent in toughness, characterized by performing secondary cooling to a temperature T [° C].
150 ≦ T ≦ 821.34 × V ^−0.3112 (Formula 1)   After holding for 1 to 30 minutes in the temperature range of 850 to 1050 ° C. according to claim 3, the tube is formed by a stretching process, and the steel tube is rotated in the circumferential direction from the temperature range of 750 ° C. or higher, while being primary. Cooling and secondary cooling are performed, The manufacturing method of the high strength seamless steel pipe for machine structures excellent in toughness of Claim 3 characterized by the above-mentioned.   5. The high-strength seamless steel pipe for machine structures having excellent toughness according to claim 3, wherein the length of the steel pipe after the seamless rolling or after the stretching process is 5 times or more of the outer diameter before accelerated cooling. Production method.

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