JPH06330170A - Manufacture of martensite system stainless seamless steel tube - Google Patents

Abstract

PURPOSE:To attain a seamless steel tube, in which flaws on the inner surface at the time of piercing are completely eliminated, by heating and holding under specific conditions the bloom which is obtained by rolling the cast billet of steel under specific conditions, then rolling the bloom to a round billet, heating to a specific temp. and holding it. CONSTITUTION:A cast billet of martensite system stainless steel is rolled with a drawing-down ratio at 1.3 or above to obtain a bloom, which is heated to the temperature to become an austenite-ferrite double-phase areas and, after holding it for 30min or more at the temp., rolled to a round billet. Then, the round billet is heated to the temp. range to become a austenite single phase area and, after holding it for 30min for more at the temp. range, it is pierced by a Mannesman piercer. By this method, even if the core segregation of the cast billet such as Cr, P, etc., is obvious, delta ferrite in the billet is effectively erased, and a seamless steel tube without flaws on the inner surface is manufactured.

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 hegging that occur during Mannesmann piercer perforation rolling of such martensitic stainless steel are caused by the fact that δ ferrite present in the round billet during perforation rolling reduces the hot workability. It is known to occur.

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, bloom having a rolled structure in the center portion by bloom rolling is allowed to progress diffusion of segregating elements Cr and P in a high temperature region which becomes an austenite-ferrite two-phase region, and finished into a round billet by billet rolling. Is heated to a temperature at which it becomes an austenite single-phase region, and at this temperature,
By holding for more than a minute and then piercing-rolling, even if the center segregation of the slab is significant, the δ ferrite is effectively erased,
The inventors have found that a seamless steel pipe having no inner surface defect can be manufactured, and have completed the present invention.

That is, according to the present invention, in producing a martensitic stainless seamless steel pipe, (1) a slab is rolled at a rolling reduction ratio of 1.3 or more, and a cast structure in the central portion is rolled as a bloom of an intermediate product. And (2) heating to a temperature at which the austenite-ferrite two-phase region is formed, and holding at this temperature for 30 minutes or more to diffuse Cr and P, (3)
Finishing rolling into a round billet having a predetermined outer diameter by billet rolling, (4) heating the round billet to a temperature in the austenite single phase region, holding at this temperature for 30 minutes or more, and then performing piercing rolling ( 5) Furthermore, in order to effectively eliminate δ ferrite, heating of the cast piece prior to bloom rolling is heated to a temperature that is in the austenite-ferrite two-phase region, and this temperature is maintained for 1 hour or more to remove Cr and P. By diffusing, 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. (Bloom rolling condition) The reason why the slab after heating is subjected to bloom rolling to a reduction ratio of 1.3 or more to obtain a bloom of an intermediate product is as follows.

During reheating after bloom rolling, when the temperature range of the austenite-ferrite two-phase region is maintained on the equilibrium diagram, bloom rolling causes the core of the slab to change from a cast structure to a rolling structure with rolling strain. The changed portion starts recrystallization, and at this time, the diffusion of Cr and P is promoted by the crystal. Here, in order to easily recrystallize in the central part of the bloom rolled material by reheating after that, in order to easily recrystallize the material, a reduction ratio at the time of slab-rolling from a cast slab to a bloom rolled material is 1.3. The above is necessary, and it is effective if it is preferably 2.0 or more.

However, if the reduction ratio is too large, the load in rolling increases, the number of passes increases, and surface defects frequently occur in bloom rolling. Therefore, it is realistic to set the reduction ratio to 10 or less. (Reheating condition) The reason for heating and soaking the bloom after bloom rolling to a temperature in the austenite-ferrite two-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 1 and 13% Cr-0.2% C steels, an austenite-ferrite two-phase region begins to appear at 1250 ° C. or higher, and
In the temperature range of 0 ° C to 1250 ° C, it is an austenite single phase region.

However, at 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, but the diffusion of these elements is faster as the holding temperature is higher, and moreover in the ferrite than in austenite. Is faster.

Therefore, in order to promote the diffusion of the segregated element, the segregated portion having a high Cr and P concentration is transformed into ferrite by heating and soaking to a higher temperature in the austenite-ferrite two-phase region. At least 30 minutes or more, preferably 2 hours or more of soaking is necessary for the segregation to diffuse, but after 10 hours, decarburization of the surface occurs and surface defects occur during billet rolling. Therefore, it is desirable to set the upper limit to 10 hours.

(Heating condition 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 finish rolling. In order to transform all the formed ferrite 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 is stably present. Reheating and soaking and holding in the temperature range where the single phase of austenite is obtained can easily transform the precipitated ferrite into austenite. At least 3 to transform ferrite to austenite
It is necessary to soak and hold for 0 minutes or more, but after 3 hours, decarburization of the surface may occur and surface defects may occur at the time of pipe making, and productivity may be significantly reduced. It is desirable to set the upper limit to 3 hours.

(Conditions for heating the slab) Prior to bloom rolling, the slab is heated and soaked and maintained at a temperature in the austenite-ferrite two-phase region for the following reason. As described above, in order to promote the diffusion of the segregation element, it is necessary to heat and soak the temperature to a higher temperature in the austenite-ferrite two-phase region to transform the segregated portion having a high Cr and P concentration into ferrite.

However, unlike bloom after rolling, since the as-cast slab has fine defects, the diffusion speed is significantly slower than that of the rolled structure. Therefore, in order to obtain the effect of segregation diffusion. It is necessary to soak and hold for at least 1 hour or more, preferably 3 hours or more, but after 20 hours, decarburization of the surface may occur and surface defects may occur during bloom rolling. Is preferably 20 hours.

[0019]

EXAMPLE FIG. 1 schematically shows a manufacturing process of a martensitic stainless seamless steel pipe of an 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 the slab 5 after passing through the slab rolling machine 4.
It becomes A. The slab 5A is heated in a heating furnace 6, lumped in a slab 7 to form a bloom 5B, and a billet rolling machine 9 into a round billet 10 in a rotary roadbed heating furnace 11.
After being heated at, the hole is punched by the Mannesmann piercer 13 to become a hollow 14. The hollow 14 is a mandrel mill 16 into which a mandrel bar 17 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 chemical composition shown in Table 1 was manufactured into a slab material of each cross section shown in Table 2. As shown in FIG. 2, the steel of this component has an austenite-ferrite two-layer structure in the range of 1250 to 1430 ° C. and 950 to 1250.
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 shows the amount of ferrite in the vicinity of the δ 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 pipe. Indicate. Of the comparison methods, No. 7 and No. 9
No. 1 was heated and held at 1230 ° C. and 1200 ° C., which are austenite single-phase structure regions, respectively, instead of the austenite-ferrite two-layer structure region during heating before billet rolling. 8, No. 10 is not an austenite single-phase structure region at the time of heating before piercing and rolling, but austenite-
No. 1 heated to 1280 ° C. which is a ferrite two-phase structure region. No. 11 has a reduction ratio of 1.18 during bloom rolling. No. 12 is austenite-
After being heated to 1280 ° C. which is a ferrite two-phase structure region, bloom rolling was performed and billet rolling was immediately performed without reheating.

As is clear from this table, the tube using the round billet manufactured according to the present invention has a significantly lower incidence of inner surface defects than the tube using the round billet manufactured by the conventional method. I understand that.

[0023]

[Table 1]

[0024]

[Table 2]

[0025]

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]

FIG. 1 is a conceptual diagram schematically showing a production process of the present invention for producing a martensitic stainless seamless steel pipe.

FIG. 2 0.2% of the ternary equilibrium diagram of Fe-Cr-C
It is a characteristic view showing a C cut surface of.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 Converter 2 Continuous casting machine 3 Ingot 4 Slab rolling machine 5A Slab 5B Bloom 6 Heating furnace 7 Slab rolling machine 8 Reheating furnace 9 Billet rolling machine 10 Round billet 11 Rotary hearth 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 (2)

[Claims] 1. A martensitic stainless steel slab is rolled to a reduction ratio of 1.3 or more to form a bloom of an intermediate product, and the bloom is then austenite-ferrite 2.
After being heated to a temperature in the phase region and kept at the temperature for 30 minutes or more, it is rolled into a round billet, and the round billet is heated to a temperature range in the austenite single phase region, and the temperature range is set to 30.
A method for producing a seamless martensitic stainless steel pipe, characterized by holding for at least minutes and then piercing and rolling. 2. The bloom according to claim 1, wherein the cast slab is heated to a temperature at which it becomes an austenite-ferrite two-phase region, kept at the temperature for 1 hour or more, and then rolled to obtain a bloom of an intermediate product. Martensitic stainless steel seamless steel pipe manufacturing method.