JP5915413B2 - ERW steel pipe excellent in low temperature toughness and method for producing the same - Google Patents

Description

  The present invention relates to an ERW steel pipe, and more particularly to improvement of workability and low temperature toughness.

In recent years, from the viewpoint of conservation of the global environment, there has been a demand for reducing the weight of automobile bodies, particularly for improving fuel efficiency. As a means of reducing the weight of automobile bodies, it is effective to apply hollow steel pipes to parts that have traditionally been used as solid products such as steel bars. Steel pipes, in particular, are relatively inexpensive and have dimensional accuracy. The application of excellent ERW steel pipes is also underway.
However, recently, the characteristics required for steel pipes as automobile parts are becoming more severe, and it is required to combine high strength, high ductility, and high toughness. In response to such a demand, for example, Japanese Patent No. 3683378 discloses a method for manufacturing a high toughness and high ductility steel pipe.

  In the technique described in Patent Document 1, a steel pipe material containing C: 0.60 wt% or less is heated to 550 to 800 ° C., the crystal grain size of the steel pipe material is set to 20 μm or less, and then the ferrite recrystallization temperature range. Drawing is performed with a reduction in area of 20% or more in the temperature range of 550 to 750 ° C, the average grain size of the cross section perpendicular to the longitudinal direction of the tube is 3 μm or less, and the structure is mainly ferrite, ferrite + pearlite, ferrite + cementite It is a manufacturing method of the high toughness high ductility steel pipe made into the steel pipe which consists of the structure which carries out. According to the technology described in Patent Document 1, a high toughness and high ductility steel pipe having a high strength of 500 MPa or more, an elongation of 20% or more, and a strength-ductility balance (tensile strength x elongation) of 10,000 MPa% or more. Is supposed to be obtained.

Japanese Patent No. 3683378

However, in the technique described in Patent Document 1, it has been necessary to perform drawing rolling in a relatively low temperature range of 750 to 550 ° C. in order to refine the average crystal grains. Thereby, a large working strain remains in the steel pipe, and particularly when the rolling reduction is high, workability and toughness may be lowered. In addition, rolling at a low temperature has a problem that the load on the equipment is large and the productivity is lowered.
An object of the present invention is to solve the problems of the prior art, and to provide an electric-welded steel pipe excellent in workability and low-temperature toughness and a method for producing the same.

In order to achieve the above-described object, the present inventors diligently studied the relationship between the metal microstructure and the low temperature toughness. As a result, an effective metal microstructure that can significantly improve low-temperature toughness is not a structure with a small average crystal grain size, but a maximum grain size surrounded by large-angle grain boundaries with a crystal orientation difference of 15 ° or more. It has been found that the metal microstructure is within the proper range and the ratio of the brittle crystal grains (brittle grains) is within the proper range. The brittle crystal grains (brittle grains) are crystal grains in which the angle between the pipe circumferential direction and the <100> orientation is θ and cos ² θ is 0.9 or more. And in order to set it as such a structure, it came to mind that it is important to perform an appropriate diameter reduction rolling according to a steel pipe composition. And it has been found that by using such a metal microstructure, it is possible to produce an electric-welded steel pipe excellent in low-temperature toughness while maintaining high workability.

First, a description will be given of experimental results performed by the present inventors and serving as the basis of the present invention.
A hot-rolled steel strip having the composition and thickness shown in Table 1 is continuously roll-formed into an open tube having a substantially circular cross section, and then both ends of the steel strip are subjected to electric resistance welding to form an ERW steel pipe (outer diameter 146 mmφ). ). The obtained ERW steel pipe was heated to a temperature (960 ° C.) higher than the Ac3 transformation point by induction heating and subjected to diameter reduction rolling under the conditions shown in Table 2. Note that the rolling end temperature of the reduced diameter rolling was limited to the Ac3 transformation point or less. For comparison, an ERW steel pipe not subjected to reduced diameter rolling was also prepared.

A test piece was collected from the obtained ERW steel pipe so that the cross-section in the pipe circumferential direction became the observation surface. The collected specimen was polished and corroded (Nital liquid corrosion) to reveal the structure, and then the metal structure was observed using a scanning electron microscope equipped with a crystal orientation analyzer (manufactured by TSL). In addition, crystal orientation analysis software (manufactured by TSL: OIM Data Analysis) was used for the analysis of the texture.
As a result of measuring the grain size distribution of the obtained steel pipe, when the diameter reduction rolling end temperature is on the high temperature side of the two-phase region, there are many coarse grains and the diameter reduction rolling end temperature decreases. In addition, a mixed grain structure having two distributions of crystal grains having the same size and coarse crystal grains as in the case where the diameter reduction rolling is not performed (as in the pipe forming using the hot-rolled steel sheet) is exhibited. Furthermore, as a result of mapping the crystal orientation with the EBSD (Electron Back Scatter Diffraction) method for the obtained steel pipe, the crystal orientation is almost random in the steel pipe not subjected to reduction rolling, but the steel pipe subjected to reduction rolling Then, it exhibits a crystal orientation distribution in which the accumulation in the <100> direction is strong in the tube circumferential direction. Then, as the temperature at which the diameter reduction finishes decreases, it becomes a mixed grain consisting of fine crystal grains and coarse crystal grains. In the fine crystal grains, there are <100> oriented crystal grains, but <111> oriented crystal grains. It was found that the proportion of crystal grains with <100> orientation increases with coarse crystal grains, while there are many crystal grains.

According to a study by H. Inagaki et al. Using a single crystal (see H. Inagaki, K. Kurihara and I. Kozasu: Trans. ISIJ, 17 (1977), 75) When crystal grains with> orientation are oriented, the test piece is considered to be brittlely broken. According to H. Inagaki's analysis, the fracture stress σf is proportional to 1 / cos ² θ, and the greater the cos ² θ, the lower the fracture stress and the higher the degree of embrittlement. Note that θ is an angle formed by the Charpy impact test piece and the <100> orientation.

Therefore, the present inventors made cos ² θ an embrittlement degree evaluation index, made crystal grains having cos ² θ of 0.9 or more embrittled, and reduced the area ratio of the embrittled grains in a diameter-rolled ERW steel pipe. The relationship between and toughness was investigated.
A Charpy impact test piece was taken from the obtained ERW steel pipe so that the pipe circumferential direction coincided with the longitudinal direction of the test piece, a Charpy impact test was conducted in accordance with the provisions of JIS Z 2242, and the fracture surface transition temperature vTrs was determined. And toughness was evaluated. A ring-shaped test piece was cut out from the steel pipe so that the circumferential direction of the pipe coincided with the longitudinal direction of the test piece, pressed and flattened, and then ground on both sides uniformly to obtain a Charpy impact test piece (thickness: 2.3). mm).

On the other hand, for the obtained electric resistance welded steel pipe, from the mapping of <100> crystal orientation of each crystal grain obtained by the EBSD method, the angle θ formed between the <100> orientation of each crystal grain and the pipe circumferential direction is obtained. The cos ² θ of the crystal grains was calculated, and the area ratio Af (cos ² θ ≧ 0.9) of the crystal grains (brittle grains) having a cos ² θ of 0.9 or more was calculated.
In addition, for the obtained ERW steel pipes, the crystal orientation mapping obtained by EBSD analysis was used to identify crystal grains surrounded by a large-angle grain boundary with a crystal orientation difference of 15 ° or more from adjacent crystal grains. The average value of the grain sizes was determined and used as the average crystal grain size, and the maximum grain size (maximum crystal grain size) among these crystal grains was determined and used as the maximum crystal grain size.

The obtained results are shown in FIG. 1 in the relationship between vTrs and Af (cos ² θ ≧ 0.9), and in FIG. 2 in the relationship between vTrs, the average crystal grain size, and the maximum crystal grain size.
From FIG. 1, it can be seen that if Af (cos ² θ ≧ 0.9) is 55% or less, it has high toughness of vTrs: −40 ° C. or less. As Af (cos ² θ ≧ 0.9) increases, vTrs increases monotonously. However, when Af (cos ² θ ≧ 0.9) exceeds 50%, vTrs shows a rapid increase. The present inventors consider the cause of the rapid increase in vTrs when Af (cos ² θ ≧ 0.9) exceeds 55% as follows.

When Af (cos ² θ ≧ 0.9) exceeds 55%, the difference in crystal orientation between adjacent crystal grains becomes small, and a connection phenomenon occurs between adjacent crystal grains, as if very coarse embrittled grains are formed. It is considered that the same behavior as that formed is formed, and a rapid increase in vTrs occurs.
For this reason, it has been found that it is first necessary to limit Af (cos ² θ ≧ 0.9) to 55% or less in order to prevent a sudden increase in vTrs and ensure the desired high toughness. It was.

  Further, FIG. 2 shows that vTrs shows a stronger correlation with the maximum crystal grain size than with the average grain size. vTrs has a low correlation with the average particle diameter, and in particular, no correlation is observed when the average particle diameter is 20 μm or less. For this reason, in order to make an ERW steel pipe with excellent low-temperature toughness, the largest of the grains surrounded by a large-angle grain boundary with a crystal orientation difference of 15 ° or more determined by EBSD analysis. It was found that it is necessary to limit the grain size (maximum crystal grain size) to 50 μm or less.

  Moreover, the influence of the end temperature of the diameter reduction rolling is shown in FIG. 3 in relation to vTrs and C content. For comparison, a hot-rolled steel sheet (without reduced diameter rolling), which is a steel pipe material, was used. From FIG. 3, when the amount of C is less than 0.10%, vTrs varies greatly depending on the end temperature of diameter reduction rolling. On the other hand, when the C content is 0.10% or more, the change in vTrs is small. This is considered to be due to the fact that when the C content is 0.10% or more, the change in the maximum crystal grain size due to the reduction diameter reduction temperature is small.

  As a result of further studies by the present inventors, the structure in which the area ratio of the above-mentioned embrittled grains is 55% or less, and the maximum crystal grain size among the crystal grains surrounded by the large tilt grain boundaries is 50 μm or less is stabilized. Therefore, the hot diameter reduction rolling is reduced diameter rolling with a thickness increase rate of + 1% or more, or the cold diameter reduction processing as a secondary process is further increased. It is preferable that the diameter reduction processing is such that +1 becomes + 1% or more.

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) By mass%, C: 0.10 to 0.50%, Si: 0.01 to 1.0%, Mn: 0.01 to 2.0%, P: 0.10% or less, S: 0.01% or less, Al: 0.001 to 0.1%, N: 0.01 The maximum grain size of the crystal grains having a composition composed of the remaining Fe and inevitable impurities, and surrounded by a large-angle grain boundary whose orientation difference from the adjacent crystal grains is 15 ° or more is 50 μm or less. Yes, the degree of embrittlement evaluation index cos ² θ is the following formula (1)
cos ² θ ≧ 0.9 (1)
(Where cos ² θ: embrittlement evaluation index, θ: angle between the <100> orientation of the crystal grains and the tube circumferential direction (°))
An ERW steel pipe having a structure in which the area ratio of crystal grains satisfying the requirements is 55% or less and excellent in low temperature toughness.
(2) In (1), in addition to the above-mentioned composition, the following groups A and B: Group A: Cr: 1% or less, Cu: 1% or less, Ni: 1% or less, Mo: 1% One or more selected from the following,
Group B: ERW steel pipe characterized by containing one or two groups selected from one or two of Ca: 0.02% or less and REM: 0.02% or less.
(3) A method of manufacturing an electric resistance welded steel pipe in which the material steel pipe is subjected to heat treatment and then subjected to reduced diameter rolling, wherein the raw steel pipe is in mass%, C: 0.10 to 0.50%, Si: 0.01 to 1.0% , Mn: 0.01 to 2.0%, P: 0.10% or less, S: 0.01% or less, Al: 0.001 to 0.1%, N: 0.01% or less, and an electric resistance welded steel pipe having a composition composed of the remaining Fe and inevitable impurities The diameter reduction rolling is performed at the rolling end temperature: 750 to 900 ° C., the cumulative diameter reduction ratio: 30 to 80%, and the following formula (2): Thickening rate = (thickness after diameter reduction rolling−before diameter reduction rolling) Wall thickness) / (wall thickness before reduced diameter rolling) x 100 (%) (2)
The maximum grain size of a crystal grain surrounded by a large-angle grain boundary whose orientation difference with an adjacent crystal grain is 15 ° or more is 50 μm or less as a rolling whose thickness increase rate defined by is + 1% or more, The embrittlement degree evaluation index cos ² θ defined by cos ² θ of the angle θ formed by the <100> orientation of the grain and the pipe circumferential direction is expressed by the following equation (1).
cos ² θ ≧ 0.9 (1)
(Where cos ² θ: embrittlement evaluation index, θ: angle between the <100> orientation of the crystal grains and the tube circumferential direction (°))
A method for producing an electric-welded steel pipe excellent in low-temperature toughness, characterized in that the steel pipe has a structure in which the area ratio of crystal grains satisfying the requirements is 55% or less.
(4) The method for producing an electric resistance steel pipe according to (3), wherein the heat treatment is performed by heating the raw steel pipe to a temperature of Ac3 transformation point to 1050 ° C. and soaking.
(5) The method for producing an electric-welded steel pipe according to (3) or (4), wherein after the diameter reduction rolling is performed, cold diameter reduction processing is further performed as secondary processing.
(6) In (5), the cold diameter reduction is performed at room temperature, the cumulative diameter reduction ratio: 1 to 30%, and the following (3) formula: wall thickness increase rate = (after cold diameter reduction processing) Thickness-Thickness before cold shrinking) / (Thickness before cold shrinking) x 100 (%) (3)
A method for producing an electric resistance welded steel pipe, characterized in that the thickness increase rate defined in (1) is + 1% or more.
(7) In any one of (3) to (6), in addition to the composition of the material steel pipe, in addition to mass%, the following group A and group B: Group A: Cr: 1% or less, Cu: 1% or less Ni: 1% or less, Mo: 1% or less selected from 1% or less,
Group B: A method for producing an ERW steel pipe, comprising one or two groups selected from one or two of Ca: 0.02% or less and REM: 0.02% or less.

  ADVANTAGE OF THE INVENTION According to this invention, the electric resistance welded steel pipe excellent in the low temperature toughness whose fracture surface transition temperature vTrs is -40 degrees C or less can be manufactured stably and easily at low cost, and there is a remarkable industrial effect.

It is a graph showing the relationship between the fracture appearance transition temperature vTrs and embrittlement particle area ratio ^{Af (cos 2 θ ≧ 0.9)} . It is a graph which shows the relationship between a fracture surface transition temperature vTrs and a crystal grain size. It is a graph which shows the influence of the diameter reduction rolling temperature on the relationship between fracture surface transition temperature vTrs and C content.

The ERW steel pipe of the present invention is in mass%, C: 0.06 to 0.50%, Si: 0.01 to 1.0%, Mn: 0.01 to 2.0%, P: 0.001 to 0.10%, S: 0.01% or less, Al: 0.001 to 0.1 %, N: 0.01% or less, and has a composition comprising the balance Fe and inevitable impurities.
First, the reasons for limiting the composition of the ERW steel pipe of the present invention will be described. Hereinafter, unless otherwise specified, mass% is simply referred to as%.

C: 0.06-0.50%
C is an element that greatly affects the crystal grain size of the steel tube after diameter reduction rolling. In the present invention, the maximum crystal grain size must be not more than a predetermined value, and it is necessary to contain 0.06% or more in order to ensure desired low temperature toughness. . On the other hand, if the content exceeds 0.50%, the toughness of the base metal is lowered, the weldability is lowered, and the desired ERW weld toughness cannot be secured stably. For this reason, C was limited to the range of 0.06 to 0.50%. In addition, Preferably it is 0.10 to 0.50%.

Si: 0.01-1.0%
Si is a solid solution strengthening element that acts as a deoxidizer and increases the steel pipe strength. In order to acquire such an effect, it is necessary to contain 0.01% or more. On the other hand, if the content exceeds 1.0%, the weldability is lowered, and stable quality of the ERW weld cannot be secured. For this reason, Si was limited to the range of 0.01 to 1.0%. In addition, Preferably it is 0.01 to 0.5%.

Mn: 0.01-2.0%
Mn is an element that contributes to an increase in the strength of the steel pipe, and it is necessary to contain 0.01% or more in order to ensure a desired high strength. On the other hand, if the content exceeds 2.0%, the quality of the ERW welds will be reduced. For this reason, Mn was limited to the range of 0.01 to 2.0%. In addition, Preferably it is 0.01 to 1.5%.
P: 0.10% or less P is an element that segregates at grain boundaries and lowers toughness, and is preferably reduced as much as possible. However, it is an element that contributes to an increase in steel pipe strength. Is acceptable up to 0.10%. For this reason, P was limited to 0.10% or less. In addition, Preferably it is 0.001 to 0.05%.

S: 0.01% or less S is an element which exists as sulfide inclusions in steel and reduces workability, toughness, etc., and is preferably reduced as much as possible, but is acceptable up to 0.01%. For this reason, S was limited to 0.01% or less. In addition, Preferably it is 0.008% or less.
Al: 0.001 to 0.1%
Al is an element that acts as a deoxidizer, and suppresses the growth of γ grains during high-temperature heating, thereby contributing to improved toughness. In order to acquire such an effect, it is necessary to contain 0.001% or more, but inclusion exceeding 0.1% may increase toughness due to an increase in Al inclusions. For this reason, Al was limited to the range of 0.001 to 0.1%. In addition, Preferably it is 0.001 to 0.05%.

N: 0.01% or less N is inevitably contained in steel, but has an action of combining with Al to suppress the growth of γ grains during high-temperature heating. However, an excessive content exceeding 0.01% increases the amount of solute N and decreases the toughness of the ERW weld. For this reason, N was limited to 0.01% or less. In addition, Preferably it is 0.008% or less.

Although the above-mentioned components are basic components, in addition to these basic compositions, one or two groups selected from Group A and Group B may be further contained.
Group A: Cr: 1% or less, Cu: 1% or less, Ni: 1% or less, Mo: 1% or less selected from the following: Group A: Cr, Cu, Ni, Mo are Any of these is an element that contributes to an increase in the strength of the steel pipe, and can be selected as necessary to contain one or more. In order to obtain such an effect, Cr: 0.01% or more, Cu: 0.01% or more, Ni: 0.01% or more, Mo: 0.01% or more are desirable, but Cr: 1%, Cu: 1% , Ni: 1%, Mo: more than 1%, respectively, toughness is significantly reduced. For this reason, when it contains, it is preferable to limit to Cr: 1% or less, Cu: 1% or less, Ni: 1% or less, and Mo: 1% or less, respectively.

Group B: Ca: 0.02% or less, REM: One or two of 0.02% or less Group B: Both Ca and REM are elements that contribute to the control of inclusion morphology, and are selected as necessary. Can be contained. In order to obtain such an effect, it is desirable to contain Ca: 0.0010% or more, REM: 0.0010% or more. However, if the content exceeds Ca: 0.02% and REM: 0.02%, the amount of inclusions is too large. , Ductility and toughness are reduced. For this reason, when it contains, it is preferable to limit to Ca: 0.02% or less and REM: 0.02% or less.

The balance other than the components described above consists of Fe and inevitable impurities. As an inevitable impurity, O: 0.01% or less is acceptable.
Next, the reason for limiting the structure of the ERW steel pipe of the present invention will be described.
The ERW steel pipe of the present invention has a structure in which the maximum grain size of the crystal grains surrounded by the large-angle grain boundaries is 50 μm or less and the area ratio Af (cos ² θ ≧ 0.9) of the embrittled grains is 55% or less.

Maximum grain size of crystal grains surrounded by high-angle grain boundaries: 50 μm or less The high-angle grain boundaries referred to here are grain boundaries whose orientation difference from adjacent crystal grains is 15 ° or more. The large-angle grain boundary is determined from the crystal orientation mapping obtained by EBSD analysis. As shown in FIG. 2, the toughness of the steel pipe has a strong correlation with the maximum grain size among the crystal grains surrounded by the large-angle grain boundaries. When the maximum grain size of the crystal grains surrounded by the large-angle boundaries exceeds 50 μm, vTrs becomes high as shown in FIG. For this reason, the maximum grain size of the crystal grains surrounded by the large tilt grain boundaries is limited to 50 μm or less.

Embrittlement particle area ratio ^{Af (cos 2 θ ≧ 0.9)} : 55% or less in the present invention, a cos ² theta as embrittlement evaluation index, cos ² theta following formula (1)
cos ² θ ≧ 0.9 (1)
(Where cos ² θ: embrittlement degree evaluation index, θ: angle between the <100> orientation of the crystal grains and the tube circumferential direction (°))
The crystal grains satisfying the above are called embrittled grains. In the present invention, when the area ratio Af (cos ² θ ≧ 0.9) of the embrittled grains exceeds 55%, the accumulation of <100> -oriented crystals in the tube circumferential direction becomes strong, and the embrittlement tendency becomes strong. As a result, the base material toughness of the steel pipe decreases.

For this reason, the area ratio Af (cos ² θ ≧ 0.9) of the embrittled grains is limited to 55% or less.
Next, a preferred method for producing the ERW steel pipe of the present invention will be described.
In the present invention, an ERW steel pipe having the above-described composition is used as a raw steel pipe, and the raw steel pipe is subjected to heat treatment and then subjected to hot diameter reduction rolling.
It is preferable that the heat treatment is a heat treatment by heating to a temperature of Ac3 transformation point to 1050 ° C. If the heating temperature is less than the Ac3 transformation point, the heating temperature is too low and subsequent reduction rolling becomes difficult. On the other hand, when the temperature is higher than 1050 ° C., the desired end diameter reduction temperature cannot be ensured and the toughness is lowered. For this reason, it is preferable that the heat treatment is a treatment in which heating is performed at a temperature of Ac3 transformation point to 1050 ° C. and soaking.

The heat-treated material steel pipe is then subjected to hot diameter reduction rolling.
The hot diameter reduction rolling is rolling with a rolling end temperature of 750 to 900 ° C., a cumulative diameter reduction ratio of 30 to 80%, and a thickness increase rate of + 1% or more.
Rolling end temperature of reduced diameter rolling: 750-900 ° C
When the rolling end temperature of hot diameter reduction rolling is less than 750 ° C., the diameter reduction rolling temperature becomes too low, the residual strain increases, adversely affects the low temperature toughness, and the productivity decreases. On the other hand, when the rolling end temperature exceeds 900 ° C. and becomes a high temperature, the surface properties are deteriorated and the productivity is also lowered. For this reason, the rolling end temperature of hot diameter reduction rolling is limited to a range of 750 to 900 ° C.

Cumulative reduction ratio of reduced diameter rolling: 30-80%
If the cumulative diameter reduction ratio of hot diameter reduction rolling is less than 30%, the amount of diameter reduction is too small to achieve refinement of crystal grains. On the other hand, if the cumulative diameter reduction exceeds 80%, the low temperature toughness is lowered due to the increase in residual strain, and the productivity is also lowered. For this reason, the cumulative diameter reduction ratio of hot diameter reduction rolling is limited to a range of 30 to 80%. In addition, Preferably it is 30 to 75%.

Thickening rate: + 1% or more
The thickness increase rate is expressed by the following equation ( 2 ): Wall thickness increase rate = (thickness after diameter reduction rolling−thickness before diameter reduction rolling) / (thickness before diameter reduction rolling) × 100 (%) ( 2 ) )
Defined by

  When the thickness increase is less than + 1%, particularly when the thickness increase is negative, it becomes impossible to effectively prevent the accumulation of <100> oriented crystal grains that are likely to become brittle. For this reason, in this invention, the thickness increase rate of hot diameter reduction rolling shall be + 1% or more. As a result, accumulation of crystals that are easily embrittled is prevented and toughness is improved. By performing rolling to increase the thickness before and after the diameter reduction rolling, the accumulation of <111> orientation crystal grains in the tube circumferential direction becomes stronger, and <100> orientation crystal grains that tend to become brittle are accumulated. Can be prevented. For this reason, the thickness increase ratio of hot diameter reduction rolling is limited to + 1% or more. In addition, Preferably it is 1.5% or more. Moreover, if it exceeds 30%, there is a concern about the generation of internal wrinkles.

In addition, although cooling after completion | finish of hot diameter reduction rolling does not need to specifically limit, it is preferable from a viewpoint of productivity to set it as air cooling or more.
In addition, in this invention, after performing hot diameter reduction rolling, you may perform cold diameter reduction processing as secondary processing.
The diameter reduction process in the cold is a process in which the cumulative diameter reduction rate is 1 to 30% at room temperature and the thickness increase rate is + 1% or more.

Cumulative reduction ratio of cold diameter reduction: 1-30%
If the cumulative diameter reduction ratio in the cold diameter reduction processing is less than 1%, the amount of diameter reduction is too small to form a desired texture. On the other hand, when the cumulative diameter reduction exceeds 30%, the toughness is greatly lowered due to an increase in processing strain, and post-heat treatment is required. For this reason, the cumulative diameter reduction ratio of cold diameter reduction processing is limited to a range of 1 to 30%. In addition, Preferably it is 1 to 20%.

Thickening rate of cold diameter reduction: + 1% or more Thickness ratio is the following formula ( 3 ): Thickening rate = (thickness after cold diameter reduction-thickness before cold diameter reduction) / (Thickness before cold shrinking) x 100 (%) ( 3 )
Defined by

  When the thickness increase is less than + 1%, particularly when the thickness increase is negative, it becomes impossible to effectively prevent the accumulation of <100> oriented crystal grains that are likely to become brittle. For this reason, in this invention, the thickness increase rate of cold diameter reduction rolling shall be + 1% or more. This prevents accumulation of <100> -oriented crystals that tend to become brittle and improves toughness. By performing processing to increase the thickness before and after the diameter reduction processing, the accumulation of <111> orientation crystal grains in the tube circumferential direction becomes stronger, and <100> orientation crystal grains that tend to become brittle are accumulated. Can be prevented. For this reason, the thickness increase rate in the cold diameter reduction process is limited to + 1% or more. In addition, Preferably it is 2% or more. Examples of cold diameter reduction include drawing, forging, and swaging.

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

  Hot-rolled steel sheets (thickness: 4 mm) having the composition shown in Table 3 are continuously roll-formed, processed into a substantially circular cross-section, and the ends in the width direction of the steel sheets are electro-welded by resistance welding, ERW steel pipe was used. The obtained electric resistance welded steel pipe was heated under the conditions shown in Table 4, subjected to hot diameter reduction rolling under the conditions of the cumulative diameter reduction rate, the wall thickness increase rate, and the rolling end temperature shown in Table 4, and air-cooled to room temperature. . In some cases, as the secondary processing, cold reduction processing was performed at room temperature under the conditions of the cumulative diameter reduction rate and the wall thickness increase rate shown in Table 4. In the diameter reduction processing, press working oil having a friction coefficient μ of 0.12 was used.

The obtained electric resistance welded steel pipe was subjected to a structure observation, a tensile test, and an impact test. The test method was as follows.
(1) Microstructure observation A specimen for microstructural observation was collected from the obtained ERW steel pipe so that the cross section in the circumferential direction of the pipe became the observation surface. The collected specimen was polished and corroded (Nital liquid corrosion) to reveal the structure, and then the metal structure was observed using a scanning electron microscope equipped with a crystal orientation analyzer (manufactured by TSL). In addition, crystal orientation analysis software (manufactured by TSL: OIM Data Analysis) was used for the analysis of the texture.

For the obtained metallographic structure, the crystal orientation is mapped by EBSD (Electron Back Scatter Diffraction) method, the crystal grains surrounded by the large-angle grain boundaries with a crystal orientation difference of 15 ° or more are identified, and the average grain size of these crystal grains The diameter and the maximum grain size among these crystal grains were determined. Further, for each crystal grain, an angle θ formed between the tube circumferential direction and the <100> orientation is obtained, cos ² θ is calculated, and the area ratio Af of the embrittled grain that is a crystal grain having cos ² θ of 0.9 or more. Was calculated.
(2) Impact test Charpy impact test specimens were collected from the obtained ERW steel pipes so that the circumferential direction of the pipes coincided with the longitudinal direction of the specimen, and subjected to Charpy impact tests in accordance with the provisions of JIS Z 2242. The surface transition temperature vTrs was determined and the toughness was evaluated. A ring-shaped test piece was cut out from the steel pipe so that the circumferential direction of the pipe coincided with the longitudinal direction of the test piece, pressed and flattened, then ground on both sides evenly and Charpy impact test piece (plate thickness: 2.5 mm) ).

  The obtained results are shown in Table 3.

  All of the examples of the present invention are ERW steel pipes having excellent low-temperature toughness with a fracture surface transition temperature vTrs of −40 ° C. or lower. On the other hand, the comparative example which deviates from the range of the present invention cannot secure desired low temperature toughness.

Claims (7)

% By mass
C: 0.10 to 0.50%, Si: 0.01 to 1.0%,
Mn: 0.01 to 2.0%, P: 0.10% or less,
S: 0.01% or less, Al: 0.001 to 0.1%,
N: The maximum grain size of a crystal grain containing 0.01% or less, having a composition composed of the remainder Fe and inevitable impurities, and surrounded by a large-angle grain boundary whose orientation difference from the adjacent crystal grain is 15 ° or more. ERW steel pipe characterized in that it has a structure in which the area ratio of crystal grains satisfying the following formula (1) is 55% or less with a brittleness evaluation index cos ² θ of 50 μm or less and excellent in low temperature toughness .
Record
cos ² θ ≧ 0.9 (1)
Where cos ² θ: embrittlement degree evaluation index,
θ: Angle formed between the <100> orientation of crystal grains and the tube circumferential direction (°) 2. The electric resistance welded steel pipe according to claim 1, further comprising one group or two groups selected from the following group A and group B by mass% in addition to the composition.
Group A: Cr: 1% or less, Cu: 1% or less, Ni: 1% or less, Mo: 1% or less selected from 1% or less,
Group B: Ca: 0.02% or less, REM: One or two of 0.02% or less A method of manufacturing an electric resistance steel pipe that is subjected to heat treatment and then subjected to reduced diameter rolling,
The material steel pipe in mass%,
C: 0.10 to 0.50%, Si: 0.01 to 1.0%,
Mn: 0.01 to 2.0%, P: 0.001 to 0.10%,
S: 0.01% or less, Al: 0.001 to 0.1%,
N: An electric resistance welded steel pipe containing 0.01% or less and having the balance Fe and inevitable impurities,
The diameter reduction rolling is adjacent to the rolling end temperature: 750 to 900 ° C., the cumulative diameter reduction ratio: 30 to 80%, and the thickness increase rate defined by the following formula (2) is + 1% or more. The maximum grain size of a crystal grain surrounded by a large-angle grain boundary whose orientation difference from the crystal grain is 15 ° or more is 50 μm or less, and the cos of the angle θ formed by the <100> orientation of the crystal grain and the tube circumferential direction ² theta is the embrittlement evaluation index cos ² theta defined and excellent low-temperature toughness below (1) crystal grain area ratio satisfying the formula is characterized in that a steel tube having a tissue is 55% or less A method for manufacturing ERW steel pipes.
Record
cos ² θ ≧ 0.9 (1)
Where cos ² θ: embrittlement degree evaluation index,
θ: Angle formed between the <100> orientation of crystal grains and the tube circumferential direction (°)
Thickening rate = (Thickness after reduced diameter rolling-Thickness before reduced diameter rolling) / (Thickness before reduced diameter rolling) x 100 (%) (2)   The method for producing an ERW steel pipe according to claim 3, wherein the heat treatment is a process of heating the material steel pipe to a temperature of Ac3 transformation point to 1050 ° C. and soaking.   The method for producing an electric resistance welded steel pipe according to claim 3 or 4, wherein after the diameter reduction rolling is performed, cold diameter reduction processing is further performed as secondary processing. The cold diameter reduction processing is characterized in that at room temperature, the cumulative diameter reduction ratio is 1 to 30%, and the thickness increase rate defined by the following formula (3) is + 1% or more. The manufacturing method of the electric-resistance-welded steel pipe according to claim 5.
Thickening rate = (Thickness after cold shrinking-Thickness before cold shrinking) / (Thickness before cold shrinking) x 100 (%) (3) 7. The material steel pipe according to claim 3, further comprising, in addition to the composition, one group or two groups selected from the following group A and group B by mass%. Manufacturing method of ERW steel pipe.
Group A: Cr: 1% or less, Cu: 1% or less, Ni: 1% or less, Mo: 1% or less selected from 1% or less,
Group B: Ca: 0.02% or less, REM: One or two of 0.02% or less

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