JP3214351B2 - Method for producing Cr-Mo based seamless steel pipe excellent in high temperature strength - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION The present invention relates to a C-Mo seamless steel pipe having excellent high-temperature strength used for high-temperature applications such as boilers and the like, which can be produced at lower cost than conventional methods.
The present invention relates to a method for manufacturing an r-Mo seamless steel pipe.

[0002]

2. Description of the Related Art In the steel industry, which requires huge equipment, various studies have been conducted on simplification of processes by applying online thermomechanical treatment from the viewpoint of saving processes and energy. In the production of seamless steel pipes, from the viewpoint of high reliability and high quality, it is a fact that quite a few products are still manufactured by off-line heat treatment, and a heat treatment furnace is installed separately from the pipe production line. It is common practice to perform a normalizing process followed by a tempering process, or a constant temperature transformation heat treatment. On the other hand, online heat treatment uses the heat of the material after hot working to immediately reheat it to the austenite temperature, which can significantly reduce the cost industrially. There is a move to introduce a heat treatment process.

[0003] As shown in Fig. 2, a Cr-Mo seamless steel pipe excellent in high-temperature strength is generally obtained by heating a billet and piercing it with a piercer, which is an inclined rolling mill, by using a plug mill or a mandrel mill. After elongation rolling in
Finishing with a sizer or stretch reducer, cooling, normalizing, tempering or Ac ₃
Austenitized at a temperature above the temperature, maintained at a predetermined temperature for a certain period of time in the cooling process, and manufactured by performing high-temperature transformation heat treatment, but in any case, it is necessary to heat the once cooled seamless steel pipe again Not economical.

As a method for producing a Cr-Mo seamless steel pipe, two direct heating type heat treatment furnaces are used, and in the first heat treatment furnace, the temperature of the pipe is kept at a constant temperature in the range of 880 to 1000 ° C. After heating for more than 10 minutes, extracting from the first heat treatment furnace and cooling it by natural cooling or forced cooling at a temperature in the range of 650 to 780 ° C., it is charged into the second heat treatment furnace and the temperature of the tube is reduced to 680 to 680 ° C. 60 at constant temperature in the range of 750 ° C
(Japanese Patent Application Laid-Open No. 4-168224), after the refining tube is reheated to 850 to 1150 ° C., the sectional area of the finishing tube is Ao (cm ² ), When the area is A (cm ² ), ε = ln (A
o / A) a method of performing a finishing process in which the working strain ε at the time of the finishing process is 0.1 or more (Japanese Unexamined Patent Publication No. 4-80350). After reheating the finish rolling tube to 850 to 1150 ° C., when the cross-sectional area of the finishing tube is Ao (cm ² ) and the cross-sectional area of the finishing tube is A (cm ² ), ε = ln ( Ao
/ A) is rolled and air-cooled so that the processing strain ε at the time of finish rolling represented by / A) becomes 0.1 or more, and then 740 to 82
Method of performing tempering heat treatment at 0 ° C. (Japanese Patent Publication No. 7-3554)
No. 7) has been proposed.

[0005]

SUMMARY OF THE INVENTION The above-mentioned Japanese Patent Application Laid-Open No. 4-168 is disclosed.
Although the method disclosed in Japanese Patent Publication No. 224 is changed from an indirect heating furnace to a direct heating furnace, it is an offline constant temperature transformation process, so it is necessary to reheat the material once cooled, which is not economical. . In addition, this method has the disadvantage that the austenitizing treatment time is long, the scale growth is large due to direct heating, and the descaling treatment is required in a subsequent step.

In the method disclosed in Japanese Patent Application Laid-Open No. 4-80350, since the finishing process is performed after reheating, uniformity of the structure in the longitudinal direction and the circumferential direction cannot be obtained depending on the size. On the other hand, in the case of a high Cr type, the strength is too high in the cooling treatment and the tempering treatment after the finish processing, so that the high-temperature creep characteristics may deteriorate.

Furthermore, the method disclosed in Japanese Patent Publication No. Hei 7-35547 does not provide uniformity of the structure in the longitudinal direction and the circumferential direction depending on the size because the finish processing is performed after reheating. On the other hand, in the case of a high Cr type, the strength is too high in the cooling treatment and the tempering treatment after the finish processing, so that the high-temperature creep characteristics may deteriorate.

SUMMARY OF THE INVENTION An object of the present invention is to solve the above-mentioned disadvantages of the prior art and to specify a steel composition, hot rolling and heat treatment conditions, thereby performing a heat treatment on a pipe-making line to provide a Cr-Mo alloy having excellent high-temperature strength. An object of the present invention is to provide a method for manufacturing a seamless steel pipe.

[0009]

Means for Solving the Problems The present inventors have intensively studied and studied to achieve the above object. As a result, even in the direct heat treatment process, the component system of the steel is specified, the conditions of finish rolling such as piercing and elongation rolling are specified, and the recrystallized grains that have been sized are obtained by performing appropriate working heat treatment. It has been determined that mechanical properties equivalent to those obtained by ordinary off-line heat treatment can be obtained, and the present invention has been achieved.

According to the present invention , C: 0.04-
0.25%, Si: less than 1.0%, Mn: 0.2-1.
0%, Cr: 1.0-12.5%, Mo: 0.2-2.
5% , Nb: less than 0.2%, V: less than 1.0% ,
And Ni : less than 1.0%, W: less than 2.5%, N:
A billet containing one or more of 0.007 to 0.080%, the balance consisting of Fe and unavoidable impurities is heated, hot-pierced, and hot-rolled to produce a seamless steel pipe. In the method, after the perforation, finish rolling at a cross-sectional compression ratio of 40% or more is performed at a finishing temperature of 900 ° C. or more, and then immediately reheated to a temperature of 950 to 1100 ° C. to continue the normalizing process. Done online, after air cooling,
The tempering process is performed at a temperature of 700 to 850 ° C. As described above, a large working strain with a cross-sectional compression ratio of 40% or more is given at a finishing temperature of 900 ° C. or more, and further, after finishing rolling, the material is reheated to a temperature of 950 to 1100 ° C. to continuously perform normalizing processing online. And after air cooling, 700 ~
By performing the tempering treatment at a temperature of 850 ° C., sized recrystallized grains can be obtained, and the same mechanical performance as that obtained by ordinary off-line heat treatment can be obtained.

In the present invention, a cross-punching machine having a cone-shaped roll is used in the punching step, and a crossing angle of 5 to 35 ° (an angle formed by the axis of the roll with respect to a horizontal plane or a vertical plane of the pass line). It is perforated. As described above, by using a cross-drilling machine and piercing at an intersection angle of 5 to 35 °, it is possible to form a thin-walled pipe, and the obtained hollow shell has a wall thickness of a conventional barrel-type roll. It can be made thinner than when drilling with a machine. Therefore, it is easy to perform strong working in the finish rolling in which both the subsequent elongation rolling and the finishing work are integrated. Therefore, even without performing a reheating treatment to reduce the deformation resistance of the hollow shell,
Strong processing with a cross-sectional compression ratio of 40% or more is possible at a finishing temperature in a relatively low temperature range of 1050 ° C. or less.

[0012]

BEST MODE FOR CARRYING OUT THE INVENTION In the finish rolling according to the present invention, if the finishing is continuously performed before the work strain imparted by the elongation rolling after piercing is recovered, both the elongation rolling and the finishing are integrated. Things. In order to perform finishing before the processing strain imparted in the elongation rolling after perforation is recovered, for example, a mandrel mill as a stretching rolling machine and a sizer as a finishing machine are conventionally arranged independently. What is necessary is just to form an integrated continuous arrangement.

The reasons for limiting the chemical composition of steel and the production conditions in the present invention are as follows. In addition,
All percentages thereafter indicate mass% unless otherwise specified. C is an element that is essential for securing room-temperature strength and high-temperature strength by solid solution hardening and carbide formation / precipitation.
If it is less than 0.25%, the effect is not sufficient, and if it exceeds 0.25%, the quenchability increases and the strength increases, but the weldability and workability deteriorate.

Si is an element necessary for deoxidizing steel,
If the content exceeds 1.0%, the toughness of the steel is reduced.
Less than.

Mn is necessary for deoxidation and desulfurization of steel and is a useful element for improving the strength and hot workability of steel and obtaining an appropriate structure. The effect is not sufficient, and in the case of high-strength Cr-Mo steel, 1.0%
If it exceeds, the strength of the steel increases, but the weldability and workability deteriorate, so the content was set to 0.2 to 1.0%.

Cr is an element added to improve the high-temperature oxidation resistance and long-time high-temperature strength of steel. If it is less than 1.0%, its effect cannot be sufficiently exerted. If it exceeds, the weldability and workability are reduced, so that
2.5%.

Mo significantly enhances the high-temperature long-term strength of steel,
In addition to strengthening by solid solution in steel, it is an element that improves the creep strength by precipitating carbides. If less than 0.2%, its effect is not sufficient, and if it exceeds 2.5%, its effect is increased. Is saturated, and the workability during pipe production is reduced due to segregation or the like.

[0018] Ni is added to adjust the component balance because high delta ferrite increases in high Cr-Mo steels and causes flaws during hot pipe making. However, since it is an expensive element, Ni is expensive. Undesirably, it was less than 1.0%.

Nb is an element that forms carbides or carbonitrides and improves the high-temperature strength. However, if it exceeds 0.2%, it becomes coarse carbides, lowering creep strength and lowering weldability and toughness. It was less than 0.2%.

V is an element that forms carbides and improves the high-temperature strength. However, if it exceeds 1.0%, it becomes coarse carbides, which lowers creep strength and lowers weldability and toughness. Less than.

W is an element which, like Mo, forms a solid solution in steel and strengthens it, and also precipitates carbides to improve high-temperature strength. However, if it exceeds 2.5%, the amount of delta ferrite increases, and Not only is it a cause of flaws during pipe making, it is an expensive element, so it is not preferable in terms of economic efficiency.
%.

N is present in steel as an unavoidable impurity,
On the other hand, the toughness is lowered, but on the other hand,
It is an element that forms a nitride in coexistence with b and improves the high-temperature strength. However, if it is less than 0.007%, its effect is not sufficient, and if it exceeds 0.080%, the weldability is deteriorated. 0.007 to 0.080%.

The heating temperature of the billet of the material in the present invention is not particularly limited as long as it is a temperature at which hot drilling can be performed by a drilling machine, but the optimum temperature depends on the material and is determined in consideration of high-temperature ductility and high-temperature strength. Suffice, but usually 1
It is in the range of 100 to 1300 ° C.

In the perforating step of the present invention, the expanded thin wall perforation is made possible, and the thickness of the obtained hollow shell is made thinner than the case where the wall is rolled using a conventional barrel-type roll inclined-type drilling machine. In order to facilitate strong working in finish rolling that combines both elongation rolling and finishing, a cross-piercing machine with a cone-shaped roll is used. If the crossing angle of the crossing drilling machine in this case is less than 5 °, it is difficult to obtain a desired thin hollow shell, and it is difficult to perform strong working in the subsequent finishing rolling,
If it exceeds 35 °, the rear end of the hollow shell will not come off from the drilling machine, so-called “butt clogging” will occur, and finish rolling will be unstable.

The finish rolling in the present invention is a process for producing a seamless steel pipe having a desired shape and dimensions by subjecting a hollow shell drilled by a punch to elongation rolling and finishing to produce a seamless steel pipe having a desired shape and dimensions. In comparison, since the processing is performed in a relatively low temperature range, it is an important step for imparting effective processing when considering the processing heat treatment. In particular, if finish processing is performed before the processing strain imparted by elongation rolling after piercing is recovered, the sum of the two processing rates is the so-called finish rolling processing rate, so the hollow shell is cut in the middle. A large working ratio can be provided by finish rolling without reheating.

In the present invention, the cross-sectional compression ratio is 4
It is necessary to perform finish rolling of 0% or more at a finishing temperature of 900 ° C. or more. If the cross-sectional compression ratio is less than 40%, the recrystallization does not proceed smoothly, the micronization effect cannot be obtained, and sometimes the crystal grains sometimes grow abnormally. . The upper limit of the processing rate of finish rolling cannot be specified because it differs depending on the material of the pipe and the capacity of the mill. However, if the processing rate is too large, flaws are likely to occur. preferable.

The finishing temperature of the finish rolling cannot be specified because it varies depending on the material of the steel pipe and the capacity of the mill. However, if the temperature is too low, the deformation resistance of the hollow shell becomes large and the cross-sectional compression ratio is 40% or more. And it becomes difficult to apply, and the energy consumption for reheating performed to obtain a desired microstructure after finish rolling becomes large,
The temperature was set to 900 ° C. or higher.

In order to perform the finishing before the processing strain imparted by the elongation rolling after the perforation is recovered, the elongating rolling mill and the finishing machine, which have been conventionally arranged independently, are replaced with an integrated continuous arrangement. do it. If the elongating mill and the finishing machine are formed as an integrated continuous arrangement, it is possible to stably secure a finishing rolling rate of 40% or more for all sizes in the manufacturing range.

The major feature of the present invention is that a recrystallization treatment (normalization) is performed between finish rolling and heat treatment, whereby recrystallization is induced by a combination of processing and heating, and the recrystallization is induced. Grain sizing can be achieved. The present invention does not require processing after reheating, unlike the conventional technique of reheating between elongation rolling during rolling and finish processing, so the reheating temperature is set to the lowest temperature at which recrystallization proceeds. Thus, a recrystallized grain which has been sized can be obtained. In addition, the recrystallization treatment between the finish rolling and the high-temperature transformation heat treatment guarantees the uniformity in the longitudinal direction (rolling direction) of the seamless steel pipe and between lots, and the secondary effect that the performance variation is significantly reduced. Can also be obtained.

The reheating temperature in the present invention is 950 ° C. in the case of the Cr—Mo steel as the object of the present invention.
If it is less than 1, the recrystallized grains are fine grains and the high-temperature creep properties are reduced, and it takes a long time to recrystallize, and the pipe-making efficiency is extremely reduced. Since it grows and becomes coarse, it becomes 950
~ 1100 ° C.

In the case of off-line normalizing, which is a conventional process, the temperature is raised from room temperature, and the residence time in the heating furnace is prolonged. Since the growth is large and the steps such as pickling and shot for removing the scale from the product are required, the economy is poor. According to the present invention, the finished steel pipe can be soaked in the reheating furnace as it is, so that the furnace time can be suppressed to less than 30 minutes, and the problem of the conventional process can be solved and the economy is excellent.

[0032] tempering temperature in the present invention, that carbides are finely dispersed, it is necessary in view of the high temperature creep performance, Ac Ac ₁ transformation point -50 ° C. or preferably less than ₁ transformation point, this In the component range described in claim 1 of the invention, since the Ac ₁ transformation point is different, 700
~ 850 ° C. The holding time is usually 60
Need more than a minute.

[0033]

EXAMPLE Steels A to C having the composition shown in Table 1 were melted by a conventional method, and a billet having an outer diameter of 187 mm obtained by slab rolling was used as a raw material. As shown in the schematic process diagram of FIG. After heating to 0 ° C., pipe making (perforation and finish rolling), reheating and normalizing were performed under the test conditions shown in Table 2, and further tempering was performed to obtain a seamless outer diameter of 150 mm and a wall thickness of 8.5 mm. Steel pipe was manufactured. A test specimen was cut out from each of the obtained seamless steel pipes, and subjected to a tensile test at room temperature and at 550 ° C., and an austenite grain size and toughness (flat test) were measured. Table 3 shows the results. The tensile test was performed in accordance with the metal material tensile test method specified in JIS Z2241 and the high-temperature tensile test method of steel materials and heat-resistant alloys specified in JIS G0567.
Measured according to the austenitic grain size test method for steel specified in 1 above. When flattened, the tube wall was checked to see if any flaws or cracks had occurred.Flaws and cracks did not occur. ○, fine cracks occurred. △, and cracks occurred as ×. displayed. H = {(1 + e) t} / (e + t / D) where
H: distance between flat plates (mm), t = thickness of tube (mm),
D: Outer diameter (mm) of pipe, e: Constant 0.08

[0034]

[Table 1]

[0035]

[Table 2]

[0036]

[Table 3]

Test No. under the test conditions shown in Table 2 1-5
Is a manufacturing process satisfying the conditions of the present invention. 6
No. to No. 11, the production process of Comparative Example in which the mark * in Table 2 was out of the range of the condition of the present invention. 12-15
These are typical heat treatment conditions applied to the production of conventional Cr-Mo steel tubes for boilers. As shown in Table 3, Test No. 3 satisfying the conditions of the present invention. Test Nos. 1 to 5 of the conventional method were all used. Test No. 1 has an olestenite particle size of 4 to 6 equivalent to that of No. 12 to 15 and has flat performance. Comparative Examples 6 to 11 are all test Nos. Performance equivalent to 11 to 13 cannot be secured.

[0038]

According to the present invention, the present invention relates to
By specifying the elongation rolling and finish processing and heat treatment conditions that are finish rolling, energy savings can be achieved by simplifying the manufacturing process and improving pipe production efficiency. An excellent Cr-Mo seamless steel pipe can be manufactured.

[Brief description of the drawings]

FIG. 1 shows test Nos. In Examples. It is a schematic process drawing of the test conditions of 1-11.

FIG. 2 is a schematic process diagram of a manufacturing process of a conventional general Cr—Mo-based seamless steel pipe.

──────────────────────────────────────────────────続 き Continuation of the front page (51) Int.Cl. ⁷ Identification code FI C22C 38/00 302 C22C 38/00 302Z 38/48 38/48 (56) References JP-A-6-10041 (JP, A) JP-A-4-80350 (JP, A) JP-A-61-238917 (JP, A) JP-B-7-35547 (JP, B2) (58) Fields investigated (Int. Cl. ⁷ , DB name) C21D 8/00-8/10 C22C 38/00-38/60 B21B 3/02 B21B 17/02

Claims (2)

(57) [Claims] 1. A mass% of C: 0.04 to 0.25%,
Si: less than 1.0%, Mn: 0.2 to 1.0%, Cr:
1.0 to 12.5%, Mo: 0.2 to 2.5% , Nb:
Less than 0.2%, V: contains less than 1.0% , and Ni :
Less than 1.0%, W: less than 2.5%, N: 0.007 to
A method for producing a seamless steel pipe by heating and hot-piercing and hot-rolling a billet containing one or more kinds of 0.080% and the balance of Fe and unavoidable impurities, Subsequent to the perforation, finish rolling at a cross-sectional compression ratio of 40% or more is performed at a finishing temperature of 900 ° C. or more, and then immediately reheating to a temperature of 950 to 1100 ° C. and normalizing is continuously performed online. After air cooling, 700-85
A method for producing a Cr-Mo based seamless steel pipe having excellent high-temperature strength, wherein a tempering treatment is performed at a temperature of 0 ° C. 2. The method according to claim 1, wherein the drilling is performed at a crossing angle of 5 ° using a crossing drilling machine.
The method for producing a Cr-Mo series seamless steel pipe having excellent high-temperature strength according to claim 1, wherein the method is performed at 35 °.