JPS6067624A - Preparation of high strength and high toughness steel pipe - Google Patents

Abstract

PURPOSE:To prepare a high strength and high toughness steel pipe, by hot rolling having a specific composition containing C, Si, Mn, Cr, Mo and Al and, if necessary, V, Ti, Nb or B and, after cooling, hardening and tempering the rolled steel. CONSTITUTION:A billet of a steel material containing, on a wt. basis, 0.15- 0.35% C, 0.05-0.50% Si, 0.25-2.00% Mn, 0.5-5.0% Cr, 0.05-2.5% Mo, 0.005- 0.10% Al, if necessary, at least one or more of 0.10% or less V, 0.05% or less Ti, 0.10% or less Nb and 0.005% or less B and comprising the remainder of Fe and inevitable impurities is heated to 1,100 deg.C or more and, after drilling hot rolled to form a seamless steel pipe. This steel pipe is forcibly cooled to a room temp. at an average cooling speed of 2 deg.C/sec or more and, thereafter, heated to a temp. range of a Ac3 transformation point or more to perform hardening and succeedingly tempered in a temp. range of a Ac1 transformation point or less. By this method, a high strength and high toughness steel pipe with tensile strength of 100kg/mm.<2> or more is obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for manufacturing high-strength, high-toughness steel pipes, particularly
The present invention relates to a method for manufacturing a seamless steel pipe having a tensile strength of 0 kgy or more and excellent low-temperature toughness.

Oil country steel pipes used for oil and natural gas extraction are required to have higher strength or pressure as the drilling depth becomes deeper, and in recent years, oil country pipes with a tensile strength of 100 kg/IIIi1 or more have been used. It is increasing.

Conventionally, high-strength oil country tubular goods are manufactured by hot rolling, reheating to a temperature above the Acs transformation point, quenching, and then tempering at a temperature below the Ac+ transformation point.
There are many eluents containing Mn, Cr, Mo, etc., and also Ti, Nb.
, V, and other lubricant-strengthening elements are also added as needed, so it was not possible to expect low filtration toughness.

In this way, conventional steel pipes with still strength have extremely low low-temperature toughness, so there is a high risk of applying force that can lead to fracture accidents, and the conditions of use and handling are severely restricted. There was a problem.

An object of the present invention is to provide a method for manufacturing a high-strength oil well steel pipe that has sufficiently excellent low-temperature toughness and can solve the problems of the prior art described above.

In order to improve the low-temperature toughness of high-strength steel pipes for oil wells, the present inventors conducted extensive research and found that by using an appropriate heat treatment process, the low-temperature toughness of high-strength oil well m'tr can be improved. The present invention has been made based on this finding.

The gist of the present invention is as follows.

That is, in the M amount ratio, C: 0.15 to 0.35%, Si: 0.05 to 0.50
%, Mn: 0.25-2.00%, Cr: 0.
5-5.0%, Mo: 0.05-2.5%, AI! , :
Contains 0.005 to 0.10%, if necessary, V: 0.10% or less, i' i: 0.05
% or less, Nb: 0.10% or less, B: 0.005%
Contains 1 yuzu or 24'i11 or more selected from the following, with the remainder consisting of Fe and unavoidable impurities.
In the method for manufacturing a J-strength high-toughness steel pipe, the above 6f: ]
, a step of heating a billet with an i 7. Acs modification of the cooling steel pipe. ! A step of heating and quenching to a temperature range below the "CAC + transformation point" and tempering at a temperature range below the "CAC+transformation point", and the tensile strength is 100 kg/m+f or more. This is a method for manufacturing a high-strength, high-toughness steel pipe characterized by the following.

The reason for limiting the chemical composition of high-strength, high-toughness steel dust in the present invention will be explained.

C; C is a parody element to obtain high strength, but 0.1
If it is less than 5%, sufficient strength cannot be achieved, and if it exceeds 0 or 35%, it will cause quench cracking. .15
It was limited to a range of ~0.35%. .

Si: Because Si deoxidizes, it is safe to use 0.05% or more, but 0.05% or more is safe.
If it exceeds 50%, workability deteriorates, so 0.05 to 0.
．． It was limited to a range of 50%.

Mn: Mn is effective in increasing hardenability and strength, but if it is less than 0.25%, it has no effect, and if it exceeds 2.00%, quench cracking susceptibility increases, so 0.25 to 2.00%. limited to the range of

Cr: Or is added to increase hardenability and tempering resistance, but its effect is small at less than 0.5%, and at 5.0%
If the temperature is exceeded, hot workability will decrease.

The lower limit was limited to 0.5% and the upper limit was limited to 5.0%.

MO = Mo has the effect of increasing hardenability and tempering resistance, but if it is less than 0.05%, the effect is small, and if it is less than 0.05%, it will significantly reduce hot workability; 0
One tear was added to the range of 5-2.5%.

A: 8 requires addition of 0.005% or more for deoxidation, so the value is set at 0.005%, and addition of more than 0.10% significantly reduces toughness, so the upper limit 0.10
%.

Above, C, St, Mn, Cr, Mo, AiV
, (1) The basic components of the high-strength, high-toughness steel pipe of the present invention are limited to each rectangular amount, and V, Ti, and Nb are further added as necessary.
, B is below the monthly tear quantity, the object of the present invention can be more effectively achieved even in a high-strength, high-toughness steel pipe containing one or more of these at the same time.

The reasons for these limitations are as follows.

■= ■Is the effect of increasing strength due to precipitation strengthening large? 0.10
If the addition exceeds %, the toughness will decrease significantly, so the upper limit should be set at 0.1.
It was set to 0%.

Ti: Ti has the effect of forming N compounds and refining crystal grains, but if it exceeds 0.05%, the toughness will decrease, so it should be added to 0.05%.
1. Nb limited to 0.5% or less: Nb, like Ti, forms N compounds and has the effect of refining crystal grains and strengthening precipitation, but addition of more than 0.0% or io% impairs toughness). Set the upper limit to 0. IO%.

B.

B is effective in improving hardenability before the machine and is sometimes added especially in the case of thick walls, but if it exceeds 0.005%, it not only reduces hardenability but also reduces toughness. Therefore, it was limited to 0.005% or less.

Next, a manufacturing method including hot rolling and heat treatment of a high-strength, high-toughness billet for an inner pipe having the above-mentioned lumber component will be explained. The heating of the billet was limited to 1100℃ or higher because l 1
Heating to less than 00'C increases resistance during hot rolling and makes it difficult to perforate and roll into seamless steel.

Before performing quenching Qb on this hot-rolled steel pipe, the low-temperature toughness after quenching and tempering is improved by performing forced cooling at a fast average cooling rate of 2°C/sec or more immediately after hot rolling. The most important feature of the present invention is to improve the performance. Since low-temperature toughness varies in various ways through chemical composition, rolling conditions, heat treatment conditions, and metal structure such as microstructure and morphology of alloying elements, it is difficult to determine the factors contributing to the improvement in low-temperature toughness by the method of the present invention. However, it seems to be greatly influenced by the following three factors.

(a) Fine grain refinement (b) Uniformity of alloying elements (c) Sufficient tempering In the conventional method, seamless steel pipes are hot-rolled and then allowed to cool to room temperature. The structure is generally a rough rolled structure consisting of ferrite, pearlite, bainite, etc. due to the γ-α transformation that occurs during cooling after rolling.
The alloying elements are non-uniform because the amount of solid solution in each structure is different. In contrast, in the method of the present invention, by performing forced cooling after hot rolling, the coarse rolled structure can be refined, and the distribution of alloying elements approaches the uniform state during hot rolling. As a result, the structure after quenching and tempering is only a fine tempered martensite. It is possible to raise the tempering temperature of rice compared to the conventional method, and the low-temperature toughness can be improved by sufficient tempering.In addition, the average cooling rate of rapid cooling after hot rolling is limited to 2℃/second or more. is 2℃
This is because if the cooling time is less than 1 second, the structure after cooling will not be sufficiently refined and the alloying elements will be unevenly distributed, so that high strength and high toughness after quenching and tempering cannot be achieved.

After hot rolling, the joint is forcibly cooled at an average cooling rate of 2°C/sec or more; 1 Jib steel pipe is the same as the conventional method 4) J! - to Ac
It is heated to a temperature range of s or higher and quenched, and the temperature is reduced to ℃Ac.
Tempering is performed at a temperature IIiα below +.

In the present invention, prior to quenching and tempering the hot rolled steel pipe in step 5, immediately after hot rolling, forced cooling is performed at an average cooling rate of 2° C./sec or more, thereby refining the crystal grains and homogenizing the alloying elements. After quenching and tempering, tensile strength is 100K
It was possible to produce a seamless steel plate θ having excellent low-temperature toughness of g/nUi1 or more.

EXAMPLE Billets of five steel types having the chemical compositions shown in Table 1 were rolled at a hot pressure of 2152 to form jointless A pipes of the appropriate sizes shown in the table. Steels A, B and D satisfy the component limiting conditions of the present invention, but steels C and E do not.

Immediately after hot rolling, the material is allowed to cool or forced to cool to room temperature as shown in Table 2, and then quenched and tempered under the conditions shown in the same table to achieve a tensile strength of approximately 110 kg/-.
Compare the low-temperature toughness and compare the results with the second one.
Shown in the table. -50'F (-45,
Adopt the V-notch L-direction Charpy test at 4.14 g m (30 ft-1
1bs) or more is considered to have good low temperature toughness and is indicated by a Q mark.
Values less than 4.14 kr;-m are marked with an X mark.

As shown in Table 2, except for the example of E steel, 2℃/
In the case of forced cooling to a temperature higher than that of paper, the tempering temperature during quenching and tempering is about 20°C higher than that in the case of natural cooling (even if the tempering temperature is about 20°C higher, it is possible to improve the low-temperature toughness without lowering the strength. In addition, high strength and excellent low-grade toughness can be obtained only when steels A, B, and 1] that satisfy the component conditions of the present invention are forcedly cooled at an average cooling rate of 2°C/second or more after hot rolling. A4
On the other hand, if the chemical composition and cooling rate after hot rolling do not satisfy the requirements of the present invention, it will be difficult to obtain such high strength and sufficient low temperature toughness. Ta.

As is clear from the above examples, the present invention is made by heating a billet with limited chemical composition to 1100°C or higher and hot rolling it into a seamless steel pipe, and then immediately forcibly cooling it to room temperature at an average cooling rate of 2°C/sec or more. , slightly quenched at a temperature of AC3 or higher,
By tempering at a temperature of AC1 or higher, the tensile strength increases to 1.
It was possible to manufacture a WI pipe with excellent low-temperature toughness of 00 kg/- or more.

Agent: Patent Attorney Takeshi Nakaji) 4t

Claims (1)

[Claims] (1) In terms of weight ratio, c: 0.15-0.35%, Si: 0.05-0.
50%, Mn: 0.25-2.00%, Cr:
0.5-5.0%, Mo: 0.05-2.5%, A
(42) Contains 0.005 to 0.10%, if necessary, v: o, io% or less, Ti: 0.05% or less,
In the method for manufacturing a high-strength, high-toughness steel pipe containing one or more selected from Nb: 0.10% or less, B: 0.005% or less, and the balance consisting of Fe and unavoidable impurities, the A step of heating a billet of steel material to 1100° C. or higher and then hot-rolling it into a seamless steel pipe; a step of cooling the hot-rolled steel pipe to room temperature by a strong 11 tll at an average cooling rate of 2° C./sec or higher; and the cooling. @'JI tube Acs
A step of heating to a temperature range above the transformation point and quenching, followed by tempering in a temperature range below the Ac + transformation point, and the tensile strength is 100 kg/- or more. A method for manufacturing high-strength, high-toughness steel pipes.