JPH1180899A - High strength steel tube excellent in workability, and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To refine crystalline grain and to improve workability and strength by providing a specific composition consisting of C, Si, Mn, Al, and the balance Fe with inevitable impurities, also providing a structure consisting of ferrite and a secondary phase of specific area ratio other than ferrite, and regulating the average crystalline grain size in the cross section perpendicular to the longitudinal direction of a steel tube to a specific value or below. SOLUTION: The steel has a composition containing, by weight, >0.30-0.70% C, 0.01-2.0% Si, 0.01-2.0% Mn, and 0.001-0.10% Al and also has a structure consisting of ferrite and a secondary phase of >30% area ratio, other than ferrite. As the secondary phase other than ferrite, martensite, bainite, and cementite can be cited, and these can be precipitated independently or in combination. The precipitated secondary phase can contribute to the improvement of strength and uniform elongation. Further, the average crystalline grain size in the cross section perpendicular to the longitudinal direction of a steel tube is regulated to <=2 μm.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-strength steel pipe having ultrafine crystal grains and excellent workability, and a method for producing the same.

[0002]

2. Description of the Related Art Steel materials are required to have not only high strength but also high ductility and toughness. Conventionally, steel materials having a good balance between strength and ductility have been demanded. Refinement of crystal grains is important as a few means capable of improving both strength, toughness and ductility. As a method of refining the crystal grains, a method of preventing the austenite grains from coarsening and refining the ferrite grains from the austenite-ferrite transformation from the fine austenite, and refining the austenite grains by processing to reduce the ferrite grains. There is a method of making finer or a method of using martensite or lower bainite by quenching and tempering.

[0003] Above all, controlled rolling in which ferrite grains are refined by austenitic hardening followed by austenite-ferrite transformation is widely used in the production of steel products. Further, a small amount of Nb is added to suppress recrystallization of austenite grains to further refine ferrite grains. By performing rolling in the non-recrystallization temperature range of austenite, austenite grains elongate and generate deformed bands in the grains, ferrite grains are generated from the deformed bands, and the ferrite grains are further refined. . In order to further refine ferrite grains, controlled cooling, in which cooling is performed during or after processing, is also being used.

[0004]

However, in the above-mentioned method, the miniaturization of the ferrite grain size to about 4 to 5 μm is a limit.
Significant process changes, including equipment remodeling, were required, and there was a limit in cost. High-strength steel pipes with a tensile strength exceeding 600 MPa are made of materials with a C content of 0.30% or more, or C
It is manufactured using a material whose content is increased and further a large amount of other alloying elements are added. However, in the high-strength steel pipe with the increased strength as described above, the elongation characteristics are deteriorated. Therefore, usually, strong working is avoided.If strong working is required, intermediate annealing is performed during the working. After that, heat treatment such as normalizing or quenching and tempering was performed. However, performing heat treatment such as intermediate annealing becomes complicated in the process.

[0005] In view of the above, it has been demanded that high-strength steel pipes can be strongly processed without intermediate annealing.
In order to improve the workability of high-strength steel pipes, further refinement of crystal grains has been desired. The present invention advantageously solves the above problems, has ultra-fine crystal grains, and has excellent tensile strength with excellent workability.
An object of the present invention is to provide a high-strength steel pipe of 600 MPa or more and a method for producing the same.

[0006]

SUMMARY OF THE INVENTION The present invention provides, in weight percent,
C: more than 0.30 to 0.70%, Si: 0.01 to 2.0%, Mn: 0.01 to 2.
0%, Al: 0.001 to 0.10%, having a composition consisting of balance Fe and unavoidable impurities, and having a microstructure consisting of ferrite and a second phase other than ferrite having an area ratio of more than 30%. The average grain size of the cross section perpendicular to the direction is 2μm
It is a high-strength steel pipe excellent in workability characterized by the following.

[0007] In the present invention, the composition may further comprise: C: 0.30
Ultra-0.70%, Si: 0.01-2.0%, Mn: 0.01-2.0%, A
l: 0.001 to 0.10%, Cu: 1% or less, N
i: 2% or less, Cr: 2% or less, Mo: 1% or less selected from the group consisting of Fe and unavoidable impurities. The composition was as follows: C: more than 0.30 to 0.70%, Si: 0.01 to 2.0%, Mn: 0.
01-2.0%, Al: 0.001-0.10%, and N
b: 1% or less, V: 0.3% or less, Ti: 0.2% or less, B:
One or more selected from 0.004% or less may be contained, and the composition may be composed of the balance of Fe and unavoidable impurities, and the composition may be C: more than 0.30 to 0.70%, S
i: 0.01 to 2.0%, Mn: 0.01 to 2.0%, Al: 0.001 to 0.1
0%, REM: 0.02% or less, Ca: 0.01%
Contains one or two selected from the following, with the balance being
The composition may be composed of Fe and unavoidable impurities.

[0008] In the present invention, the composition may further comprise C: 0.30
Ultra-0.70%, Si: 0.01-2.0%, Mn: 0.01-2.0%, A
l: 0.001 to 0.10%, Cu: 1% or less, N
i: 2% or less, Cr: 2% or less, Mo: 1% or less, and one or more selected from the group consisting of: Nb: 1% or less;
V: 0.3% or less, Ti: 0.2% or less, B: 0.004% or less.
And a composition consisting of unavoidable impurities, and the composition is as follows: C: more than 0.30 to 0.70%, Si: 0.01 to 2.0
%, Mn: 0.01 to 2.0%, Al: 0.001 to 0.10%,
Further, Cu: 1% or less, Ni: 2% or less, Cr: 2% or less,
Mo: 1% or more selected from 1% or less,
And REM: 0.02% or less, Ca: 0.01% or less selected from the group consisting of Fe and unavoidable impurities, and the composition may be
C: more than 0.30 to 0.70%, Si: 0.01 to 2.0%, Mn: 0.01 to 2.
0%, Al: 0.001 to 0.10%, and Nb: 1%
Below, V: 0.3% or less, Ti: 0.2% or less, B: 0.004%
One or more selected from the following, and RE
M: 0.02% or less, Ca: 0.01% or less 1
It may be a composition containing one or two species and the balance being Fe and unavoidable impurities.

Further, in the present invention, the above-mentioned composition is represented by C: 0.30
Ultra-0.70%, Si: 0.01-2.0%, Mn: 0.01-2.0%, A
l: 0.001 to 0.10%, Cu: 1% or less, N
i: 2% or less, Cr: 2% or less, Mo: 1% or less, Nb: 1% or less, V: 0.3
% Or less, Ti: 0.2% or less, B: 0.004% or less, and one or more selected from REM: 0.02% or less, Ca: 0.01% or less. , And the balance may be composed of Fe and unavoidable impurities.

Further, the present invention provides any one of the above-mentioned compositions, and has an average crystal grain diameter di of a cross section perpendicular to the longitudinal direction of the steel pipe.
(Μm), a material steel pipe having an outer diameter of ODi (mm) is heated and subjected to reduction rolling at an average rolling temperature of θm (° C) and a total reduction ratio of Tred (%) to obtain a product pipe having an outer diameter of ODf (mm). In the method for manufacturing a steel pipe, the reduction rolling is performed in a temperature range of 400 to 750 ° C.
The relationship between the average crystal grain diameter di (μm), the average rolling temperature θm (° C.), and the total diameter reduction rate Tred (%) is expressed by the following equation (1).

[0011]

(Equation 2)

(Where, di: average grain size of the material steel pipe (μm), θm: average rolling temperature (° C.) = (Θi + θf)
/ 2, θi: rolling start temperature, θf: rolling end temperature, Tre
d: Total diameter reduction ratio (%) = (ODi-ODf) × 100 / ODi, ODi:
Ultrafine grains having an average crystal grain size of 2 μm or less in a cross section perpendicular to the longitudinal direction of the steel pipe, characterized by being subjected to reduction rolling satisfying the outer diameter (mm) of the material steel pipe, ODf: the outer diameter of the product pipe (mm). It is a method of manufacturing a high-strength steel pipe having the same. In the present invention, the heating or soaking of the material steel pipe is preferably performed at 750 ° C. or less. In the present invention, it is preferable that the reduction rolling is performed under lubrication.

[0013]

DESCRIPTION OF THE PREFERRED EMBODIMENTS In the present invention, a steel pipe is used as a raw material. The manufacturing process of the raw steel pipe is not particularly limited. Electric resistance welded steel pipes (electrically welded pipes) by electric resistance welding method using high frequency, solid-phase welded steel pipes, forged welded steel pipes by heating both edges of the open pipe to the solid-state pressure welding temperature range and pressure welding
And seamless steel pipes manufactured by Mannesmann piercing and rolling can be suitably used.

Next, the reasons for limiting the chemical composition of the raw steel pipe and the product steel pipe will be described. C: more than 0.30 to 0.70% C is an element that precipitates as a solid solution or carbides in the matrix and increases the strength of steel. Contributes to higher strength and increased uniform elongation.
In order to secure the desired strength of tensile strength of 600 MPa or more,
C needs to be contained in an amount of more than 0.30% or more, but if it exceeds 0.70%, ductility deteriorates. Therefore, C is 0.30
Limited to the range of ~ 0.70%.

Si: 0.01-2.0% Si acts as a deoxidizing agent and forms a solid solution in the matrix to increase the strength of steel. This effect is observed at a content of 0.01% or more, preferably 0.10% or more, but a content of more than 2.0% deteriorates the ductility. Therefore, Si is 0.01 to 2.0
%. Preferably, it is in the range of 0.10 to 1.5% from the viewpoint of strength-ductility balance.

Mn: 0.01 to 2.0% Mn is an element that increases the strength of steel, and includes fine precipitation of cementite as a second phase or martensite,
Promotes bainite precipitation. Such an effect is 0.01
% Or more, but more than 2.0% deteriorates the ductility. For this reason, Mn was limited to the range of 0.01 to 2.0%. From the viewpoint of strength-ductility balance and weldability, Mn is preferably in the range of 0.2 to 1.3%.

Al: 0.001 to 0.10% Al has an effect of making crystal grains fine. In order to refine the crystal grains, the content must be at least 0.001% or more, but if it exceeds 0.10%, the amount of oxygen-based inclusions increases and the cleanliness deteriorates. For this reason, Al was limited to the range of 0.001 to 0.10%. Incidentally, the content is preferably 0.015 to 0.06%.

In addition to the above basic composition, the following alloy element group may be added alone or in combination. Cu: 1% or less, Ni: 2% or less, Cr: 2% or less, Mo: 1%
One or more selected from the following Cu, Ni, Cr, Mo all improve the hardenability of steel,
It is an element that increases strength.
More than one species can be added. These elements have the effect of lowering the transformation point and miniaturizing the ferrite grains or the second phase. However, the hot workability deteriorates when a large amount of Cu is added, so the upper limit is 1%. Ni improves toughness as the strength increases, but adding more than 2% saturates the effect and makes it economically expensive, so the upper limit was 2%. Cr, Mo
Since the addition of a large amount deteriorates weldability and ductility and is economically expensive, the upper limits are 2% and 1%, respectively. Preferably, Cu: 0.1 to 0.6%, Ni: 0.1 to 1.0%
%, Cr: 0.1-1.5%, Mo: 0.05-0.5%.

Nb: 0.1% or less, V: 0.3% or less, Ti: 0.
2% or less, B: one or more selected from among 0.004% or less Nb, V, Ti, and B are all precipitated as carbides, nitrides, or carbonitrides to reduce the crystal grain size. Contributes to high strength. Especially in steel pipes with joints that are heated to high temperatures,
It has the effect of miniaturizing the crystal grains in the heating process at the time of bonding and the effect of preventing the hardening of the bonded portion by acting as a ferrite precipitation nucleus in the cooling process. One or more of these can be added as necessary. However, if added in large amounts, weldability and toughness / ductility deteriorate, so Nb is 0.1% or less, V is 0.3% or less, and Ti is
B was limited to 0.2% or less, and B was limited to 0.004% or less. Preferably, Nb: 0.005 to 0.05%, V: 0.05 to 0.1%, Ti:
0.005 to 0.05%, B: 0.0005 to 0.0020%.

One or two kinds of REM and Ca selected from among REM: 0.02% or less and Ca: 0.01% or less, both have an effect of adjusting the shape of inclusions and improving workability. Further, in a steel pipe having a joint, which precipitates as a sulfide, an oxide, or a sulfate, there is also an effect of preventing hardening of the joint, and one or two kinds can be added as necessary. However, REM: 0.02%, Ca: 0.01%
If it exceeds, the number of inclusions becomes too large, the cleanliness decreases, and the ductility decreases. Therefore, REM: 0.02% or less, Ca: 0.01%
Limited to the following. REM: less than 0.004%, Ca: 0.00
Since the effect is small at less than 1%, it is preferable to set REM: 0.004% or more and Ca: 0.001% or more.

The raw steel pipe and the product steel pipe are composed of the above-mentioned components, the balance being Fe and unavoidable impurities. As inevitable impurities, N: 0.010% or less, O: 0.006% or less, P: 0.025% or less, and S: 0.020% or less are allowed. N: 0.010% or less N is an amount necessary for bonding with Al and refining crystal grains, 0.01%
Up to 0% is acceptable, but higher contents degrade ductility, so it is preferable to reduce it to 0.010% or less.

O: 0.006% or less O deteriorates cleanliness as an oxide, so it is preferable to reduce O as much as possible, but up to 0.006% is acceptable. P: 0.025% or less P segregates at crystal grain boundaries and degrades toughness, so it is preferable to reduce P as much as possible, but up to 0.025% is acceptable.

S: not more than 0.020% S is preferably reduced as much as possible because sulphide deteriorates the cleanliness, but up to 0.020% is acceptable. Next, the structure of the product steel pipe will be described. The structure of the product steel pipe of the present invention is a steel pipe comprising ferrite and a second phase other than ferrite having an area ratio of more than 30%, and having an average crystal grain size of 2 μm or less in a section perpendicular to the longitudinal direction of the steel pipe.

The second phase other than ferrite includes martensite, bainite, and cementite, and these may be precipitated alone or in combination. The area ratio of the second phase is more than 30%. The precipitated second phase contributes to the improvement of strength and uniform elongation and improves the strength and ductility of the steel pipe, but such an effect is small when the area ratio of the second phase is 30% or less. The area ratio of the second phase other than ferrite is more than 30%, preferably 60%
It is preferable to set the following. If it exceeds 60%, ductility deteriorates due to coarsening of cementite.

When the average crystal grain size exceeds 2 μm, there is no remarkable improvement in ductility and no remarkable improvement in workability can be obtained.
The average crystal grain size in the present invention, the cross section perpendicular to the longitudinal direction of the steel pipe, corroded with a nital solution, observed under an optical microscope or an electron microscope, and obtained a circle equivalent system of 200 or more grains,
The average value was used. The particle size of the second phase was measured using the pearlite colony boundary when the second phase was pearlite and the packet boundary when bainite or martensite was used.

Next, a method of manufacturing a steel pipe according to the present invention will be described. A raw steel pipe having the above composition and having an average crystal grain diameter di (μm) and an outer diameter ODi (mm) of a cross section perpendicular to the longitudinal direction of the steel pipe is heated to 750 ° C. or lower, preferably 400 ° C. to 750 ° C. After soaking, average rolling temperature θm
(° C), reduction rolling of total reduction ratio Tred (%)
ODf (mm) product tube.

As the reduction rolling method, reduction rolling by a plurality of grooved rolling mills called a reducer is preferable. FIG. 1 shows an example of an equipment row suitable for implementing the present invention. In FIG. 1, a plurality of stand reduction rolling devices 21 having a grooved roll are shown. The number of stands of the rolling mill is appropriately determined by the combination of the material steel pipe diameter and the product pipe diameter. As the hole type roll, any of generally known two rolls, three rolls and four rolls can be suitably applied.

The heating or soaking method of the reduction rolling is not particularly limited, but it is preferable to use a heating furnace or induction heating. Among them, the induction heating method is preferable because the heating rate is large and the production efficiency or the growth of crystal grains is suppressed. The heating or soaking temperature is preferably 750 ° C. or less, which is a temperature range where crystal grains are not coarsened. In the present invention, of course, even when the heating or soaking temperature of the raw steel pipe exceeds the above-mentioned temperature, the crystal grain size of the product pipe becomes fine. However, if the heating or soaking temperature exceeds 750 ° C, the surface properties deteriorate, and if it is less than 400 ° C, a suitable rolling temperature cannot be secured.
It is preferable that the temperature is not less than 750 ° C.

The rolling temperature of the reduction rolling is in a temperature range of 400 to 750 ° C. By rolling in this rolling region, when the second phase in the material steel pipe structure is pearlite, the layered cementite in the pearlite is divided and refined, whereby the elongation characteristics of the product steel pipe are secured and workability is improved. . Further, when the second phase in the material steel pipe structure is bainite, the processed bainite recrystallizes to form a fine bainitic ferrite structure, thereby securing the elongation characteristics of the product steel pipe and improving workability. I do.

If the rolling temperature exceeds 750 ° C., the growth of the grains after recrystallization becomes remarkable, it becomes difficult to form fine grains, and the ductility decreases. Further, when the rolling temperature is lower than 400 ° C., it becomes a blue embrittlement region, and the rolling becomes difficult, or the recrystallization becomes insufficient, and the work strain tends to remain, so that the ductility and toughness decrease. For this reason, the rolling temperature of the reduction rolling is set to a temperature range of 400 to 750 ° C. The temperature is preferably from 600 to 700 ° C.

In the reduction rolling, the average crystal grain diameter di of the cross section perpendicular to the longitudinal direction of the steel pipe of the raw steel pipe is set within the above-mentioned rolling temperature range.
(Μm), the average rolling temperature θm (° C.) of the reduction rolling and the total diameter reduction rate Tred (%) are expressed by the following equation (1).

[0032]

(Equation 3)

The rolling is performed so as to satisfy the following. Here, the average rolling temperature θm (° C.) is θm = (θi + θf) / 2,
(Θi: rolling start temperature, θf: rolling end temperature), the total diameter reduction rate Tred (%) is: Tred = (ODi−ODf) × 100 / ODi,
(ODi: Outer diameter of material steel pipe (mm), ODf: Outer diameter of product pipe (m
m)).

If the relationship among di, θm and Tred does not satisfy the expression (1), the average crystal grain (cross section perpendicular to the longitudinal direction of the steel pipe) of the product pipe does not become fine grains of 2 μm or less.
The drawing rolling of the steel pipe in the present invention is a rolling process in a biaxial stress state, and a remarkable grain refinement effect can be obtained. On the other hand, in the rolling of a steel sheet, a free end exists not only in the rolling direction but also in the sheet width direction (direction perpendicular to the rolling direction).
Rolling is performed in an axial stress state, and there is a limit to grain refinement.

In the present invention, the reduction rolling is preferably performed under lubrication. By performing the rolling by lubricating rolling, the strain distribution in the thickness direction becomes uniform, and the distribution of the crystal grain size becomes uniform in the thickness direction. In non-lubricated rolling, strain concentrates only on the material surface, and crystal grains in the thickness direction tend to be non-uniform. The lubricating rolling may be performed using a normal rolling oil such as a mineral oil or a mineral oil and a synthetic ester, and the rolling oil need not be particularly limited.

After rolling, the product tube is cooled, preferably to 300 ° C. or less. As a cooling method, air cooling may be used, but a generally known cooling method such as water cooling, mist cooling, or forced air cooling using a quenching device 24 for the purpose of suppressing grain growth even slightly can be applied. The cooling rate is preferably 1 ° C./sec or more. In the present invention, a cooling device may be installed on the inlet side of the reduction rolling device 21 or in the middle of the reduction rolling device 21 to adjust the temperature.

The raw steel pipe used as the raw material in the present invention may be a seamless steel pipe, an electric resistance welded steel pipe, a forged steel pipe, a solid-phase pressure welded steel pipe, or the like. Further, the manufacturing process of the ultrafine-grained steel pipe of the present invention may be continuous with the above-described raw steel pipe manufacturing line. Fig. 2 shows an example of a continuous solid-state pressure welded steel pipe production line.
Shown in The strip 1 discharged from the uncoiler 14 is connected to the preceding strip by the joining device 15, preheated by the preheating furnace 2 via the looper 17, and then opened by the forming apparatus 3 including forming rolls. The edge portion of the open pipe 7 is heated to a temperature range lower than the melting point by the induction heating device 4 for edge preheating and the induction heating device 5 for edge heating. You.

Next, the material steel pipe 8 is, as described above,
After heating or soaking to a predetermined temperature in a soaking furnace 22, the scale is removed by a descaling device 23, squeezed and rolled by a squeezing rolling device 21, cut by a cutting machine, straightened by a tube straightening device 19, and a product pipe 16. Becomes The temperature of the steel pipe is measured by a thermometer 20. According to the manufacturing method described above, the area ratio with ferrite is
Having a structure consisting of more than 30% of a second phase other than ferrite,
A high-strength steel pipe having ultrafine grains having an average crystal grain diameter of a cross section perpendicular to the longitudinal direction of the steel material of 2 μm or less can be obtained without intermediate annealing.

[0039]

【Example】

(Example 1) A steel material having the chemical composition shown in Table 1 was hot-rolled into a 4.5 mm-thick steel strip. The strip 1 was preheated to 600 ° C. in a preheating furnace 2 using the equipment row shown in FIG. 2 and then continuously formed into an open pipe 7 by a forming apparatus 3 including a plurality of forming roll groups. Then open tube 7
Of both edges is 1000 with induction heating device 4 for edge preheating.
After preheating to ℃, both edge portions were further heated to 1450 ℃ by an induction heating device 5 for edge heating, butted by a squeeze roll 6 and solid-phase pressed to obtain a material steel pipe 8 of φ110 × T4.5mm.

Then, the raw steel pipe is heated to a heating soaking temperature shown in Table 2 by a seam cooling and pipe heating device 22, and then a predetermined outer diameter is reduced by a reduction rolling device 21 in which a plurality of reduction rolling mills having a three-roll structure are installed. Product tube. The number of stands of the rolling mill used was 6 when the outer diameter of the product tube was 60.3 mm, and 16 when the outer diameter of the tube was 42.7 mm. No.1
The product pipe of No.-2 was lubricated and rolled using rolling oil in which synthetic ester was mixed with mineral oil when drawing and rolling.

After the rolling, the product tube was air-cooled. The crystal grain size and tensile properties of these product tubes were investigated, and the results are shown in Table 2. Regarding the crystal grain size, a cross section (C cross section) perpendicular to the longitudinal direction of the steel pipe was observed at 5 times or more at a magnification of 5000 times, and the average crystal grain size of the ferrite and the second phase was measured. For the tensile properties, JIS No. 11 test pieces were used. The elongation (El) is given by considering the size effect of the test piece: El = El ₀ × (× (a ₀ / a)) ^0.4 (El ₀ : measured elongation, a ₀ = 100 mm ² , a: test piece cross-sectional area mm
² ) The converted value obtained from the above was used.

[0042]

[Table 1]

[0043]

[Table 2]

From Table 2, it can be seen that the present invention within the scope of the present invention (No. 1-
2, No. 1-4 to No. 1-7, No. 1-10) are fine grains having an average crystal grain size of 2 μm, and have high elongation, high toughness, and tensile strength of 600 MPa or more. It is a steel pipe with excellent balance between strength, toughness and ductility. In addition, the lubricated rolling N
In o.1-2, the variation of the crystal grains in the thickness direction was small. In comparison, a comparative example (No. 1) out of the scope of the present invention
-1, No. 1-3, No. 1-8, No. 1-9), the crystal grains became coarse and the ductility was deteriorated.

The structure of the product tube within the scope of the present invention was a structure having ferrite and cementite having an area ratio of more than 30% as a second phase. (Example 2) A material steel pipe having a chemical composition shown in Table 3 was used.
After reheating with the induction heating coil to the temperature shown in Table 4, 3
A product tube having an outer diameter shown in Table 4 was obtained by a rolling mill having a roll structure. The number of stands of the rolling mill used was 16 stands.

The characteristics of these product tubes were investigated, and the results are shown in Table 4. The characteristics of the product tube were examined in the same manner as in Example 1 for the structure, crystal grain size, and tensile characteristics.

[0047]

[Table 3]

[0048]

[Table 4]

From Table 4, it can be seen that the present invention within the scope of the present invention (No. 2-1)
No. 2-6) is a steel pipe having an average ferrite grain size of 2 μm or less, a tensile strength of 600 MPa or more, a high elongation, and an excellent balance between strength and ductility.
In comparison, comparative examples (No. 2-7,
In No. 2-8), the crystal grains became coarse and the strength was reduced, and the target tensile strength was not obtained.

The structure of the product tube within the scope of the present invention was a structure having ferrite and pearlite, cementite, bainite, or martensite having an area ratio of more than 30% as the second phase. According to the present invention, the ductility has never been before-
Although a high-strength steel pipe having an improved strength balance can be obtained, the steel pipe of the present invention is also excellent in secondary workability, for example, bulge workability such as hydroforming, and is a steel pipe suitable for bulge work.

Among the steel pipes of the present invention, in a welded steel pipe or a solid-phase welded steel pipe subjected to seam cooling, the hardened seam portion has the same level of hardness as that of the base pipe portion by drawing and rolling, and the bulge workability has not been achieved conventionally. It is significantly improved.

[0052]

According to the present invention, a steel material having ultra-fine crystal grains of 2 μm or less, high tensile strength of 600 MPa or more, and excellent in toughness and ductility can be easily produced without intermediate annealing.
The use of steel materials can be expanded, and industrially significant effects can be expected.

[Brief description of the drawings]

FIG. 1 is a conceptual diagram showing an example of an equipment row suitable for implementing the present invention.

FIG. 2 is a conceptual diagram showing an example of a solid-state pressure welded steel pipe manufacturing facility suitable for carrying out the present invention and a continuous equipment row.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Strip steel 2 Preheating furnace 3 Forming apparatus 4 Induction heating apparatus for edge preheating 5 Induction heating apparatus for edge heating 6 Squeeze roll 7 Open pipe 8 Material steel pipe 14 Uncoiler 15 Joining apparatus 16 Product pipe 17 Looper 18 Cutting machine 19 Straightening apparatus 20 Thermometer 21 Reducing device 22 Soaking furnace (seam cooling and tube heating device) 23 Descaling device 24 Rapid cooling device 25 Reheating device 26 Cooling device

 ──────────────────────────────────────────────────続 き Continuing from the front page (72) Inventor Akira Ito 1-1-1, Kawasaki-cho, Handa-shi, Aichi Prefecture Inside the Chita Works, Kawasaki Steel Works (72) Inventor Motoaki Itani 1-1-1, Kawasaki-cho, Handa-shi, Aichi Prefecture Kawasaki Steel Corporation Chita Works (72) Inventor Yuji Hashimoto 1-1-1 Kawasaki-cho, Handa-shi, Aichi Prefecture Kawasaki Steel Corporation Chita Works (72) Inventor Nochika Okabe 1-1-1, Kawasaki-cho, Handa-shi, Aichi Prefecture Address Kawasaki Steel Corporation Chita Works (72) Inventor Taro Kanayama 1-1-1, Kawasaki-cho, Handa-shi, Aichi Prefecture Kawata Steel Corporation Chita Works

Claims (7)

[Claims] 1. The composition contains C: more than 0.30 to 0.70%, Si: 0.01 to 2.0%, Mn: 0.01 to 2.0%, and Al: 0.001 to 0.10% by weight, with the balance being Fe and unavoidable impurities. Workability characterized by having a composition, a structure comprising a ferrite and a second phase other than ferrite having an area ratio of more than 30%, and having an average crystal grain size of 2 μm or less in a section perpendicular to the longitudinal direction of the steel pipe. Excellent high strength steel pipe. 2. In addition to the above-mentioned composition, Cu: 1% or less, Ni: 2% or less, Cr: 2% or less, Mo: 1% by weight%
The high-strength steel pipe according to claim 1, comprising one or more selected from the following. 3. In addition to the above composition, Nb: 1% or less, V: 0.3% or less, Ti: 0.2% or less, B:
3. The high-strength steel pipe according to claim 1, comprising one or more kinds selected from 0.004% or less. 4. The composition according to claim 1, further comprising one or two selected from the group consisting of REM: 0.02% or less and Ca: 0.01% or less in weight% in addition to said composition. 3. The high-strength steel pipe according to any one of 3. 5. A material steel pipe having the composition according to claim 1 having an outer diameter ODi (mm) and an average crystal grain size di (μm) of a cross section perpendicular to the longitudinal direction of the steel pipe. Soak, average rolling temperature θm (° C), total diameter reduction Tred
(%) In a method of producing a steel pipe having a product pipe with an outer diameter of ODf (mm), which is drawn at 400 to 750 ° C.
And the average grain size di (μm), the average rolling temperature θm (° C.), and the total diameter reduction Tred.
(%) Is a reduction rolling satisfying the following expression (1), characterized in that it has ultra-fine grains having an average crystal grain size of 2 μm or less in a section perpendicular to the longitudinal direction of the steel pipe, and has excellent workability. Manufacturing method of high strength steel pipe. (Equation 1) Here, di: average crystal grain size (μm) of the material steel pipe, θm: average rolling temperature (° C.) = (Θi + θf) / 2, θi: rolling start temperature (° C.), θf: rolling end temperature (° C.), Tred : Total diameter reduction rate (%) = (ODi-ODf) x 100 / ODi ODi: Material steel pipe outer diameter (mm) ODf: Product pipe outer diameter (mm) 6. The heating or soaking of said material steel pipe is performed at 750 ° C.
The method for producing a high-strength steel pipe according to claim 5, wherein the temperature range is set as follows. 7. The method for producing a high-strength steel pipe according to claim 5, wherein the reduction rolling is rolling under lubrication.