JPS61157633A - Manufacture of steel bar for drill collar - Google Patents

Abstract

PURPOSE:To manufacture a steel bar for a drill collar having a low hydrogen content and a metallic structure which can be hardened and tempered in succession by hot rolling a steel material, cooling the resulting steel bar to the bainite formation temp., and holding it at the temp. for a proper time or cooling it slowly. CONSTITUTION:A steel material of a low-alloy steel such as a medium-C Cr-Mo steel is hot rolled, and the resulting steel bar is cooled to the bainite formation temp. to form an easily austenitizable bainite-base structure. The steel bar is then held at about 400 deg.C as the bainite formation temp. for >=10hr or cooled slowly at <40 deg.C/hr cooling rate for >=10hr to reduce the hydrogen content to <=about 1ppm at which quenching crack is remarkably inhibited. A steel bar for a drill collar having high strength and toughness and causing no quenching crack is obtd.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a method for manufacturing a steel bar used for drill collars, which are part of drill strings for oil and natural gas drilling.

[Conventional technology]

The drill collar acts as a weight to prevent the bit from digging into the ground, and its diameter is usually 31/2" (79.4 mm).
) ~ 11″ (279,4mm) and the length of one piece is 9m
They are connected in lengths of 100 to 200 m and are inevitably subject to severe impacts and repeated stress.

For this reason, drill collars are required to have high strength and toughness. For example, tensile strength is 95 kgf/mn+" or more, yield strength is 70 kgf/m+m" or more, elongation is 13% or more, and Charpy absorbed energy (vE).

) is required to be 5.5kfg-m or more.

Low alloy steel (AIS) is usually used to obtain these mechanical properties.
I4145 equivalent steel) is used and is manufactured by quenching and tempering in a continuous heat treatment furnace.

What is required of the raw steel bar during this heat treatment is that quench cracking does not occur. In the case of minute quench cracks, they can be removed by using a grinder, but if they are even larger, a large loss will result.

The causes of quench cracking include hydrogen in the steel and giant inclusions, and it is desirable to keep both levels as low as possible. Conventionally, as a measure to reduce hydrogen, dehydrogenation treatment is performed at the molten steel stage by various degassing methods, and the bar is allowed to cool after rolling (conventional method A), or in addition to degassing treatment at the molten steel stage, it is also carried out immediately after rolling. A slow cooling method (referred to as conventional method B) was used.

[Problem that the invention seeks to solve]

In Method A, the hydrogen content of the steel bar does not necessarily reach a sufficiently low level because the hydrogen absorbed by the billet during heating before rolling cannot be allowed to cool after rolling. This point B
In @, the hydrogen content in the steel decreases, but the metal structure of the steel bar becomes a coarse ferrite/pearlite structure, resulting in the following problems: In other words, the quenching and tempering of the drill collar is continuous with insufficient heating time. Because it is heated in a furnace, it is rapidly cooled as soon as it reaches the target temperature for productivity reasons.

Therefore, when a steel bar with a coarse ferrite/pearlite structure is used, these structures are often not fully converted into austenite and remain, resulting in poor quenching and failure to obtain the target mechanical properties, which requires reheat treatment. As a result of conducting various experiments to find a solution to this problem, it became clear that this problem could be achieved by changing the structure of the steel bar to a rapidly quenched structure that is stronger than benite.

As shown in Figures 1 and 2, if the ferrite/pearlite steel bar is heated at the normal heating temperature of 850°C, the ferrite/pearlite will remain even after quenching if a sufficient holding time is not taken. The present invention provides a method for manufacturing a raw steel bar for color coloring, which can be completely quenched and hardened without holding time.

[Means and effects for solving the problems] The present invention cools the steel material to the hot rolling mill bainite forming temperature, and then holds it at this temperature for 10 hours or more, or heats it at 40°C/Hr.
The present invention relates to a method for manufacturing a steel bar for drill collars, characterized in that the steel bar is slowly cooled for 10 hours or more at a cooling rate of less than 10 hours. The steel material according to the present invention has a C: 0.40 to 0.5, for example.
0%, Si: 0.10-0.50%, Mn:
0.60-1.50%.

P: 0.030% or less, S: 0.030% or less.

5oQAQ: 0.010-0.050%, Cr: 0°8
0-1. Self-O%, Mo: 0.15 to 0.50% So-called medium coal Cr with the balance consisting of iron and inevitable impurities
- Low alloy steel such as Mo steel.

The reasons for the limitations of the present invention will be described below.

Cooling the steel material to the benite formation temperature after hot rolling is
This is to prevent the formation of coarse ferrite/varad that is difficult to austenite during heating during quenching, and to produce a steel bar with a benite structure that is easy to austenite. By cooling to about 400° C., a benite-based structure can be obtained up to 279.4 mmφ, which is the maximum size of a drill collar steel bar.

Furthermore, in the benite transformation region, it is maintained for 10 hours or more or slowly cooled at a cooling rate of less than 40°C/hour.

This is because the hydrogen content is reduced to below lppm, at which the occurrence of quench cracking is drastically reduced.

Next, the composition of the steel material according to the present invention will be described.

Regarding C, if it is less than 0.40%, hardenability will be insufficient and the specified toughness will not be obtained, and if it exceeds 0.50%, the toughness and ductility will deteriorate and quench cracking will easily occur, so 0.40%
It is desirable that the content be in the range of 0.50% or less. Si has a deoxidizing effect on steel and is used as a deoxidizing agent.
If it is less than 0%, it is insufficient, and if it exceeds 0.50%, it becomes saturated, so 0.10 to 0.50% is desirable.

Mn is an element that increases hardenability along with C and Cr and is used as a reinforcing element, but if it is less than 0.60%, hardening will be insufficient in combination with the amounts of C and Cr, and the specified strength will not be obtained.

Further, in consideration of alloy cost, the upper limit is preferably 1.5%. Both P and S are elements that deteriorate toughness.
, o s o %, it causes significant deterioration.

5olAQ has the effect of preventing coarsening of austenite crystal grains during heating during quenching, and for this purpose 0.
0.010% or more is required, and if it exceeds 0.050%, the effect will be saturated.

Cr, like C and Mn, is used as an element to improve hardenability, and if it is less than 0.80%, its effect is small and 1,5
If it exceeds 0%, the alloy cost will increase.

Mn is an element that increases hardenability and temper softening resistance and improves toughness, but if it is less than 0.15%, the effect is small, and the upper limit is preferably 0.50% in consideration of cost.

〔Example〕

Next, Table 1, which describes examples of the present invention, shows the chemical compositions of the test materials.

Table 1 (wt%) The steel shown in Table 1 was melted in a converter, dehydrogenated using a droplet degassing device, and then cast into a steel ingot to form 247X3 by blooming.
A 00 mm steel slab was produced and hot rolled into a 150 mmφ steel bar.

Cooling after hot rolling was carried out by the method of the present invention, conventional method A, and conventional method B. Microstructure and hydrogen analysis were conducted at the stage of raw steel bars, and the mechanical properties of the continuous heat treatment machine were investigated.

In the production according to the present invention, a hot-rolled steel bar was allowed to cool to 400°C, gradually cooled from 400°C to 300°C at a rate of 5°C/Hr, and then allowed to cool again to room temperature.

In addition, in conventional method A, the steel bar after hot rolling is allowed to cool to room temperature, and in conventional method B, immediately after hot rolling (approximately 750°C), the steel bar is
The mixture was gradually cooled to 600° C. at a rate of 1 hr and then allowed to cool in the atmosphere. Metal structure and hydrogen analysis specimens are 25m from the steel bar surface.
Collected from inside m.

Next, continuous heat treatment was carried out under the same conditions as in normal operation or at a slightly higher temperature.6 In other words, the steel bar was heated while passing through the heating furnace at a constant speed so that the passing time in the heating furnace was 30 minutes, and the temperature was raised to 860°C on the exit side of the furnace. . The steel bar is quenched with water spray and then heated in the same heating furnace for 30 minutes until the temperature on the exit side reaches 65%.
The temperature was controlled at 0°C.

A mechanical test piece was taken from the inside of the heat-treated steel bar at 25 mm, similar to the metallographic test piece and the hydrogen analysis test piece. (
The test pieces were JIS No. 4 for both tensile and impact test pieces. , ) [Effects of the Invention] Figure 3 shows the metal structure of the raw steel bar before quenching and tempering.

The metal structures of the steel bars manufactured by the present invention and conventional method A are both benite-based structures, but when manufactured by conventional method B, the metal structures are coarse ferrite-pearlite structures.

Table 2 shows mechanical properties and hydrogen analysis values.

Both the tensile properties and impact values of the steel of the present invention and the conventional steel A fully satisfy the required values. However, the hydrogen content of the steel of the present invention is 0.5 ppm, whereas the hydrogen content of the conventional steel A is 2.3 ppm.
pm and has a high risk of quench cracking. (In fact, microcracks have been detected as a result of visual observation.) In addition, in the case of conventional steel B, since it was slowly cooled after hot rolling, the hydrogen content was as low as the present invention steel, at 0.7 ppm, but coarse ferrite and Because it exhibits a pearlite structure and is insufficiently austenitized during heat treatment, it cannot meet the required values for both strength and impact value.

That is, the steel bar produced according to the present invention has a low hydrogen content and mechanical properties that fully satisfy the required values.

Table 2

[Brief explanation of drawings]

FIG. 1 is a diagram showing the influence of heating during quenching, that is, austenitization conditions, and the structure of the raw steel bar on the metal structure after quenching. FIG. 2 is a diagram showing the influence of the heating conditions during quenching and the structure of the raw steel bar on the hardness after quenching. FIG. 3 is a photographic diagram showing the metallographic structure of the present invention steel, conventional steel A, and conventional steel B before heat treatment. Patent Applicant Nippon Steel Corporation Representative Patent Attorney Yasushi Furushima ζ2nd Drawing Power U Shisho Jun (0C> Cleanliness of the section!) (No change in content) 7F 3 Group 1, Indication of the incident 1981 Patent Application No. 274968 2゜Name of the invention Method for producing steel bars for drill collars 3 Relationship with the case of the person making the amendment Patent applicant address 2-6-3 Otemachi, Chiyoda-ku, Tokyo Name (6
65) Nippon Steel Corporation Representative Takeshi 1) Yutaka 4, Agent Address: 1-12-1 Nishi-Shinbashi, Minato-ku, Tokyo, 8F, Daiichi Mori Building April 10, 1985 Date of dispatch of procedural amendment order: April 30, 1932 68 Drawing subject to amendment 7, contents of amendment

Claims (1)

[Claims] A method for manufacturing a steel bar for drill collars, which comprises cooling a steel material to a benite forming temperature in a hot rolling mill, and then holding it at that temperature for 10 hours or more, or slowly cooling it at a cooling rate of less than 40°C/Hr for 10 hours or more. .