KR910006027B1 - Making process for steel plate for oil well - Google Patents

Abstract

A making process for high toughness and strength steel for an oiler is characterized by its composition of 0.20-0.30 wt.% C, 0.003- 0.009 wt.% N, 0.1-0.4 wt.% Si, <=0.01 wt.% P, <= 0.01 wt.% S, 0.01-0.1 wt.% Al, 0.02-0.05 wt.% Nb, 0.01-0.03 wt.% Ti, and balance Fe and heating it at 1200-1300 deg.C; hot-rolling it at its finishing temperature ranges of 900-950 deg.C; quenching it at a speed of 10 deg.C/sec; coiling it round roller at temperature range from 550 deg.C to 600 deg.C. Its toughness and tensile strength can be increased by the addition of small amount of Nb and Ti element to C-Mn steel. It is applied for casing and tubing pipe for an oiler.

Description

Manufacturing method of high toughness oil well steel pipe steel

The present invention relates to a method for producing a steel pipe steel for oil wells having high toughness.

API-55 steel pipes for casing or tubing pipes used in oil wells require high tensile strength compared to yield strength, so about 0.3% of carbon is added as an alloying element. Due to overall poor workability, there is a disadvantage in quality that micro cracks are frequently generated during the slitting or bending of the hot rolled coil material for the manufacture of pipes.

Therefore, as a method for addressing the disadvantage of high carbon content and poor toughness and weldability, a method of manufacturing by adding a small amount of alloying element Nb to medium carbon steel or winding a low temperature of medium carbon C-Mn steel has been proposed.

The production of API K-55 oil refined steel pipe has low impact value due to the high carbon content, and the impact value is largely dependent on the amount of ferrite, particle size, and pearlite structure. In the case of the material material during rolling and austenite transformation, there is a considerable difficulty in maintaining proper transformation conditions depending on the direction, such as plate thickness, and the mechanical properties change largely depending on the winding temperature. In addition, the low temperature winding method of C-Mn steel without the addition of trace alloy elements Nb, V, and Ti is carried out at a low cooling rate of 15 ° C./sec at a cooling rate after finishing rolling, so that the low temperature winding is performed at about 500 ° C. Although the price is low and good quality characteristics can be obtained, a large cooling capacity of the run out table is basically required.

The present invention is to prepare a high toughness high strength steel pipe steel material by adding a small amount of Nb and Ti as a suitable alloy component and wound at an appropriate temperature in order to solve the above problems as described in detail as follows.

In the present invention, C: 0.20% -0.30%, N: 0.003-0.009%, Si: 0.1-0.4%, Mn: 1.0-1.5%, P and S: 0.01% or less, Al: 0.01-0.1% , Slab composed of Nb: 0.02-0.05%, Ti: 0.01-0.03% and balance Fe was heated to 1200-1300 ℃ and hot rolled to finish rolling temperature in the range of 900-950 ℃, then 10 ℃ / sec It is characterized by the configuration of winding at 550-600 ℃ by cooling at a cooling rate of.

EMBODIMENT OF THE INVENTION Hereinafter, this invention is demonstrated in detail.

The addition of Nb and Ti, the carbonitride-forming elements, on the medium carbon microalloy suppresses the growth of austenite grains while Ti reacts with nitrogen to form TiN in a hot rolling furnace, while Nb reacts with carbon and nitrogen. Nb carbonitride is precipitated during hot rolling to suppress recrystallization grain growth to create fine austenite grains, and to produce fine ferrite and pearlite after transformation to obtain mechanical properties with excellent strength and toughness. It is preferable to add 0.02-0.05% and 0.01-0.03% of Ti, respectively.

In addition, the heating temperature is a reheating temperature of the conventional medium carbon steel, but if it is 1200 ℃ or less, precipitates such as Nb and Ti are not sufficiently dissolved, so that it is difficult to obtain a uniform solid solution. Since high temperature embrittlement occurs in a product, it is preferable to make the temperature into 1200-1300 degreeC.

When the coiling temperature is 600 ° C. or higher, the ferrite grain size becomes coarse and the coarse ferrite is reduced on the fine ferrite or bainite phase of the second phase of the metamorphic structure, and thus the strength and toughness are lowered. Therefore, 550-600 ° C. is preferable. The austenite grain size during rolling depends on the rolling temperature and the rolling reduction. The lower the rolling temperature and the larger the rolling reduction, the finer austenite grains are obtained. Thus, Ar ₃ haejumyeon row immediately above the finish rolling in the pre-transformation temperature is to occur, but the preferred ferrite grain refinement, and more specifically, it is preferable that the finish rolling temperature to 900-950 ℃.

On the other hand, austenite transformation during cooling and precipitation reaction during transformation are important factors affecting the mechanical properties of microalloyed steel. At this time, as the yield strength or the thickness of the product increases among the mechanical properties, it is very difficult to maintain the proper transformation conditions. Therefore, by adding an appropriate element, the transformation temperature is lowered or the equipment is supplemented with the appropriate cooling conditions. In other words, the faster the cooling rate, the smaller the ferrite grain size and the lower the amount of pearlite, the more the yield strength and tensile strength value are increased as some transform to bainite or martensite structure.

However, the higher the cooling rate, the higher the cooling capacity of the runout table is required, and thus the manufacturing cost is increased. Therefore, the cooling rate is preferably about 10 ° C / sec in consideration of such economic aspects.

Hereinafter, the present invention will be described in detail through examples.

EXAMPLE

The C-Mn steel, the Nb-added steel, the V-added steel, and the Nb + Ti-added steel having the composition shown in Table 1 were prepared by dissolving the medium carbon wire and adding necessary elements. Each cast ingot was heated to 1250 ° C. and maintained for a certain period of time to completely dissolve the alloying element precipitate in the heating furnace, and then finish rolling was usually performed directly under Ar ₃ transformation temperature by controlled rolling.

After finishing rolling, the run-out table converts the cooling rate into 10 ℃ / sec and changes the winding temperature to 200-700 ℃, and then collects test materials wound in each winding temperature range to obtain tensile strength, yield strength, impact value and Elongation was measured and shown in Table 2 below.

TABLE 1

TABLE 2

As shown in Table 2, the present invention (14, 15, 16) has a yield strength, tensile strength and elongation that satisfies the API K-55 quality requirements, unlike the comparative material (1-13) Able to know.

These mechanical properties are characterized by grain refinement by TiN precipitates in a furnace, grain refinement by precipitates of Nb (C, N) or (Nb, Ti) (C, N) during rolling, precipitation strengthening during transformation and fine pearlite or part It is achieved by the transformation strengthening effect by bainite.

As described above, the present invention not only does not significantly affect the mechanical properties depending on the coiling temperature or the cooling rate by adding Nb and Ti in combination, and in particular, by adding Ti, the TiN precipitate refines the particle size of the austenite. When the austenite transformation, the lamellar spacing of pearlite is narrowed, so it is possible to manufacture high-strength, high-toughness oil pipe steel with excellent impact value compared to steel, and it is not necessary to strictly control the cooling conditions, and to install facilities with low cooling ability. There is an effect that can be applied in the hot rolling process.

Claims (1)

By weight%, C: 0.20-0.30%, N: 0.003-0.009%, Si: 0.1-0.4%, Mn: 1.0-1.5%, P and S: 0.01% or less, Al: 0.01-0.1%, Nb: Slab composed of 0.02-0.05%, Ti: 0.01-0.03% and balance Fe was heated to 1200-1300 ° C and hot-rolled to finish rolling temperature in the range 900-950 ° C, and then cooled at 10 ° C / sec. Method for producing a high toughness high strength steel pipe steel material characterized in that the cooling by the 550-600 ℃ winding.