JP2000096143A - Manufacturing method of steel pipe - Google Patents

Abstract

(57) [Summary] [Problem] To provide a method for manufacturing a steel pipe which is excellent in a balance between ductility and strength and can manufacture a product pipe having a small variation in these properties. SOLUTION: C: 0.005 to 0.70%, Si: 0.01 to 3.0
%, Mn: 0.01~4.0%, Al : 0.001 to steel containing 0.10%, after heating the heating or soaking the Ac ₃ transformation point to 400 ° C.,
Rolling is performed at a transformation point of Ac _{3 to} 400 ° C. and a cumulative reduction ratio of 20% or more. Then, the temperature θ (° C.) × time τ (min) is 1200
After keeping the temperature as described above, it is cooled.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of manufacturing a steel pipe, and more particularly to a method of manufacturing a steel pipe capable of imparting excellent mechanical properties and a good straight shape to the steel pipe.

[0002]

2. Description of the Related Art In order to increase the strength of steel, Mn,
Addition of alloy elements such as Si, heat treatment such as controlled rolling, controlled cooling, quenching and tempering, and addition of precipitation hardening elements such as Nb and V are utilized. However, steel materials need to have high ductility and toughness as well as strength.
For some time, there has been a demand for a steel material in which strength, ductility, and toughness are improved in a well-balanced manner.

[0003] Refinement of crystal grains is important as a few means capable of improving both strength, ductility and toughness. As a method for refining crystal grains, coarse austenite grains are prevented, and fine austenite is converted to austenite-.
There are a method of making ferrite grains fine by utilizing ferrite transformation, a method of making austenite grains fine by working to make ferrite grains fine, and a method of using martensite and lower bainite by quenching and tempering.

[0004] Above all, controlled rolling in which ferrite grains are refined by strong working in the austenite region and subsequent 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 processing in the austenite non-recrystallization temperature range, the austenite grains elongate and deformed bands are formed in the grains, and ferrite grains are generated from the deformed bands, and the ferrite grains are further refined. In order to further refine the ferrite grains, a step of cooling during or after the processing, that is, a controlled cooling is also used.

[0005]

However, in the above-mentioned method, the size reduction of the ferrite grain size to about 4 to 5 μm is a limit, and the process is too complicated to apply to the production of a steel pipe. For these reasons, in order to improve the toughness and ductility of the steel pipe, further refinement of the ferrite crystal grain size by a simple process has been demanded.

[0006] In response to this demand, the present inventors have conducted intensive studies, and hot-rolled a steel pipe material having a specific chemical composition to obtain a fine structure having a grain size of 3 μm or less and to achieve a balance between ductility and strength. The knowledge that an excellent product pipe can be obtained was obtained. However, if the steel pipe is water-cooled after warm drawing,
Alternatively, even in the case of air cooling, there is a problem that the rolling distortion remains and the ductility of the product tube becomes insufficient or varies.

Conventional methods for eliminating such ductility deficiencies and variations include increasing the rolling temperature and reducing the diameter reduction ratio. However, in these methods, there is a problem that it is difficult to refine the crystal grains and the strength tends to be insufficient. SUMMARY OF THE INVENTION An object of the present invention is to provide a method of manufacturing a steel pipe which can solve the above-mentioned problems of the prior art advantageously, has excellent ductility-strength balance, and can manufacture a product pipe having small variations in these characteristics.

[0008]

SUMMARY OF THE INVENTION The present invention provides, in weight percent,
C: 0.005 to 0.70%, Si: 0.01 to 3.0%, Mn: 0.01 to 4.
0%, Al: 0.001 to 0.10%, or
Cu: 1% or less, Ni: 2% or less, Cr: 2% or less, Mo: 1%
One or more selected from the following, and / or one or more selected from Nb: 0.1% or less, V: 0.3% or less, Ti: 0.2% or less, B: 0.004% or less 2 or more, and / or REM: 0.02% or less, Ca: 0.01%
One or two selected from the following, with the balance Fe
And a steel pipe having a chemical composition consisting of unavoidable impurities,
Ac ₃ After heating heating or soaking the transformation point to 400 ° C., Ac ₃ performs the reducing rolling cumulative radial contraction rate of 20% or more transformation point to 400 ° C., subsequently, temperature theta (° C.) × time tau (min) is This is a method for producing a steel pipe, which comprises cooling the temperature after maintaining the temperature at 1200 or more.

[0009]

DETAILED DESCRIPTION OF THE INVENTION In the present invention, a steel pipe having a specific chemical composition (hereinafter simply referred to as "composition") is used as a rolled material (raw pipe). Is not particularly limited. Electric resistance welding method using cold or hot high-frequency current (element name: ERW pipe, hot ERW pipe in case of hot), heating both edges of open pipe to solid-state pressure welding temperature range Solid-state pressure welding method (pipe name:
Any of solid-state pressure welded pipe), forged welding method (base pipe name: forged pipe), and Mannesmann piercing and rolling method (base pipe name: seamless pipe) can be suitably used.

Next, the reasons for limiting the composition of the raw tube will be described. C: 0.005 to 0.70% C is an element that increases the strength of steel by forming a solid solution in the matrix or precipitating as carbides, and also precipitates cementite, pearlite, bainite, as a hard second phase.
Martensite contributes to high strength and ductility (uniform elongation). In order to secure the desired strength and obtain the effect of improving ductility due to cementite and the like precipitated as the second phase, C
Is required to be contained at least 0.005%, more preferably at least 0.04%, but if it exceeds 0.70%, the ductility deteriorates. For this reason, C is limited to the range of 0.005 to 0.70%.

Si: 0.01-3.0% Si acts as a deoxidizing agent and forms a solid solution in the matrix to increase the strength of steel. This effect is observed when the content is 0.01% or more, preferably 0.1% or more, but the content exceeding 3.0% deteriorates the ductility. From this, Si is 0.01
Limited to the range of ~ 3.0%. Preferably, it is in the range of 0.10 to 1.5% from the viewpoint of strength-ductility balance.

Mn: 0.01 to 4.0% 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 content exceeding 4.0% deteriorates ductility. For this reason, Mn was limited to the range of 0.01 to 4.0%. From the viewpoint of strength-elongation balance, Mn is
The range is preferably from 0.2 to 1.3%, more preferably from 0.6 to 1.3%.
The range is 1.3%.

Al: 0.001 to 0.10% Al has an effect of making crystal grains fine. To refine the crystal grains, the content must be at least 0.001% or more. However, if the content exceeds 0.10%, the amount of oxide-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 alloying 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, and Mo are all elements that increase the strength, and one or more may be added as necessary. 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 with increasing strength, but adding more than 2% saturates the effect and increases cost, so 2%
Was set as the upper limit. If large amounts of Cr and Mo are added, the weldability and ductility will deteriorate and the cost will increase.
Was set as the upper limit. Preferably, Cu: 0.1-0.6%, N
i: 0.1 to 1.0%, Cr: 0.1 to 1.5%, Mo: 0.05 to 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 precipitate as carbides, nitrides, or carbonitrides, and refine crystal grains and increase strength. Element, especially in steel pipes with joints heated to high temperatures,
There is also an effect of miniaturizing grains in a heating process at the time of joining and a precipitation nucleus of ferrite in a cooling process to prevent hardening of a joined portion, and one or more kinds can be added as necessary.
However, when added in large amounts, both weldability and toughness deteriorate, so that Nb is 0.1%, V is 0.3%, Ti is 0.2%, and B is 0.004%.
% Was the upper limit. Preferably, Nb: 0.005
~ 0.05%, V: 0.05 ~ 0.1%, Ti: 0.005 ~ 0.10%,
B: 0.0005 to 0.002%.

One or two kinds of REM and Ca selected from REM: 0.02% or less and Ca: 0.01% or less, both have an effect of adjusting the shape of inclusions and improving workability. Furthermore, it has the effect of preventing sulphide, oxide or sulphate oxide from being hardened at the joint part in the steel pipe having the joint part, and one or more kinds can be added as necessary. REM exceeds 0.02% or Ca is 0.
If it exceeds 01%, the amount of inclusions becomes too large and the cleanliness decreases,
Ductility deteriorates. REM is less than 0.004% and Ca is less than 0.004%.
Below 1%, the effect of this effect is small, so REM:
The content is preferably 0.004% or more and Ca: 0.001% or more.

The composition part (remaining part) other than the above component elements is:
Consists of Fe and inevitable impurities. As inevitable impurities, N: 0.010% or less, O: 0.006% or less, P: 0.02%
5% or less, S: 0.020% or less is allowable. N: 0.010% or less N is an amount necessary for bonding with Al and refining crystal grains.
Up to 010% is acceptable, but more than 10% will reduce ductility, so it is preferred to reduce it to 0.010% or less.
More preferably, N is 0.002 to 0.006%.

O: 0.006% or less O degrades 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 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 desirably reduced as much as possible because it increases sulfides and deteriorates cleanliness. However, S is allowable up to 0.020%. Next, the reduction rolling step of the present invention will be described. The reduction rolling is preferably performed by a three-roll type reduction mill (reducer), but is not limited to the three-roll type. The reducer is preferably one that can be continuously rolled with a plurality of stands arranged in tandem. The number of stands is appropriately determined according to the target dimensions at the reducer entrance side and exit side of the rolled tube.

In the present invention, a steel pipe (raw pipe) having the above composition is heated or soaked to an Ac ₃ transformation point to 400 ° C.
Rolling is performed at a transformation point of Ac _{3 to} 400 ° C. and a cumulative reduction ratio of 20% or more. Then, the temperature θ (° C.) × time τ (min) is 1200
After keeping the temperature as described above, it is cooled. If the heating or soaking temperature (hereinafter collectively referred to as the heating temperature) exceeds the Ac ₃ transformation point, the surface properties deteriorate and the crystal grains become coarse. For this reason, the heating temperature of the raw tube should be lower than the Ac ₃ transformation point, preferably lower than (Ac ₁ + 50 ° C.), and more preferably lower than 750 ° C. If the heating temperature is less than 400 ° C., it is difficult to secure a suitable rolling temperature.
Preferably, the temperature is 400 ° C. or higher.

The rolling of the heated or soaked raw tube is performed by
It is preferable to use a three-roll type rolling mill, but the present invention is not limited to this. It is preferable that the rolling mill be capable of continuous rolling in which a plurality of stands are arranged in tandem. The number of stands can be appropriately determined according to the dimensions of the raw tube and the product tube. The rolling temperature of the reduction rolling is Ac _{3 to} 400 ° C. in the ferrite recovery / recrystallization temperature range, preferably (Ac ₁ + 50 ° C.).
To 400 ° C, more preferably 750 to 400 ° C.

When the rolling temperature exceeds the Ac ₃ transformation point, ferrite grains after recrystallization grow remarkably, and ductility does not improve at the expense of strength. Therefore, the rolling temperature is set to the Ac ₃ transformation point or lower, preferably (Ac ₁ + 50 ° C.) or lower, more preferably 750 ° C. or lower. On the other hand, if the rolling temperature is lower than 400 ° C., the material becomes brittle due to blue embrittlement and the material may be broken during rolling. Further, when the rolling temperature is lower than 400 ° C., the deformation resistance of the material increases and rolling becomes difficult. In addition, recrystallization becomes insufficient and processing strain tends to remain. For this reason, the rolling temperature of the reduction rolling is from Ac _{3 to} 400 ° C., preferably (Ac ₁ + 50 ° C.).
It was limited to 400 ° C, more preferably in the range of 750 ° C to 400 ° C. Particularly preferred is a range of 700 to 600 ° C.

The cumulative diameter reduction rate in the reduction rolling is set to 20% or more. Cumulative diameter reduction rate (= ｛(base tube outside diameter-product tube outside diameter) /
If (the outer diameter of the base tube) x 100%) is less than 20%, the refinement of the crystal grains by recovery and recrystallization is insufficient, and the steel tube does not become highly ductile. Also, the rolling speed is low and productivity is poor. Therefore, the cumulative diameter reduction rate needs to be 20% or more. In addition,
When the cumulative diameter reduction rate is 60% or more, the microstructure becomes remarkable in addition to the increase in strength due to work hardening, and the balance between strength and ductility is excellent even in a low-component steel pipe with a low alloy addition amount in the above composition range. A steel pipe excellent in both strength and ductility can be obtained.
For this reason, the cumulative diameter reduction rate is more preferably set to 60% or more.

In the reduction rolling, it is preferable that the rolling includes at least one rolling pass having a diameter reduction ratio of 6% or more per pass. If it is less than 6%, the crystal grains are not sufficiently refined by recovery / recrystallization. Also, 6%
In the above, the temperature rise due to the heat generated by the processing is recognized, and the reduction of the rolling temperature can be prevented. In order to further refine the crystal grains, the diameter reduction ratio per pass is particularly preferably 8% or more.

The product tube drawn and rolled under the above conditions is shown in FIG.
(c) By maintaining the temperature so that the temperature × time (θ ° C. × τmin) after rolling in the temperature history of the tube shown in (c) is 1200 or more, the balance between strength and ductility is excellent, and variations in these characteristics are small. Steel pipe can be obtained. In order to perform the above-mentioned heat preservation, it is preferable that after the raw tube 1 is squeezed and rolled, cut into the straight tube 2 and transferred by the cooling floor, the cooling floor is covered with the heat retaining furnace 7 (FIG. 1 (a)). If the coil 3 is to be wound, it is preferable to cover the coil 3 with a heat retaining furnace 8 (FIG. 1B). This can be done with simple modifications to existing equipment. In FIG. 1, reference numeral 4 denotes a heating / soaking furnace, 5 denotes a reducer (reducing mill), and 6 denotes a cutting machine.

After keeping the temperature, it may be cooled according to a conventional method.
This cooling may be air cooling or water cooling.

[0027]

EXAMPLES (Example 1) Among steels having compositions shown in Table 1, steels A to E were used as raw tubes (φ62.0 mm × T5.0 mm (φ: outer diameter,
T: wall thickness, the same shall apply hereinafter)), and after heating these blanks to 705 ° C., a rolling temperature of 700 to 655 ° C. and a rolling speed (final) by a three-roll reducer arranged in a 16-stand tandem arrangement. (Outside of stand) Rolling under 400m / min condition to produce product tube of φ25.4mm × T4.5mm.
As shown in (b), it was wound around a coil in a heat-retaining furnace, cooled under the conditions shown in Table 2, and then cooled. For comparison, a product cooled without performing heat retention after rolling, and a product having heat retention conditions outside the present invention were also manufactured.

In the solid-state pressure-welded pipe described in Table 2 as “solid phase”, a hot-rolled steel strip is preheated to 600 ° C. in a preheating furnace and then continuously formed into a tube by a plurality of forming rolls. Then, the joint was preheated to 1000 ° C. by induction heating, heated to 1450 ° C. in an unmelting temperature range, and upset with a squeeze roll to form a tube. The ERW tube described as "ERW" is formed by a conventional method in which a hot-rolled steel strip is continuously formed into a tubular shape with a plurality of forming rolls, the joint is heated to a melting temperature range by induction heating, and then upset by a squeeze roll. Piped.

Table 2 shows the results obtained by examining the tensile properties and crystal grain size of the product thus obtained. The tensile test uses a JIS No. 11 test piece, and the elongation value is calculated in consideration of the test piece size effect by a conversion formula El = El ₀ (√ (a ₀ / a)) ^0.4 (where,
El ₀ : measured elongation, a ₀ : constant 292 mm ² , a: specimen cross-sectional area (m
m ² )). As for the crystal grain size, a cross section perpendicular to the longitudinal direction of the steel pipe was corroded with a nital solution, the structure was observed with an optical microscope or an electron microscope, the equivalent circle diameter of 200 or more grains was obtained, and the average value was used. With respect to the grain size of the structure other than ferrite, the grain size was measured with the pearlite colony boundary in the case of pearlite, and the packet boundary in the case of bainite and martensite.

As shown in Table 2, each product has a crystal grain size of 3 μm.
In the present invention, which was cooled after being kept at a temperature of θ (° C.) × τ (min): 1200 or more after rolling, the strength (T
S)-excellent ductility (El) balance and small variation in TS and El (n number is 50), but the comparative example in which cooling was performed without keeping the temperature after rolling, and the condition for keeping the temperature deviated from the present invention. In the comparative example, the balance between TS and El is poor and the variation is large. (Example 2) For steels F to J among the steels having the compositions shown in Table 1, a billet made of continuous casting was heated and pierced and rolled with a Mannes mandrel mill to obtain "SML" in Table 3.
Seamless pipe (φ71.5mm x T11.0mm x length 15m)
After piercing and rolling, it was cooled to 560 ° C, heated to 680 ° C, and rolled at a temperature of 680-645 ° C with a three-roll reducer in a tandem arrangement of 18 stands at a rolling speed of 500m / min.絞 り 33.0mm
× T10.0mm product tube, after rolling, cut into a straight pipe of a predetermined length as shown in Fig. 1 (a), and then transported by a walking beam in a heat preservation furnace under the conditions shown in Table 3. It was kept warm and then cooled. For comparison, a product cooled without performing heat retention after rolling was also manufactured.

The results obtained by examining the tensile properties and the crystal grain size of the product thus obtained in the same manner as in Example 1 are shown in Table 3.
Table 3 shows that all the products have a crystal grain size of 3 μm or less. However, in the example of the present invention, which was cooled after having been kept at a temperature of θ (° C.) × τ (min): 1200 or more after rolling, the strength (TS) -ductility (El) The balance is excellent and the variation in TS and El (the number of n is 30) is small, whereas the comparative example in which cooling is performed without keeping the temperature after rolling is poor, the balance between TS and El is poor and the variation is large.

[0032]

[Table 1]

[0033]

[Table 2]

[0034]

[Table 3]

[0035]

As described above, according to the present invention, the crystal grain size is 3 μm.
When the diameter is equal to or less than m, an excellent effect of being able to manufacture a steel pipe having excellent strength-ductility balance and small variation in these properties can be obtained.

[Brief description of the drawings]

FIG. 1 is a schematic diagram showing a heat retaining embodiment of the present invention.

[Description of Signs] 1 Raw pipe 2 Straight pipe 3 Coil 4 Heating / soaking furnace 5 Reducer (reducing rolling mill) 6 Cutting machine 7, 8 Heat retention furnace

 ──────────────────────────────────────────────────続 き Continuing from the front page (72) Inventor Masanori Nishimori 1-1-1, Kawasaki-cho, Handa-shi, Aichi Prefecture Inside the Chita Works of 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 Kawasakicho, Handa-shi, Aichi Prefecture Kawasaki Steel Corporation Chita Works F-term (reference) 4K032 AA01 AA02 AA04 AA05 AA06 AA08 AA11 AA12 AA14 AA16 AA17 AA19 AA21 AA22 AA23 AA24 AA26 AA27 AA29 AA31 AA32 AA35 AA36 AA40 BA03 CA01 CB01 CC01 CC02 CD01 CD02 CD03 CD05 CD06

Claims (1)

[Claims] C. 0.005 to 0.70%, Si: 0.01 to 3.0%, Mn: 0.01 to 4.
0%, Al: 0.001 to steel containing 0.10%, after heating the heating or soaking the Ac ₃ transformation point to 400 ° C., Ac ₃ transformation point to 400
Draw rolling at 20% or more of cumulative diameter reduction at ℃
A method for producing a steel pipe, comprising: maintaining a temperature so that a temperature θ (° C.) × time τ (min) becomes 1200 or more, and then cooling.