JPH06306466A - Production of seamless martensitic stainless steel tube - Google Patents

Abstract

PURPOSE:To reduce the rate of occurrence of internal defects at piercing, to reduce the man-hour of improvement, to improve productivity, and to reduce costs at the time of producing a seamless martensitic stainless steel tube. CONSTITUTION:A cast billet of martensitic stainless steel is heated to an austenite ferrite two phase region temp. After soaking and holding at this temp. for >=3hr, the cast billet is rolled into a round billet, and this round billet is heated to a temp. in an austenite single phase region. After soaking and holding at this temp. range for >=1hr, piercing is done. By this method, the good-quality steel tube can inexpensively be obtained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a martensitic stainless seamless steel pipe used for oil well pipes, line pipes and the like.

[0002]

2. Description of the Related Art Martensitic stainless steel represented by SUS420 steel type is widely used as a material for oil country tubular goods and the like because it exhibits excellent corrosion resistance in a corrosive environment containing CO ₂ . When manufacturing stainless steel seamless steel pipes, a mannequin piercer is made by heating a slab cast by a mold or a continuous casting machine and using a solid round billet finished to a predetermined diameter by slab rolling and hole rolling. Generally, the piercing-rolling is performed by a typical inclined rolling method.

However, the martensitic stainless steel has poor hot workability, and therefore, during the piercing and rolling process with a Mannesmann piercer, which imposes severe deformation on the material, rolling defects such as cracks and whiskers occur on the inner surface. Yield and productivity were significantly impaired. Rolling defects such as cracks on the inner surface and whiskers that occur during Mannesmann piercer piercing and rolling of martensitic stainless steel are caused by the decrease in hot workability of δ ferrite present in the round billet during piercing and rolling. Is known to do.

For the purpose of preventing the occurrence of defects on the inner surface during piercing and rolling, for example, as disclosed in Japanese Patent Laid-Open No. 63-134630, a round billet before piercing and rolling is heated at 950 ° C to 1150 ° C. A method has been proposed in which δ-ferrite in a round billet is eliminated by soaking and holding in the temperature range for 30 minutes or more, preferably in the temperature range of 1000 ° C to 1100 ° C for 1 hour or more.

However, in the method of soaking the round billet before piercing and rolling at a high temperature for a long time as described above, the time spent in the heating furnace of the pipe making line becomes long and the productivity in pipe making decreases, Not only does scale loss and fuel consumption increase, but the decarburization of the outer surface of the round billet progresses, and on the contrary, the precipitation of δ ferrite on the outer surface is promoted and the hot workability deteriorates. There is a problem that the yield is significantly reduced due to the occurrence of rolling defects such as cracks.

Therefore, the present inventors have completely avoided the generation of defects on the inner surface during Mannesmann piercer piercing and rolling, and are capable of producing high productivity, low cost, and high yield martensitic stainless steel round billets. For the purpose of providing a manufacturing method, after the method disclosed in Japanese Patent Application Laid-Open No. 2-182825, that is, a slab, which is a material of a round billet, is subjected to primary rolling such as slabbing to form a rolling structure in the central portion, We proposed a method of effectively eliminating δ-ferrite by keeping the temperature equal to or lower than the δ-ferrite precipitation temperature on the equilibrium diagram and finishing it into a round billet by ordinary billet rolling.

[0007]

However, in the above method, since it is impossible to completely eliminate δ ferrite in the billet, a product with a large amount of drawing deformation in a stretch reducer during pipe making is Even a very small amount of residual δ ferrite leads to internal defects. Furthermore, when the center segregation of Cr, P, etc. of the cast slab, which is the material of the round billet, is remarkable, it is necessary to lengthen the soaking hold time during heating before billet rolling in order to effectively eliminate δ ferrite. However, there is a problem in that the productivity during the production of round billets is reduced. The present invention has been made in view of the above circumstances, completely avoids the occurrence of defects on the inner surface during Mannesmann piercer perforation rolling, high productivity, low cost, and manufactured with high yield. It is an object of the present invention to provide a method for producing a martensitic stainless seamless steel pipe.

[0008]

Means for Solving the Problems The present inventors have found that the amount of δ ferrite precipitated in a round billet of a martensitic stainless steel immediately before piercing and rolling and the amount of center segregation of slab, bloom rolling, billet rolling, piercing As a result of intensive investigations and studies on the relationship with the heating conditions before rolling, it was found that the diffusion and homogenization of the ferrite former elements of mainly Cr and P segregated in the steel are important for the disappearance of δ ferrite. I found it. Further, soaking and holding for 3 hours or more in a high temperature region, which is the austenite-ferrite two-phase region, is a segregating element, C.
The slab with r and P diffused is finished into a round billet by billet rolling, the round billet is heated to a temperature in the austenite single-phase region, held at the temperature for 1 hour or more, and then pierced and rolled. The present invention has been accomplished by finding that it is possible to effectively eliminate δ-ferrite and produce a seamless steel pipe having no inner surface defect even if the center segregation of is remarkable.

That is, according to the present invention, in producing a martensitic stainless seamless steel pipe, (1) a slab is heated to a high temperature region which is an austenite-ferrite two-phase region, and is soaked at this temperature for 3 hours or more. Hold, (2) finish rolling to a round billet with a predetermined outer diameter by billet rolling, (3) heat this round billet to a temperature that is in the austenite single phase region, hold at this temperature for 1 hour or more, and then punch By rolling, the above object is achieved.

[0010]

The reason for limiting the heating conditions and processing conditions in the present invention as described above will be described below. (Slab heating condition) The cast slab is austenite-ferrite 2
The reason for heating and soaking and maintaining the temperature in the phase region is as follows.

FIG. 2 is a 0.2 wt% C cutaway diagram of the ternary equilibrium diagram of Fe-Cr-C. For example, SUS420J which is a typical steel type of martensitic stainless steel
In the 1 and 13% Cr-0.2% C steels, the austenite-ferrite two-phase region begins to appear at 1250 ° C. or higher, and 950
The austenite single-phase region is in the temperature range of ℃ to 1250 ℃.

However, in the bloom center segregated portion after bloom rolling, C which is a ferrite former is partially present.
There is a region where the r and P concentrations are high, and even if the temperature is maintained at the austenite single-phase region on the equilibrium diagram, the diffusion rate of these segregated Cr and P is very slow, and ferrite is locally precipitated. . In order to eliminate these precipitated ferrites, it is necessary to diffuse segregated Cr and P while holding them at high temperature for a long time. The higher the holding temperature, the faster the diffusion of these elements, and more rapidly in ferrite than in austenite. Therefore, in order to promote the diffusion of the segregation element, heating and soaking are performed at a higher temperature in the austenite-ferrite two-phase region, and the segregation portion having a high Cr and P concentration is transformed into ferrite. In order for the segregation to diffuse, soaking and holding for at least 3 hours, preferably 5 hours or more are necessary, but after 30 hours, decarburization of the surface occurs and surface defects occur during billet rolling. Therefore, it is desirable to set the upper limit to 30 hours.

(Heating conditions before piercing / rolling) In heating before piercing / rolling, precipitation occurs in the billet, including those precipitated when heated / soaked to a temperature in the austenite-ferrite two-phase region before billet rolling. In order to transform all the existing ferrites into austenite, it is necessary to soak and maintain the temperature in a temperature range where a new ferrite does not precipitate and in which austenite stably exists.
By reheating and soaking and holding in a temperature range where a single phase of austenite is obtained, the precipitated ferrite can be easily transformed into austenite. In order to transform ferrite into austenite, it is necessary to soak and hold for at least 1 hour or more, but after 3 hours, decarburization of the surface may occur and surface defects may occur during pipe forming. To significantly reduce productivity,
It is desirable to set the upper limit to 3 hours.

[0014]

EXAMPLE FIG. 2 schematically shows the manufacturing process of the martensitic stainless seamless steel pipe of the example. The stainless steel refined in the converter 1 is continuously cast in the continuous casting machine 2 or cast in the ingot 3 and then passed through the decomposition rolling machine 4 and the slab 5 is cast.
Becomes The slab 5 is heated in a heating furnace 6, decomposed by a decomposition rolling mill 7, and turned into a round billet 10 by a billet rolling mill 9,
After being heated in the rotary roadbed heating furnace 11, it is perforated by the Mannesmann piercer 13 to become a hollow 14. The hollow 14 is a mandrel mill 16 into which a mandrel bar is inserted and the shell 15 is rolled by a mandrel mill roll 18, then reheated in a reheating furnace 19 and finish-formed by a stretch reducer 20 to form a finished pipe 21.

13% Cr steel having the composition shown in Table 1 was manufactured into a slab material of each cross section shown in Table 2. As shown in FIG. 1, the steel of this component has an austenite-ferrite dual phase structure of 950 to 1250 in the range of 1250 to 1430 ° C.
It becomes an austenite single phase structure in the range of ° C. This slab was finished into billets of 175 mmφ under the conditions shown in Table 2 according to the manufacturing process shown in FIG.
73.0mmφ × 5.51m with a relatively large drawing deformation in the stretch reducer by the mandrel mill method
Manufactured into mt seamless steel pipe. Table 2 also shows the amount of δ ferrite in the vicinity of the center segregated portion in the billet immediately before piercing and rolling, and the results of investigating the occurrence rate of inner surface defects by performing visual inspection and ultrasonic flaw detection on the rolled tube. N among comparison methods
o. 7, No. 8, No. 11 is an austenite-ferrite two-phase structure region, not the austenite single-phase structure region at the time of heating before piercing and rolling.
What was heated and maintained at 1280 ° C. and 1270 ° C., No.
9, No. 10 is a cast austenite-ferrite 2
It was heated to 1200 ° C. which is an austenite single-phase structure region, not a phase structure region.

As is clear from this table, the tube produced by the method of the present invention has a significantly reduced incidence of internal defects as compared with the tube produced by the comparative method.

[0017]

[Table 1]

[0018]

[Table 2]

[0019]

As described above, according to the present invention, it is possible to reduce the occurrence rate of inner surface defects during piercing and rolling, reduce the number of maintenance steps, improve productivity, and reduce cost, and obtain high-quality martensitic stainless steel. A seamless steel pipe can be manufactured at low cost.

[Brief description of drawings]

1% of Fe-Cr-C ternary equilibrium diagram
It is a characteristic view showing a C cut surface of.

FIG. 2 is a conceptual diagram schematically showing a manufacturing process for manufacturing a martensitic stainless seamless steel pipe.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Converter 2 Continuous casting machine 3 Ingot 4 Decomposition rolling machine 5A Slab 6 Heating furnace 7 Decomposition rolling machine 9 Billet rolling machine 10 Round billet 11 Rotary hearth type heating furnace 13 Mannesmann piercer 14 Hollow 15 Shell 16 Mandrel mill 17 Mandrel bar 18 Mandrel mill roll 19 Reheating furnace 20 Stretch reducer 21 Finished pipe

Claims (1)

[Claims] 1. A slab of martensitic stainless steel is heated to a temperature in the austenite-ferrite two-phase region, soaked and held at that temperature for 3 hours or more, and then rolled into a round billet. A method for producing a martensitic stainless seamless steel pipe, which comprises heating to a temperature range of a single-phase region, soaking the temperature range for 1 hour or more, and then piercing-rolling.