KR20000068335A - Ultrafine-grain steel pipe and process for manufacturing the same - Google Patents

Abstract

The present invention relates to an ultra-fine grain steel pipe and a method for manufacturing the same, and to provide a steel pipe and a method for manufacturing the same, which have excellent toughness and ductility, excellent ductility-strength balance and impact resistance due to miniaturization of ferrite crystal grains. Limit the Al to an appropriate range and, if necessary, add a Cu, Ni, Cr, Mo, Nb, Ti, V, B, etc. material steel pipe having an average grain size (di) (μ) below the Ac3 transformation point. Heating and drawing rolling of the average rolling temperature (θm) (° C) and total axis diameter (Tred) (%) of drawing rolling at a temperature range of 400 to Ac3 transformation point, and the relationship between di, θm and Tred satisfies a predetermined relational expression. It is characterized by being possible to manufacture a steel pipe having ultra-fine particles.

Description

ULTRAFINE-GRAIN STEEL PIPE AND PROCESS FOR MANUFACTURING THE SAME

In order to increase the strength of the steel, addition of alloying elements such as Mn and Si, heat treatment such as controlled rolling, controlled cooling, quenching and tempering, or addition of precipitation hardening elements such as Nb and V is used. However, steel materials need not only high strength but also high ductility and toughness, and conventionally, steel materials having improved balance of strength, ductility, and toughness have been desired.

The miniaturization of crystal grains is important as one of the few means that can improve strength, ductility and toughness together. As a method for refining crystal grains, a method of preventing coarsening of austenite particles and refining ferrite crystal grains using austenite ferrite transformation from fine austenite, and a method of refining ferrite crystal grains by refining austenite particles by processing Or a method of using martensite and lower bainite by quenching and tempering.

Especially, the control rolling which refine | miniaturizes a ferrite particle by the steel processing in austenite area | region and subsequent austenite ferrite transformation is widely used for steel materials manufacture. Further, addition of a small amount of Nb suppresses recrystallization of the austenite particles to further refine the ferrite particles. By processing in the unrecrystallized temperature range of austenite, austenite particles are stretched to produce strain bands in the particles, and ferrite particles are generated from the strain bands to further refine the ferrite particles. In addition, in order to refine the ferrite particles, controlled cooling, which is performed during or after the processing, is also used.

However, in the above-described method, the miniaturization of ferrite grains to about 4 to 5 탆 is limited, and the process is complicated to apply to the production of steel pipes. For this reason, further refinement of ferrite crystal grains has been desired in a simple process for improving the toughness and ductility of steel pipes. In addition, in the above-described method, in order to manufacture steel pipes having improved impact resistance characteristics for the purpose of improving safety of automobiles, which have recently become more demanding, a large-scale process modification including modification of facilities is required, and in terms of cost, There was a limit.

Moreover, in order to improve the sulfide stress corrosion cracking property of the line pipe steel pipe, the present condition is performing hardness control by reducing an impurity or adjusting an alloying element.

Conventionally, in order to improve fatigue resistance, heat treatments such as quenching, tempering, high frequency quenching, carburizing, or expensive alloying elements such as Ni, Cr, and Mo have been added.

However, this method has a problem in that weldability deteriorates and costs increase.

High strength steel pipes with a tensile strength exceeding 600 MPa are manufactured using materials having a C content higher than 0.30% or materials having a high C content and a large amount of other alloying elements added thereto. However, in the case of high strength steel pipes having high strength in this manner, the stretching characteristics are deteriorated. Therefore, in general, when steel processing is avoided and steel processing is required, intermediate annealing is performed during processing, and then normalizing or quenching and tempering. Heat treatment such as was further performed. However, performing heat treatment such as intermediate annealing becomes complicated in process.

From this, it is desired to enable steel processing of high strength steel pipes without performing intermediate annealing, and further refinement of crystal grains has been desired to improve the workability of high strength steel pipes.

An object of the present invention is to advantageously solve the above-mentioned problems and to provide a steel pipe excellent in ductility and impact resistance characteristics without a significant process change and a method of manufacturing the same. In addition, an object of the present invention is to provide a steel pipe having ultrafine particles excellent in toughness and ductility, in which ferrite crystal grains are made finer to 3 μm or less, preferably 2 μm or 1 μm or less, and a manufacturing method thereof.

Another object of the present invention is to provide a high-strength steel pipe having a tensile strength of 600 MPa or more having ultrafine crystal grains and excellent workability and a method of manufacturing the same.

The present invention relates to a steel pipe having ultrafine crystal grains, having high strength, high toughness and high ductility, and having excellent impact shock resistance, and a method of manufacturing the same.

1 is a graph showing the relationship between elongation and tensile strength of steel pipes,

2 is a graph showing the effect of the tensile strain rate on the relationship between the tensile strength of the steel pipe and the ferrite particle diameter,

3 is an electron micrograph showing a metal structure of a steel pipe as an embodiment of the present invention;

4 is a conceptual diagram showing an example of a preferred installation example of the practice of the present invention;

FIG. 5 is a conceptual diagram showing an example of a facility example which is continuous with a solid state welded steel pipe manufacturing facility which is preferred for the practice of the present invention; FIG.

6 is a graph showing the relationship between the total axial diameter ratio and the average grain size of the raw material steel pipe to the crystal grain size of the product tube according to the first embodiment of the present invention;

It is a schematic explanatory drawing which shows the shape of the test piece of the sulfide stress cracking test.

* Explanation of symbols for the main parts of the drawings

1: steel strip 2: preheating

3: forming processing equipment 4: induction heating device for edge preheating

5: Induction heating device for edge heating 6: Squeeze roll

7: Open pipe 8: Material steel pipe

14: uncoiler 15: bonding apparatus

16: Product Tube 17: Looper

18: cutter 19: pipe straightener

20: thermometer 21: drawing rolling apparatus

22: crack furnace (core cooling and tube heating) 23: descaling device

24: quenching device 25: reheater

26: chiller

MEANS TO SOLVE THE PROBLEM The present inventors earnestly examined the manufacturing method of the steel pipe which can produce high-strength steel pipe which is excellent in ductility at high pipe speed, and when drawing-rolling in the ferrite recovery and recrystallization temperature area | region to a steel pipe which limits composition, it is excellent in the strength-ductility balance. It has been found that a high ductility high strength steel pipe can be manufactured.

First, the experimental result used as the basis of this invention is demonstrated.

A full-rolled steel tube (ø42.7 mm D x 2.9 mm t) containing 0.09 wt% C-0.40 wt% Si-0.80 wt% Mn-0.04 wt% Al was heated at each temperature of 750 ° C. to 550 ° C. The drawing rolling which variously changed the outer diameter of the product pipe | tube to ø33.2-15.0 mm was made into the product pipe | tube which performed at the rolling exit side speed 200m / min. After rolling, the tensile strength (TS) and the stretching (El) of the product pipe were measured and shown in Fig. 1 in the relation of the stretching-strength (indicated in the figure). In addition, (circle) is an example which shows the same relationship of the extension | strength-strength of the electric resistance steel pipe which does not perform the drawing-rolling welded of various sizes.

In addition, the value of the stretching (El) in consideration of the test piece size effect,

E1 = E10 × (√ (a0 / a)) 0.4

Here, the calculated value calculated using E10: measured stretching, a0: 292 mm <2>, a: test piece cross-sectional area (mm <2>) was used.

It can be seen from FIG. 1 that when the raw material steel pipe is heated to 750 ° C. to 550 ° C. and subjected to drawing rolling, a high elongation is obtained at the same strength as compared to the elongation-strength relationship of the electric sealing steel pipe in the bonded state. That is, the present inventors have obtained the knowledge that a high-strength steel pipe having excellent ductility-strength balance can be produced by heating the raw material steel pipe having a limited composition to 750 to 400 ° C. and carrying out drawing rolling.

In addition, it was found that the steel pipe manufactured by the above production method had fine ferrite particles of 3 µm or less. The present inventors drastically changed the strain rate to 2000 s ⁻¹ and investigated the relationship between the tensile strength (TS) and the ferrite particle diameter in order to investigate the impact resistance characteristics. As a result, as shown in FIG. 2, when the ferrite particle diameter became 3 micrometers or less, TS was found to increase remarkably, especially at the time of impact shock deformation with a large deformation rate. In other words, it was also found that steel pipes having fine ferrite particles were excellent in ductility-strength balance and had significantly improved impact resistance characteristics.

In addition, the present invention for obtaining a steel pipe having ultra-fine particles is heated and average rolled material steel pipe of the average crystal grain diameter (di) of the ferrite of the cross section perpendicular to the steel pipe length direction at the outer diameter (ODi) (mm) In the manufacturing method of the steel pipe which carries out drawing rolling of temperature (theta) m (degreeC) and total axis diameter Tred (%), and makes a product pipe of an outer diameter (ODf) (mm), the said drawing rolling is performed. The relationship between the average grain size di (μm), the average rolling temperature (θ m) (° C.) and the total axis diameter Tred (%) in the temperature range of 400 ° C. or higher or less than the cracking temperature is expressed by the following equation. Equation 1

di≤ (2.65-0.003 × θm) × 10 ^{{(0.008 + θm / 50000) × Tred}}

Where di is the average grain size of the material steel pipe (µm), θm is the average rolling temperature (° C) = (θi + θf) / 2, θi is the rolling start temperature, θf is the rolling finish temperature, and Tred is the total axis diameter. (%) = (ODi-ODf) x 100 / ODi, ODi: perpendicular to the longitudinal direction of the steel pipe, characterized in that the drawing rolling satisfies the material steel pipe outer diameter (mm) and ODf: product pipe outer diameter (mm). It is a manufacturing method of the steel pipe which has the ultrafine particle whose average crystal grain diameter of the ferrite of a cross section is 1 micrometer or less. Moreover, in this invention, it is preferable to perform the said drawing rolling in the temperature range of 400-750 degreeC. In the present invention, it is preferable that the heating or cracking of the raw material steel pipe be less than or equal to the Ac3 transformation point, and the heating or cracking of the raw material steel pipe is made based on the Ac1 transformation point of the raw material steel pipe, and is lower than (Ac1 + 50 ° C.). It is preferable to set it as a temperature range, and it is preferable to make it the rolling under lubrication.

It is preferable to make the said drawing rolling into the rolling containing at least 1 pass or more of the rolling path | passes whose axis diameters per pass are 6% or more, and it is preferable to make the cumulative axis diameters 60% or more.

In addition, in the present invention, which is a steel pipe having ultrafine particles having an average crystal grain diameter of 1 μm or less, and a method of manufacturing the same, the material steel pipe is preferably used as a steel pipe containing C: 0.70 wt% or less. The material steel pipe in a weight percent, C: 0.005 to 0.30%, Si: 0.01 to 3.0%, Mn: 0.01 to 2.0%, Al: 0.001 to 0.10% and has a composition consisting of the remaining Fe and unavoidable impurities In addition, in the present invention, in addition to the above composition, the following A to C group,

Group A: Cu: 1% or less, Ni: 2% or less, Cr: 2% or less, Mo: 1% or less,

Group B: Nb: 0.1% or less, V: 0.5% or less, Ti: 0.2% or less, B: 0.005% or less,

Group C; REM: 0.02% or less, Ca: 0.01% or less

It is good also as a steel pipe containing 1 or more types of each said group from 1 group chosen from 2 or more groups.

In addition, the present inventors found that in the above-described method for manufacturing a steel pipe, by restricting the composition of the material steel pipe within an appropriate range, it is possible to manufacture a steel pipe having high strength, high toughness and excellent stress corrosion cracking resistance, and thus, a pipe for line pipe Thought came to be usable as advantageously.

In order to improve stress corrosion cracking resistance, steel pipe for line pipe has conventionally been subjected to hardness control by reducing impurities such as S and adjusting alloy elements. However, this method has a problem in that the strength is high and the cost is increased.

By restricting the composition of the material steel pipe to an appropriate range and drawing and rolling in a ferrite recrystallization region, dispersion of fine ferrite and fine carbide is obtained, thereby obtaining high strength and high toughness, and also limiting alloy elements and welding hardenability. In addition, it is possible to reduce the occurrence and growth of cracks, thereby improving the stress corrosion corrosion cracking resistance.

That is, the present invention contains C: 0.005 to 0.10%, Si: 0.01 to 0.5%, Mn: 0.01 to 1.8%, Al: 0.001 to 0.10% by weight, Cu: 0.5% or less, Ni: 0.5% or less At least one selected from Cr: 0.5% or less, Mo: 0.5% or less, or at least one selected from Nb: 0.1% or less, V: 0.1% or less, Ti: 0.1% or less, and B: 0.004% or less. Drawing that satisfies the above formula 1 for a material steel pipe containing one or two selected from two or more, or REM: 0.02% or less, Ca: 0.01% or less, and having a composition consisting of residual Fe and unavoidable impurities By rolling, steel pipes having excellent ductility and impact resistance characteristics and excellent stress corrosion cracking resistance can be produced.

In addition, the present inventors found that the steel pipe can be manufactured with high strength, high toughness and excellent fatigue resistance by further limiting the composition of the material steel pipe within the appropriate range in the method of manufacturing the steel pipe, and can be advantageously used as a high fatigue strength steel pipe. Thought came.

By drawing and rolling material steel pipes with a limited composition within an appropriate range in the ferrite recovery and recrystallization region, dispersion of fine ferrite and fine precipitates is obtained, high strength and high toughness can be obtained, and alloy elements can be restricted and welded. Hardenability is reduced, fatigue crack generation and advancing can be suppressed, and fatigue resistance is improved.

That is, the present invention is a material steel pipe containing C: 0.06 to 0.30%, Si: 0.01 to 1.5%, Mn: 0.01 to 2.0%, Al: 0.001 to 0.10% by weight, and having a composition consisting of residual Fe and unavoidable impurities. By carrying out the drawing rolling that satisfies the above Equation 1, it is possible to produce a steel pipe excellent in ductility, impact resistance and fatigue resistance.

In addition, the composition contains by weight% 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 has a composition consisting of residual Fe and inevitable impurities. Ultrafine particles having an average crystal grain diameter of 2 micrometers or less in a cross section perpendicular to the steel pipe length direction and having a particle diameter of 1 micrometer or less in a cross section perpendicular to the steel pipe length direction. It is possible to obtain a high strength steel pipe having excellent workability, which has fine particles.

In this invention, a steel pipe is used as a raw material. It does not specifically limit about the manufacturing method of a raw material steel pipe. Electric resistance welded steel pipe (electrically sealed steel pipe) by electric resistance welding method using high frequency current, open edges of open tube in the solid state welding temperature range, solid state welded steel pipe by pressure welding, single welded steel pipe and seamless steel pipe by Mannesmann type punched rolling All can be used preferably.

Next, the reason for limitation of chemical composition of the raw material steel pipe and the product steel pipe will be explained.

C: 0.70% or less

C is an element that precipitates as a solid solution or carbide in the matrix and increases the strength of the steel, and fine cementite, martensite, and bainite precipitated as a hard second phase contribute to the improvement of ductility (same stretching). In order to secure the desired strength and to obtain the effect of improving the ductility by cementite or the like precipitated as the second phase, C is required to be 0.005% or more, preferably 0.04% or more. Further, C is preferably 0.30% or less, and more preferably 0.10% or less. From these things, the preferable thing was limited to 0.005 to 0.30%, and the more preferable thing was 0.04 to 0.30% of range.

In order to improve the stress corrosion cracking resistance for line pipes, C is preferably 0.10% or less. If it exceeds 0.10%, the stress corrosion cracking resistance deteriorates for hardening of the welded portion.

In order to improve fatigue resistance for high fatigue strength steel pipes, C is preferably 0.06 to 0.30%. If it is less than 0.06%, fatigue resistance deteriorates because of lack of strength.

In order to secure a desired strength of 600 MPa or more, C needs to contain more than 0.30%, but when it contains more than 0.70%, the ductility deteriorates. For this reason, C was limited to 0.30%-0.70% of range.

Si: 0.01 to 3.0%

Si acts as a deoxidation element and simultaneously solidifies in the base to increase the strength of the steel. This effect is acknowledged to be 0.01% or more and preferably 0.1% or more, but a content exceeding 3.0% deteriorates the ductility. In high strength steel pipes, the upper limit is 2.0% for reasons of ductility. From this, Si was limited to 0.01 to 3.0% or 0.01 to 2.0% of range. Moreover, preferable is 0.1 to 1.5% of range.

Moreover, in order to improve stress corrosion cracking resistance for line pipes, it is preferable to make Si 0.5% or less. If it exceeds 0.5%, the weld hardens and the stress corrosion cracking resistance deteriorates.

In addition, in order to improve fatigue resistance for high strength steel pipes, Si is preferably 1.5% or less. If the content exceeds 1.5%, inclusions are generated, thereby deteriorating fatigue properties.

Mn: 0.01 to 2.0%

Mn is an element which increases the strength of the steel and in the present invention promotes the fine precipitation of cementite as the second phase, or the precipitation of martensite and bainite. If it is less than 0.01%, the desired strength cannot be secured, and fine precipitation of cementite or precipitation of martensite and bainite is inhibited. Moreover, when it exceeds 2.0%, intensity will increase too much and ductility deteriorates. For this reason, Mn was limited to 0.01 to 2.0% of range. In addition, from the viewpoint of the strength-stretching balance, Mn is preferably in the range of 0.2 to 1.3%, more preferably in the range of 0.6 to 1.3%.

Moreover, in order to improve stress corrosion cracking resistance for line pipes, it is preferable to make Mn 1.8% or less. If it exceeds 1.8%, the weld hardens and the stress corrosion cracking resistance deteriorates.

Al: 0.001-0.10%

Al has an effect of making the crystal grain diameter smaller. At least 0.001% or more is required to refine the crystal grains, but when it exceeds 0.10%, the oxygen-based inclusions increase and the cleanliness deteriorates. For this reason, Al was limited to 0.001 to 0.10% of range. Moreover, preferable is 0.015 to 0.06%. In addition to the basic composition of the above-described raw material steel pipe, one kind or two or more kinds of each of the above groups may be added to the group 1 or 2 or more selected from the group of alloy elements of the A to C group described below.

Group A: Cu: 1% or less, Ni: 2% or less, Cr: 2% or less, Mo: 1% or less

Cu, Ni, Cr and Mo are all elements which improve the hardenability of steel and increase strength, and can add 1 type, or 2 or more types as needed. This element has the effect of lowering the transformation point and miniaturizing the ferrite particles or the second phase. However, when a large amount of Cu was added, hot workability deteriorated, so 1% was the upper limit. Ni increased the strength and improved toughness, but added 2% or more, so that the effect was saturated and economically expensive, the upper limit was 2%. When Cr and Mo are added in large amounts, weldability and ductility deteriorate and are economically expensive, so 2% and 1% are the upper limits, respectively.

Further, preferred are Cu: 0.1 to 0.6%, Ni: 0.1 to 1.0%, Cr: 0.1 to 1.5%, and Mo: 0.05 to 0.5%.

In addition, in order to improve the stress corrosion cracking resistance for line pipes, it is preferable to limit Cu, Ni, Cr, and Mo to 0.5% or less, respectively. If a large amount is added in excess of 0.5%, the weld hardens, thereby deteriorating the stress corrosion cracking resistance.

Group B: Nb: 0.1% or less, V: 0.5% or less, Ti: 0.2% or less, B: 0.005% or less

Nb, V, Ti, and B are elements that precipitate as carbides, nitrides, or carbonitrides and contribute to the refinement and strength of crystal grains. Particularly, in steel pipes having a joint heated at a high temperature, the crystal grains during the heating process at the time of joining are refined. B) It acts as precipitation nuclei of ferrite in the cooling process and also prevents hardening of the joints. One or two or more kinds can be added as necessary. However, when a large amount is added, weldability and toughness deteriorate, Nb is 0.1%, V is 0.5%, 0.3% is preferable, 0.2% is Ti, 0.005% is preferable, and 0.004% was preferable for the upper limit, respectively. Further, preferred are Nb: 0.005 to 0.05%, V: 0.05 to 0.1%, Ti: 0.005 to 0.10%, and B: 0.0005 to 0.002%.

In addition, in order to improve stress corrosion cracking resistance for line pipes, Nb, V and Ti are preferably limited to 0.1% or less, respectively. When a large amount of Nb, V and Ti is added in excess of 0.1%, the stress corrosion cracking resistance deteriorates due to precipitation hardening.

Group C: REM: 0.02% or less, Ca: 0.01% or less

Both REM and Ca have the function of improving the workability by adjusting the shape of inclusions, and also acting to prevent hardening of the joint in a steel pipe which precipitates as a sulfide, oxide or sulfur oxide and has a joint. Can be added. If REM: 0.02% and Ca: 0.01% are exceeded, the inclusions are excessively large, resulting in a decrease in cleanliness and deterioration of ductility. In addition, since REM: less than 0.004% and Ca: less than 0.001%, the effect by this action decreases. Therefore, it is preferable to make REM: 0.004% or more and Ca: 0.001% or more.

The material steel pipe and the product steel pipe are made of balance Fe and unavoidable impurities in addition to the above components. As unavoidable 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

Although N can tolerate up to 0.010% of the amount necessary to combine with Al to refine the crystal grains, it is preferable to reduce the content to 0.010% or less, because more content deteriorates ductility. In addition, N is more preferably 0.002% to 0.006%.

O: 0.006% or less

Since O degrades the cleanliness as an oxide, it is desirable to reduce it as much as possible, but can tolerate up to 0.006%.

P: 0.025% or less

P is preferably reduced as much as possible because it segregates in the grain boundary and degrades toughness, but it can tolerate up to 0.025%.

S: 0.020% or less

S is preferably reduced as much as possible because it increases sulfide and degrades cleanliness, but it can tolerate up to 0.020%.

Next, the organization of the product steel pipe will be described.

1) The steel pipe of the present invention is a steel pipe having excellent ductility and impact resistance characteristics, which is mainly composed of a structure mainly composed of ferrite whose average grain size of ferrite is 3 µm or less.

If the ferrite particle diameter exceeds 3 µm, significant improvements in ductility, characteristics of impact loads with high deformation rates, and impact resistance characteristics are not obtained. Preferably, the ferrite average crystal grain is 1 탆 or less.

In the present invention, the ferrite average crystal grains were corroded in a nital liquid in a cross section perpendicular to the length direction of the steel pipe, and the structure was observed by an optical microscope or an electron microscope to obtain a circular equivalent diameter of 200 or more ferrite particles, and the average value was used.

The structure mainly containing ferrite as used in the present invention includes a structure of ferrite alone, in which the second phase is not precipitated, and a structure composed of ferrite and second phases other than ferrite.

Second phases other than ferrite include martensite, bainite and cementite and may be precipitated alone or in combination. The area ratio of the second phase is 30% or less. The precipitated second phases all contribute to the improvement of elongation at the time of deformation and improve the ductility and impact resistance properties of the steel pipe, but this effect is less when the area ratio of the second phase exceeds 30%.

2) The structure of the high-strength steel pipe of the present invention is composed of ferrite and a second phase other than ferrite having an area ratio of more than 30%, and the average crystal grain diameter of the cross section perpendicular to the longitudinal direction of the steel pipe is 2 µm or less. Second phases other than ferrite include martensite, bainite and cementite, which 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 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. It is preferable that the area ratio of 2nd phases other than ferrite is more than 30%, and 60% or less is preferable. If it exceeds 60%, ductility deteriorates for coarsening of cementite.

If the average crystal grain size exceeds 2 mu m, workability is remarkably improved without significant improvement in ductility. Preferably, the ferrite average crystal grain size is 1 탆 or less. In the present invention, the average crystal grain diameter was obtained by corroding a cross section perpendicular to the length of the steel pipe in a nitrile solution and microscopically inspecting it with an optical microscope or an electron microscope to obtain a circular equivalent diameter of 200 or more particles, and using the average value. The particle diameter of the second phase was measured using a pearlite colony boundary when the second phase was pearlite and a packet boundary when bainite and martensite were used as particle systems.

An example of the steel pipe structure of the present invention is shown in FIG. 3.

Next, the manufacturing method of the steel pipe of this invention is demonstrated.

The raw material steel pipe of the above composition is heated to a heating temperature of Ac 3 to 400 ° C., preferably (Ac 1 + 50 ° C.) to 400 ° C., and more preferably 750 to 400 ° C.

When the heating temperature exceeds the Ac3 transformation point, the surface properties deteriorate and crystal grains coarsen. For this reason, the heating temperature of a raw material steel pipe should be Ac3 transformation point or less, preferable (Ac1 + 50 degreeC) or less, and more preferably 750 degreeC or less. If the heating temperature is less than 400 ° C, the desired rolling temperature cannot be secured, and therefore the heating temperature is preferably 400 ° C or more.

Next, the drawn raw steel pipe is subjected to drawing rolling.

It is preferable to perform drawing rolling by the rolling mill of a 3 roll system, but it is not limited to this. It is preferable that the drawing mill is provided with a stand and rolled continuously. The number of stands can be appropriately determined by the size of the material steel pipe and the size of the product steel pipe.

The rolling temperature of the drawing rolling is in the range of Ac3 to 400 ° C in the ferrite recovery and recrystallization temperature range, preferably (Ac1 + 50 ° C) to 400 ° C, and more preferably 750 to 400 ° C. If the rolling temperature exceeds the Ac3 transformation point, ultrafine particles cannot be obtained, and the ductility does not improve except for the decrease in strength. For this reason, rolling temperature shall be Ac3 transformation point or less, preferable (Ac1 + 50 degreeC) or less, and more preferably 750 degreeC or less. On the other hand, when rolling temperature is less than 400 degreeC, it may be embrittled by clear blue brittleness, and a material may break during rolling.

In addition, when the rolling temperature is less than 400 ° C, the deformation resistance of the material increases, making rolling difficult, inadequate recovery and recrystallization, and processing deformation easily remain. For this reason, the rolling temperature of drawing rolling was Ac3-400 degreeC, the preferable thing was (Ac1 + 50 degreeC)-400 degreeC, and the more preferable thing was limited to the range of 750-400 degreeC. Preferable is 600-700 degreeC.

The cumulative shaft diameter in the drawing rolling is set to 20% or more.

If the cumulative axis diameter (= (material steel pipe outside diameter-product steel pipe outside diameter) / (material steel pipe outside diameter) × 100%) is less than 20%, the crystal grains due to recovery and recrystallization are insufficient and the ductile steel pipe is not rich. Do not. In addition, the piping speed is low, the production efficiency is low. For this reason, in this invention, cumulative shrinkage rate was made into 20% or more. In addition, when the cumulative shaft diameter is 60% or more, the microstructure of the structure is remarkable in addition to the increase in strength due to work hardening, and even in low-component steel pipes with low alloying amounts in the above-described composition range, the balance between strength and ductility is excellent. All are excellent steel pipes. From this, it is more preferable that the cumulative shaft shrinkage is 60% or more.

In drawing rolling, it is preferable to set it as the rolling which contains the rolling path | pass of 6% or more of axial-diameters per pass | pass at least 1 pass or more.

When the axial diameter per pass of the drawing rolling is less than 6%, the crystal grains due to recovery and recrystallization are insufficient. In addition, at 6% or more, the temperature rise due to the processing heat is recognized and the reduction of the rolling temperature can be prevented. In addition, it is more preferable that the shrinkage ratio per one pass be 8% or more, which has a great effect by crystal grain refinement.

Drawing rolling of the steel pipe in this invention becomes the rolling process of a biaxial stress state, and the remarkable crystal grain refinement | miniaturization effect can be acquired. On the other hand, in the rolling of the steel sheet, in addition to the rolling direction, the free end also exists in the plate axis direction (rolling perpendicular direction), and there is a limit to the refinement of crystal grains by rolling in a uniaxial stress state.

In addition, in this invention, it is preferable to make drawing rolling into rolling under lubrication. By drawing drawing rolling under lubrication (lubrication rolling), the strain distribution in the thickness direction becomes uniform, and the distribution of the crystal grain diameter becomes uniform in the thickness direction. When lubrication-free rolling is performed, the strain concentrates only on the material surface layer portion due to the shear effect, and the crystal grains in the thickness direction tend to be nonuniform. Lubrication rolling may be normally performed using the well-known mineral oil or the rolling oil which mixed synthetic ester with mineral oil, and does not need to specifically limit a rolling oil.

After drawing rolling, the steel is cooled to room temperature. Although the cooling method is good by air cooling, conventionally well-known cooling methods, such as water cooling, mist cooling, forced air cooling, are applicable in order to suppress particle growth at all. The cooling rate is 1 ° C / sec or more, preferably 10 ° C / sec or more. Further, depending on the required characteristics of the product, a stepwise cooling method such as correction during cooling may be used.

In addition, in the present invention, in order to stabilize the crystal grain diameter of the product steel pipe to 1 μm or less and high strength steel pipe to 2 μm or less, it is preferable to perform the following drawing rolling on the material steel pipe.

The average crystal grain diameter (di) of the ferrite of the cross section perpendicular to the steel pipe length direction at the outer diameter (ODi) (mm), and in the case of a high strength steel pipe, The raw material steel pipe is heated or cracked, drawing rolling of average rolling temperature (θm) (° C) and total axis diameter (Tred) (%) is made to product tube of outer diameter (ODf) (mm).

The drawing rolling method is preferably drawing rolling by a plurality of hole rolling mills called reducers. 4 shows an example of a preferred installation example of the embodiment of the present invention. 4 shows a drawing rolling apparatus 21 of a plurality of stands with hole rolls. The number of stands of the rolling mill is suitably determined by the combination of the material steel pipe diameter and the product pipe diameter. The hole rolls can normally be suitably applied to all of the known 2 rolls, 3 rolls or 4 rolls. The method of heating or cracking the drawing 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 from the viewpoint of high heating speed and suppression of production efficiency or growth of crystal grains (FIG. 4 illustrates a reheating apparatus 25 of the induction heating method). The heating or cracking temperature is based on the Ac3 transformation point, which is a temperature range in which the crystal grains do not coarsen, or on the basis of the material steel pipe Ac1 transformation point, and is preferably (Ac1 + 50 ° C) or less, and preferably 600 to 700 ° C. In the present invention, of course, even when the heating or cracking temperature of the material steel pipe exceeds the above-mentioned temperature, the crystal grain diameter of the product pipe becomes fine.

When the second phase in the material steel pipe structure is pearlite by rolling in this rolling region, the layered cementite in pearlite is divided into fine particles, thereby securing the stretching characteristics of the product steel pipe and improving workability. In addition, when the second phase in the material steel pipe structure is bainite, the processed bainite is recrystallized to form a fine bainitic ferrite structure, thereby securing the stretching property of the product steel pipe and improving workability.

The rolling temperature of drawing rolling is made into the temperature range of 400 degreeC or more heating or cracking temperature or less, and preferably 750 degreeC or less. At temperatures above the Ac3 transformation point, or above (Ac1 + 50 ° C), or at temperatures above 750 ° C, ferrite plus austenite biphasic zones containing large amounts of austenite, or ausate single phase and It becomes difficult to become a ferrite structure or a structure mainly containing ferrite, and reduces the effect of crystal grain refinement by ferrite processing. In addition, when the rolling temperature exceeds 750 ° C., the growth of the ferrite particles after recrystallization becomes remarkable, and it becomes difficult to become fine particles. Further, when the rolling temperature is less than 400 ° C, it becomes a clear heat embrittlement region, which makes the rolling difficult, or the recrystallization insufficient, and the work deformation tends to remain, thereby reducing the ductility and toughness. For this reason, the rolling temperature of drawing rolling is 400 degreeC or more, Ac3 transformation point or less, or (Ac1 + 50 degreeC) or less, and it is preferable to set it as the temperature range of 750 degrees C or less. Moreover, 560-720 degreeC is preferable and 600-700 degreeC is more preferable.

Drawing rolling is the average crystal grain diameter (di) of the ferrite of the cross section orthogonal to the steel pipe length direction of the material steel pipe within the rolling temperature range, the average rolling temperature (θm) (℃) of the drawing rolling and the total axis diameter ratio (Tred) (%) Is a drawing rolling which satisfy | fills Formula (1).

When the relationship between di, θ m and Tred does not satisfy Equation 1, the ferrite average crystal grains (cross section perpendicular to the steel pipe length direction) of the product tube do not become 1 micrometer or less. Similarly, in high strength steel pipes, the average crystal grains (cross section perpendicular to the steel pipe longitudinal direction) do not become fine particles of 2 m or less.

Rolling exit speed 200m / min, average rolling temperature, 550 ° C, 700 ° C in a drawing-rolling machine in which a 22-roll continuous 4 roll mill was made of JIS STKM 13A equivalent steel pipe (ODi = 60,3 mm, thickness: 3.5 mm). Fig. 6 shows the relationship between the total axial diameter ratio and the average crystal grain diameter of the raw material steel pipe in the case of rolling product tubes having various diameters in Figs.

An oblique line region satisfying the equation (1) is a region in which the crystal grains of the product tube can be 1 μm or less.

After the drawing rolling, the product tube 16 is preferably cooled to 300 ° C or lower. Although the cooling method is good by air cooling, the conventionally well-known cooling methods, such as water cooling using the quenching apparatus 24, mist cooling, forced air cooling, are applicable for the purpose of suppressing a grain growth at all. The cooling rate is 1 ° C / sec or more, preferably 10 ° C / sec or more.

In the present invention, the cooling device 26 may be provided at the inlet side of the drawing rolling device 21 or in the middle of the drawing rolling device 21 to control the temperature. In addition, the descaling apparatus 23 may be provided on the inlet side of the drawing rolling apparatus 21.

The material steel pipe made of the material of the present invention may be a seamless steel pipe or an electric wire steel pipe, a single welded steel pipe, a solid-state pressure steel pipe, or the like. In addition, the manufacturing process of the ultrafine grain steel pipe of this invention may be continued with the manufacturing line of the above-mentioned raw material steel pipe. 5 shows an example of the series of continuous solid pressure welded pipes.

The steel strip 1 from the uncoiler 14 is connected to the preceding steel strip by the joining device 15 and preheated to the preheating furnace 2 through the looper 17, and then formed into a forming roll group ( 3) in the open tube 7 and the edge preheating induction heating device 4 and the edge heating induction heating device 5 by heating the edge of the open tube 7 in the temperature region below the melting point squeeze roll (6) And press-bonded to form a steel tube (8).

Next, the raw material steel pipe 8 is heated or cracked at the predetermined temperature in the crack furnace 22 as described above, and then descaled by the descaling device 23 and drawn by the drawing rolling device 21. And cut with a cutter and calibrated in the pipe straightening device 19 to become a product pipe 16. The temperature of the steel pipe is measured by a thermometer (20).

Moreover, it is preferable to make it the rolling under lubrication as mentioned above also in said drawing rolling.

According to the above-described manufacturing method, a steel pipe having a structure mainly composed of ferrite and having ultrafine particles having an average crystal grain diameter of ferrite having a cross section perpendicular to the steel longitudinal direction of 1 μm or less is obtained. In addition, according to the above-described manufacturing method, there is an effect that the hardness of the core portion such as the electric resistance steel pipe, the single welded steel pipe, the solid state pressure welded steel pipe is uniform.

In addition, a high-strength steel pipe having a structure composed of ferrite and a second phase other than pearlite of more than 30% in area ratio and having ultrafine particles having an average crystal grain diameter of 2 mu m or less in a right angle cross section in the longitudinal direction of the steel is obtained without intermediate annealing.

(Example 1)

The steel tube having the chemical composition shown in Table 1 was heated to an induction heating coil at the temperature shown in Table 2, and then made into a product tube under the rolling conditions shown in Table 2 in a drawing roll mill having a three-roll structure. The solid pressure welded steel pipe shown in Table 2 is preheated to 2.6 ℃ thick hot rolled steel at 600 ℃ and continuously formed with a plurality of forming rolls to make an open tube, and then the preheated open edges of both open tubes to 1000 ℃ by induction heating. In addition, both edge portions were heated to 1450 ° C., which is an unmelting temperature region by induction heating, were joined by a squeeze roll, solid-phase welded, and a steel pipe having a diameter of 42.7 mm x 2.6 mm was used. On the other hand, the seamless steel pipe was used as a seamless steel pipe by heating the continuous casting billet and piped with Mannesmann mandrel miller.

Tensile properties, impact impact properties, and structure of this tube were investigated and the results are shown in Table 2. Tensile characteristics were JIS 11 test piece. The yield stress was the value of the lower yield point when the yield phenomenon was clearly observed, and 0.2% PS otherwise.

In addition, the value of stretching takes into account the size effect of the test piece,

E1 = E10 × (√ (a0 / a)) ^0.4

(E10: measured extension, a0: 292 mm2, a: test piece cross-sectional area (mm2))

The conversion value calculated | required using was used.

The impact impact characteristics were evaluated as the impact impact absorption energy by obtaining the absorbed energy up to 30% of the strain amount from the stress-strain curve obtained by performing a high-speed tensile test of the strain rate 2000s ^-1 .

In addition, the impact impact characteristic is actually represented by the deformation energy of the material at a strain rate of 1000 to 2000 s ⁻¹ when the vehicle collides, and the higher the energy, the better the impact resistance characteristics.

From Table 2, the invention examples (No. 1-No. 16, No. 19-No. 22) of the range of this invention are steel pipes which are excellent in the balance of ductility and strength. It has high tensile strength and high impact impact absorption energy at high strain rates. On the other hand, Comparative Examples No. 17, No. 18, and No. 23, which deviate from the scope of the present invention, are inferior in ductility or strength, have poor strength-ductility balance, and deteriorate impact shock characteristics.

In Comparative Examples No. 17 and No. 18, the shrinkage ratio was outside the range of the present invention, the ferrite particles were coarsened, the strength ductility balance was deteriorated, and the impact shock absorption energy was lowered.

(Example 2)

The raw material steel pipe having the chemical composition shown in Table 3 was heated to an induction heating coil at the temperature shown in Table 4, and then made into a product pipe under the rolling conditions shown in Table 4 in a drawing roll mill having a three-roll structure. In addition, the product method of the raw material steel pipe was made the same as the actual example 1.

About this product pipe | tube, tensile property, impact-resistant shock characteristic, and a structure | tissue were investigated similarly to an Example, and the result is shown in Table 4.

From Table 4, examples of the present invention (Nos. 2-1 to No. 2-3, Nos. 2-6 to No. 2-8, and Nos. 2-10 to No. 2 to 14) of the present invention range from ductility. It is made of steel pipe with excellent balance of strength. It also has high tensile strength and high impact shock absorption energy at high strain rates. On the other hand, Comparative Examples No. 2-4, No. 2-5, and No. 2-9, which deviate from the scope of the present invention, have any decrease in ductility or strength, poor strength-ductility balance, and also impair impact impact characteristics. do.

According to the present invention, a steel pipe having a ductility-strength balance is improved and excellent in impact resistance properties is obtained, but the steel pipe of the present invention is also excellent for secondary workability, for example, bulge workability such as hydrofoam. It is a preferred steel pipe.

Among the steel pipes of the present invention, in the welded steel pipe (sealing steel pipe) or the solid-state pressure welded steel tube subjected to the core cooling, the hardened core portion becomes the hardness of the same level as that of the mother tube portion by drawing rolling, and the bulge workability is remarkably improved than before.

(Example 3)

The raw material steel pipe having the chemical composition shown in Table 5 was heated with an induction heating coil at the temperature shown in Table 6, and then made into a product tube under the rolling conditions shown in Table 6 in a drawing roll mill having a three-roll structure. As the raw material steel pipe in this embodiment, a steel pipe having a thickness of 110 mm x 4.5 mm was used using a hot rolled steel sheet produced by controlled rolling and controlled cooling.

Tensile characteristics, impact impact characteristics, structure and sulfide stress cracking properties of the tube were investigated, and the results are shown in Table 6. In the same manner as in Example 1, JIS 11 test piece was used for the tensile property. In addition, the drawing value was determined using E1 = E10 × (√ (a0 / a)) ^0.4 (where E10: measured stretching, a0: 292 mm2, a: test piece cross-sectional area (mm2)) in consideration of the size effect of the test piece. The calculated value was used.

In addition, similarly to Example 1, the impact impact characteristics were evaluated as the impact impact absorption energy by using the high-speed tensile test of the strain rate 2000s ^-1 , and obtaining the absorption energy up to 30% of the deformation amount from the obtained stress-strain curve.

Incidentally, the impact impact characteristic is represented by the deformation energy of the material at a strain rate of 1000 to 2000 s ⁻¹ when the vehicle actually collides, and the higher the energy is, the better the impact resistance is.

In addition, the sulfide stress corrosion cracking property was a tensile strength of 120% of the yield strength in a NACE bath (0.5% acetic acid + 5% saline, H ₂ S saturation, temperature 25 ℃, 1 atmosphere) using the C ring test piece shown in FIG. The stress was applied and evaluated by examining the presence or absence of fracture during the 200 hr test period. The C-ring test piece was cut out from the T direction (circumferential direction) of the product tube base material part. Each test was performed on the same conditions.

In Table 6, examples of the present invention (No. 3-1 to No. 3-3, No. 3-5 to No. 3-8, No. 3-10, No. 3-12) of the present invention range from ductility. It is made of steel pipe with excellent balance of strength. It has high tensile strength and high impact shock absorption energy at high strain rates. In addition, it is a steel pipe that is excellent in sulfide stress cracking resistance and has excellent characteristics for line pipe. On the other hand, the comparative examples (No. 3-4, No. 3-9, No. 3-11) outside the scope of the present invention are either ductility or strength is lowered, the strength-ductility balance is poor, and impact resistance characteristics Deterioration occurs in the test in the NACE bath, and sulfide stress corrosion cracking deteriorates.

In Comparative Example No. 3-4, the shrinkage ratio is beyond the scope of the present invention, the ferrite particles coarsen, the strength ductility balance is deteriorated, the impact shock absorption energy is lowered, and the sulfide stress corrosion cracking is deteriorated.

In Comparative Examples NO. 3-9 and No. 3-11, the rolling temperature of the drawing rolling is outside the scope of the present invention, the coarse pearlite particles coarsen, the strength ductility balance is deteriorated, the impact shock absorption energy is lowered, and the sulfide stress is reduced. Corrosion cracking is deteriorating.

(Example 4)

As shown in Table 7, the raw material steel pipe having the chemical composition was heated to an induction heating coil at the temperature shown in Table 8, and then made into a product tube under the rolling conditions shown in Table 8 in a drawing roll mill having a three-roll structure. The raw material steel pipe of this embodiment is formed by forming a hot rolled steel strip into a plurality of forming rolls and forming an open pipe, and then welding both edges of the open pipe by induction heating to form a ø110 mm x 2.0 mm thick electric rod steel pipe, and continuous The cast billet was heated and piped with a Mannesmann mandrel-type miller to form a seamless steel pipe having a thickness of 110 mm x 3.0 mm.

Tensile properties, impact impact properties, structure and fatigue properties of this tube were investigated and the results are shown in Table 8. Tensile characteristics and impact impact characteristics were carried out in the same manner as in Example 1.

Fatigue characteristics were determined by using a test tube of the product tube as it was, and a fatigue strength vibration test (repetition rate: 20㎐) was performed in the air, and fatigue strength was obtained.

From Table 8, this invention example (No. 4-1, No. 4-3, No. 4-6-No. 4-9) of the range of this invention consists of a steel pipe excellent in the balance of ductility and strength. It has high tensile strength and high impact shock absorption energy at high strain rates. Moreover, it is the steel pipe which is excellent in fatigue-resistant property and has the outstanding property as a high fatigue strength steel pipe. On the other hand, the comparative examples (No. 4-2, No. 4-4, No. 4-5) out of the scope of the present invention are decreasing in fatigue strength.

In Comparative Example No. 4-2, drawing rolling was not performed, and in Comparative Example No. 4-5, the axial diameter ratio was out of the range of the present invention, and in Comparative Example No. 4-4, the rolling temperature of drawing rolling was the present invention. The ferrite particles are coarsened, the strength ductility balance is deteriorated, the impact shock absorption energy is lowered, and the fatigue resistance is deteriorated.

(Example 5)

Steel material A1 which has the chemical composition shown in Table 9 was made into the steel strip of 4.5 mm thickness by hot rolling. After preheating the steel strip 1 at 600 ° C. in the preheating furnace 2 using the example of the installation shown in FIG. 5, the open tube 7 is continuously molded in a molding processing apparatus 3 including a plurality of forming roll groups. ) Next, both edges of the open tube 7 are preheated to 1000 ° C. in the edge preheating induction heater 4, and then both edges are heated to 1450 ° C. in the edge heating induction heater 5 and squeezed. It was made of the raw material steel pipe 8 of ø88.0 * 4.5mm thickness by welding with the roll 6 and solid-phase contact.

Next, after the raw steel pipe is subjected to a heat cracking temperature as shown in Table 10 in the deep cooling and tube heating apparatus 22, a predetermined outside in the drawing rolling apparatus 21 provided with a drawing rolling machine having a plurality of three-roll structures. It was made into the product pipe of diameter size. The number of the stands of the used rolling mill was 6 stands, when the outer diameter of the product pipe was ø60.3 mm, and 16 stands when ø42.7 mm.

In addition, No.5-2 product pipe lubricated and rolled using the rolling oil which mixed synthetic ester with mineral oil at the time of drawing rolling.

The product tube was air-cooled after drawing rolling.

Crystal tube diameter, tensile characteristics, and impact characteristics were investigated for this tube, and the results are shown in Table 10. The crystal grain diameters were observed by 5 times or more at a magnification of 5000 times with respect to a cross section (C section) perpendicular to the longitudinal direction of the steel pipe, and the average grain size of ferrite was measured. Tensile characteristics were JIS 11 test piece. In addition, the stretching (E1) in consideration of the size effect of the test piece

E1 = E10 × (√ (a0 / a)) ^0.4

The converted value calculated | required by (E10: measured extension, a0 = 100mm <2>, a: test piece cross-sectional area mm <2>) was used. The impact characteristics (toughness) were evaluated by the Charpy impact test using the ductile fracture rate of the C section at -150 degreeC. In the seal pipe Charpy impact test, a 2 mmV notch was inserted at a right angle to the seal pipe length direction of the seal pipe so as to destroy the impact and obtain a ductile fracture rate.

From Table 10, examples of the present invention (No. 5-2, No. 5-4 to No. 5-7, No. 5-9 to No. 5-11, and No. 5-13) of the present invention range from The average crystal grains are all fine particles having a diameter of 1 µm, and are made of steel pipes having high stretching and toughness and excellent balance of strength, toughness and ductility. Moreover, in No. 5-2 which performed lubrication rolling, the variation of the crystal grain in the thickness direction was few. On the other hand, in the comparative examples (No. 5-1, No. 5-3, No. 5-8, No. 5-12) outside the scope of the present invention, the crystal grains coarsen and the ductility and toughness deteriorate. In addition, the structure of the product pipe of the scope of the present invention was ferrite + pearlite, ferrite + cementite, or ferrite + bainite.

(Example 6)

Steel B1 having the chemical composition shown in Table 9 was dissolved in a converter to obtain billets by the continuous casting method. The billet was heated and piped with a Mannesmann mandrel miller to form a seamless steel pipe having a thickness of 110.0 mm x 6.0 mm. This seamless steel pipe was reheated to the temperature shown in Table 11 by an induction heating coil, and was used as the product pipe of the outer diameter shown in Table 11 in the drawing-rolling machine of a 3-roll structure. In addition, the number of the stands of the used rolling mill was set to 24 stands when the outer diameter of the product pipe | tube was ø31.8mm, and it was 28 stands when ø25.4mm.

The properties of this product tube were examined and the results are shown in Table 11. The characteristics of the product tube were examined in the same manner as in Example 5 regarding the structure, grain size, tensile properties, and toughness.

Table 11 shows examples of the present invention (No. 6-1, No. 6-3, No. 6-6, No. 6-7, No. 6-9) in which the average grain size of ferrite is 1 It is made of steel pipes having a thickness of less than or equal to and excellent in stretching and toughness and excellent balance of strength, toughness and ductility. On the other hand, in the comparative examples (No. 6-2, No. 6-4, No. 6-5, No. 6-8) outside the scope of the present invention, ferrite crystals are coarsened and ductility and toughness deteriorate.

In addition, the structure of the product tube of the scope of the present invention was ferrite + pearlite, ferrite + cementite, or ferrite + bainite.

(Example 7)

The raw material steel pipe having the chemical composition shown in Table 12 was heated to an induction heating coil at the temperature shown in Table 13, and then made into a product pipe under the rolling conditions shown in Table 3 in a drawing roll mill having a three-roll structure. In addition, the number of the stands of the used rolling mill was 24 stands for the seamless steel pipe, and 16 stands for the solid-state pressure pipe and the sealing tube.

Solid pressure welded steel pipes shown in Table 13 are preheated 2.3 mm thick hot rolled steel strip to 600 ° C and continuously formed with a plurality of forming rolls to form an open tube, followed by induction heating of both edges of the open tube and preheating to 1000 ° C. After that, both edge portions were further heated to 1450 ° C. below the melting point by induction heating, fused with a squeeze roll, and subjected to solid-phase pressure welding to form steel pipes having a predetermined diameter. On the other hand, as a seamless steel pipe, a continuous cast billet was heated, a Mannesmann mandrel-type miller was piped into a seamless steel pipe having a thickness of 110.0 × 4.5 mm.

The properties of this product tube were investigated and the results are shown in Table 13. The properties of the product tube were examined in the same manner as in Example 1 regarding the structure, grain size, tensile properties, and toughness. From Table 13, this invention example of the range of this invention consists of a steel pipe whose average crystal grain diameter of a ferrite becomes 1 micrometer or less, is high in stretch and toughness, and is excellent in the balance of strength, toughness, and ductility. In addition, the structure of the product tube of the present invention was ferrite + pearlite, ferrite + pearlite + bainite, ferrite + cementite, ferrite + martensite.

(Example 8)

The steel material which has the chemical composition shown in Table 14 was made into the steel strip of 4.5 mm thickness by hot rolling. After preheating this steel strip 1 to 600 degreeC in the preheating furnace 2 using the example of an installation shown in FIG. 5, it shape | molded continuously in the shaping | molding apparatus 3 which consists of several shaping | molding rolls, and the open tube 7 I did. Next, both edges of the open tube 7 are preheated to 1000 ° C. in the edge preheating induction heating device 4, and then both edges are heated to 1450 ° C. by the edge heating induction heating device 5. It was made by squeeze roll 6, solid-state pressure welding, and the raw material steel pipe 8 of (phi) 110 * T4.5mm.

Next, after the raw material steel pipe was subjected to the heat cracking temperature shown in Table 15 in the deep cooling and tube heating device 22, the predetermined rolling diameter was obtained in the drawing rolling device 21 provided with a drawing rolling machine having a plurality of three-roll structures. It was made with product tube of. The number of the stands of the used rolling mill was 6 stands when the outer diameter of the product pipe was ø60.3 mm, and 16 stands when ø42.7 mm.

In addition, the product pipe of No. 1-2 was lubricated and rolled using the rolling oil which mixed synthetic ester with mineral oil at the time of drawing rolling.

After the drawing rolling, the product tube was air cooled.

About this product pipe | tube, crystal grain diameter and tensile property were investigated and the result is shown in Table 15.

The crystal grain diameters were observed by 5 times or more at a magnification of 5000 times for a cross section (C section) perpendicular to the longitudinal direction of the steel pipe, and the average crystal grains of the ferrite and the second phase were measured. Tensile characteristics were JIS 11 test piece. In addition, the stretching (E1) in consideration of the size effect of the test piece,

E1 = E10 × (√ (a0 / a)) ^0.4

The converted value calculated by (E10: measured stretching, a0 = 100 mm <2>, a: test piece cross-sectional area mm <2>) was used.

In Table 15, examples of the present invention (No. 1-2, No. 1-4 to No. 1-7, No. 1-10) in the scope of the present invention extend the average crystal grains to 2 micrometers. It is made of steel pipe with high toughness, tensile strength of 600MPa or more and excellent balance of strength, toughness and ductility.

Moreover, in No. 1-2 which performed lubrication rolling, the variation of the crystal grain in the thickness direction was not small. On the other hand, in the comparative examples (No. 1-1, No. 1-3, No. 1-8, No. 1-9) outside the scope of the present invention, the crystal grains coarsen and the ductility deteriorates.

In addition, the structure of the product pipe | tube of the scope of the present invention was a structure which has ferrite and cementite with an area ratio of more than 30% as a 2nd phase.

(Example 9)

After reheating the raw material steel pipe having the chemical composition shown in Table 16 with the induction heating coil at the temperature shown in Table 17, a three-roll drawing mill was used as the product pipe having the outer diameter shown in Table 17. In addition, the number of the stands of the used rolling mill was 16 stands.

The properties of this product tube were investigated and the results are shown in Table 17. The characteristics of the product tube were examined in the same manner as in Example 8 regarding the structure, grain size, and tensile characteristics.

Table 17 shows examples of the present invention (No. 2-1 to No. 2-6) in which the average crystal grain size of ferrite is 2 µm or less, the tensile strength is 600 MPa or more, the elongation is high, and the strength is also high. It is made of steel pipe with excellent balance of ductility. In contrast, in Comparative Examples (No. 2-7 and No. 2-8) outside the scope of the present invention, the crystal grains coarsened, the strength decreased, and the target tensile strength was not obtained.

In addition, the structure of the product pipe | tube of the scope of the present invention was a structure which has ferrite and ferrite, cementite, bainite, or martensite more than 30% by area ratio as a 2nd phase.

According to the present invention, a high-strength steel pipe having a conventional ductility-strength balance is obtained, but the steel pipe of the present invention is also excellent in secondary workability, for example, bulge workability such as hydroforming, and thus is suitable for bulge processing.

Among the steel pipes of the present invention, in the welded steel pipe or the high-tension welded steel tube subjected to core cooling, the hardened core portion becomes the same level of hardness as the mother tube portion by drawing rolling, and the bulge workability is remarkably improved compared with the conventional one.

Advantageous Effects According to the present invention, high strength steel pipes having excellent ductility and impact resistance properties can be manufactured with high productivity and easily, and the use of steel pipes can be expanded, which has a particular effect in the industry. In addition, according to the present invention, a high strength, high toughness line pipe steel pipe having excellent stress corrosion cracking resistance and a high strength high ductility steel pipe having excellent fatigue resistance can be manufactured at a low cost by reducing the amount of alloying elements. In addition, according to the present invention, it is possible to easily manufacture steel materials having ultra-fine crystal grains of 1 µm or less and excellent in high strength, toughness and ductility, and the use of steel materials can be expanded. In addition, it is possible to easily produce a high strength, excellent toughness and ductility of steel having ultrafine crystal grains of 2 µm or less and a tensile strength of 600 MPa or more without intermediate annealing.

Claims (28)

Heat or crack the material steel pipe with the outer diameter (ODi) (mm) and the average grain size of the ferrite in the cross section perpendicular to the steel pipe length direction (di) (μm), and the average rolling temperature (θm) (℃), total axis diameter In the manufacturing method of the steel pipe which carries out drawing rolling of (Tred) (%) and makes a product pipe of an outer diameter (ODf) (mm), The average grain size di (μm), the average rolling temperature (θm) (° C) and the total axis diameter (Tred) (%) in the temperature range of the drawing rolling is 400 ° C or more and below the heating or cracking temperature. The method of manufacturing a steel pipe, characterized in that the relationship of the drawing comprises a drawing rolling that satisfies the following formula (1). (Equation 1) di≤ (2.65-0.003 × θm) × 10 ^{{(0.008 + θm / 50000) × Tred}} Where di is the average 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 Shaft Diameter (%) = (ODi-ODf) × 100 / ODi ODi: Material Steel Pipe Outer Diameter (mm) ODf: Product tube outer diameter (mm) The method of claim 1, The manufacturing method of the steel pipe characterized by having the ultrafine particle whose average crystal grain diameter of the ferrite of the cross section orthogonal to the longitudinal direction of the steel pipe after drawing rolling is 1 micrometer or less. The method of claim 1, The structure after the drawing rolling is made of ferrite or a second phase other than ferrite with a ferrite and an area ratio of 30% or less, and has ultrafine particles having a particle diameter of 3 μm or less in the cross section perpendicular to the steel pipe length direction. Method for manufacturing steel pipe The method of claim 1, The structure after the drawing rolling is composed of ferrite or a second phase other than ferrite and the ferrite is 30% or less in area ratio, and has ultrafine particles having a particle diameter of the ferrite in a cross section perpendicular to the longitudinal direction of the steel pipe of 1 μm or less. Method for manufacturing steel pipe The method of claim 1, Fabrication of steel pipes characterized in that the structure after drawing rolling is composed of ferrite and a second phase other than ferrite in excess of 30% by area ratio and has an average crystal grain diameter of 2 mu m or less in a cross section perpendicular to the longitudinal direction of the steel pipe. Way. The method of claim 1, The structure after the drawing rolling is made of ferrite and a second phase other than ferrite in excess of 30% by area ratio, and has ultrafine particles having a particle diameter of 1 μm or less in the cross section perpendicular to the longitudinal direction of the steel pipe. Manufacturing method. The method according to any one of claims 1 to 6, Drawing rolling is performed at the Ac3 transformation point-400 degreeC, The manufacturing method of the steel pipe characterized by the above-mentioned. The method according to any one of claims 1 to 6, A method for producing a steel pipe, wherein the raw material steel pipe is heated to a heating temperature of Ac 3 transformation point to 400 ° C. before drawing rolling and then drawing rolling at a drawing rolling temperature of 400 deg. The method according to any one of claims 1 to 6, A method for producing a steel pipe, wherein the raw material steel pipe is heated to a heating temperature of 400 to 750 ° C before drawing rolling, and then drawing rolling is performed at a drawing rolling temperature of 400 to 750 ° C. The method according to claim 1 to 6, A method for producing a steel pipe, wherein the drawing rolling is rolling under lubrication. The method according to any one of claims 1 to 6, A method for producing a steel pipe, characterized in that the drawing rolling comprises at least one pass of a rolling pass having an axial diameter of 6% or more per pass. The method according to any one of claims 1 to 6, A method for producing a steel pipe, wherein the cumulative shaft diameter in the drawing rolling is 60% or more. The method according to any one of claims 1 to 6, By weight C: 0.005 to 0.30% Si: 0.01 to 3.0% Mn: 0.01 to 2.0% Al: 0.001-0.10% And a drawing roll is carried out using a raw material steel pipe having a composition consisting of a balance Fe and unavoidable impurities. The method according to any one of claims 1 to 6, By weight C: 0.005 to 0.30% Si: 0.01 to 3.0% Mn: 0.01 to 2.0% Al: 0.001 to 0.10% Moreover, 1 type (s) or 2 or more types chosen from Cu: 0.5% or less, Ni: 0.5% or less, Cr: 0.5% or less, Mo: 0.5% or less, or One or two or more selected from Nb: 0.1% or less, V: 0.1% or less, Ti: 0.1% or less, B: 0.004% or less, or REM: 0.02% or less, Ca: 0.01% or less of the steel pipe manufacturing method characterized in that further comprising one or two selected from the material steel pipe having a composition consisting of the remaining Fe and unavoidable impurities . The method according to any one of claims 1 to 6, By weight C: over 0.30 to 0.70% Si: 0.01 to 2.0% Mn: 0.01 to 2.0% Al: 0.001-0.10% And a drawing roll is carried out using a raw material steel pipe containing a composition consisting of a balance Fe and unavoidable impurities. The method according to any one of claims 1 to 6, By weight C: over 0.30 to 0.70% Si: 0.01 to 2.0% Mn: 0.01 to 2.0% Al: 0.001-0.10% 1 or 2 or more types selected from Cu: 0.5% or less, Ni: 0.5% or less, Cr: 0.5% or less, Mo: 0.5% or less, or One or two or more selected from Nb: 0.1% or less, V: 0.1% or less, Ti: 0.1% or less, B: 0.004% or less, or Manufacture of steel pipes comprising drawing and rolling using a material steel pipe further containing one or two selected from REM: 0.02% or less and Ca: 0.01% or less and having a composition consisting of residual Fe and unavoidable impurities. Way. External diameter (ODi) (mm), material steel pipe of average crystal grain diameter (di) of ferrite in the cross section perpendicular to the steel pipe longitudinal direction is heated or cracked, and the average rolling temperature (θm) (℃), total axis diameter A method for producing a steel pipe (Tred) (%) which is subjected to drawing rolling to a product pipe having an outer diameter (ODf) (mm), wherein the drawing rolling is carried out at a temperature range of 400 ° C. or higher and a heating or cracking temperature below the average. The relationship between the crystal grain diameter (di) (mu m), the average rolling temperature (θ m) (° C.), and the total axis diameter (Tred) (%) is expressed by weight% by drawing rolling satisfying the following formula (1). C: 0.005 to 0.30% Si: 0.01 to 3.0% Mn: 0.01 to 2.0% Al: 0.001-0.10% Ultrafine particle steel pipe, characterized in that it has a composition consisting of residual Fe and unavoidable impurities. (Equation 1) di≤ (2.65-0.003 × θm) × 10 ^{{(0.008 + θm / 50000) × Tred}} Here, di: average grain size 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 Shaft Diameter (%) = (ODi-ODf) × 100 / ODi ODi: Material Steel Pipe Outer Diameter (mm) ODf: Product tube outer diameter (mm) The method of claim 17, The component system of the steel pipe C: 0.005 to 0.10% Si: 0.01 to 0.5% Mn: 0.01 to 1.8% Al: 0.001-0.10% Ultrafine grain steel pipe containing, and having a composition consisting of the balance Fe and unavoidable impurities. The method of claim 17, The component system of the steel pipe By weight C: 0.06 to 0.30% Si: 0.01 to 1.5% Mn: 0.01 to 2.0% Al: 0.001-0.10% Ultrafine grain steel pipe containing, and having a composition consisting of the balance Fe and unavoidable impurities. The method according to any one of claims 17 to 19, An ultrafine grain steel pipe, wherein the average grain size of the ferrite in the cross section perpendicular to the longitudinal direction of the steel pipe after the drawing rolling has ultrafine particles of 1 µm or less. The method according to any one of claims 1 to 19, The structure after the drawing rolling is composed of ferrite or a second phase other than ferrite and ferrite having an area ratio of 30% or less, and has ultrafine particles having a particle diameter of 3 μm or less in the cross section perpendicular to the longitudinal direction of the steel pipe. Ultra-fine grain steel pipe. The method according to any one of claims 17 to 19, The structure after the drawing rolling is composed of ferrite or a second phase other than ferrite and 30% or less of ferrite and an area ratio, and has ultrafine particles having a particle diameter of the ferrite in a cross section perpendicular to the longitudinal direction of the steel pipe of 1 μm or less. Ultra-fine grain steel pipe. The method according to any one of claims 17 to 19, The structure after drawing rolling consists of ferrite and a second phase other than ferrite which is more than 30% by area ratio, and has the ultrafine particle whose average crystal grain diameter of the cross section orthogonal to a steel pipe length direction is 2 micrometers or less. Steel pipe. The method according to any one of claims 17 to 19, The structure after the drawing rolling consists of ferrite and a second phase other than ferrite which is over 30% by area ratio, and has ultrafine particles having a particle diameter of the ferrite in a cross section perpendicular to the longitudinal direction of the steel pipe of 1 μm or less. Grain steel pipe. By weight C: over 0.30 to 0.70% Si: 0.01 to 2.0% Mn: 0.01 to 2.0% Al: 0.001-0.10% It has a composition consisting of residual Fe and unavoidable impurities, the structure consists of ferrite and a second phase other than ferrite of more than 30% by area ratio, the average crystal grain diameter of the cross section perpendicular to the longitudinal direction of the steel pipe is 2㎛ or less High strength steel pipe with excellent workability. The method of claim 25, By weight C: over 0.30 to 0.70% Si: 0.01 to 2.0% Mn: 0.01 to 2.0% Al: 0.001-0.10% And one or two or more selected from Cu: 0.5% or less, Ni: 0.5% or less, Cr: 0.5% or less, Mo: 0.5% or less, or One or two or more selected from Nb: 0.1% or less, V: 0.1% or less, Ti: 0.1% or less, B: 0.004% or less, or A high strength steel pipe, characterized in that drawing rolling is carried out by using a material steel pipe further containing one or two selected from REM: 0.02% or less and Ca: 0.01% or less and having a composition consisting of residual Fe and unavoidable impurities. By drawing rolling of Equation 1 By weight C: over 0.30 to 0.70% Si: 0.01 to 2.0% Mn: 0.01 to 2.0% Al: 0.001-0.10% Or one or two or more selected from Cu: 0.5% or less, Ni: 0.5% or less, Cr: 0.5% or less, Mo: 0.5% or less, or Nb: 0.1% or less, V: 0.1% or less, Ti: 0.1% or less, B: 0.004% or less One or two more selected from REM: 0.02% or less, Ca: 0.01% or less, and have a composition consisting of residual Fe and unavoidable impurities, and the structure is composed of ferrite and a second phase other than ferrite having an area ratio of more than 30%. A high strength steel pipe, characterized in that the average crystal grain diameter of the cross section perpendicular to the longitudinal direction of the steel pipe is 2 μm or less. The method according to any one of claims 25 to 27, The structure after drawing rolling is composed of ferrite and a second phase other than ferrite in excess of 30% in area ratio, and has ultrafine particles having a particle diameter of the ferrite in a cross section perpendicular to the longitudinal direction of the steel pipe of 1 μm or less. Grain steel pipe.