JPS58224155A - Seamless two-phase stainless steel pipe and its manufacture - Google Patents

Abstract

PURPOSE:To obtain a material for a seamless two-phase stainless steel pipe manufactured by a plug mill system by adding specified amounts of C, Si, Mn, Cr, Ni, Cu, Mo, N, Al, si and P to a steel and by further adding specified amounts of one or more among a rare earth element, Ca and B. CONSTITUTION:The composition of this two-phase stainless steel is composed of, by weight, <0.03% C, <2% Si, <2% Mn, 20-30% Cr, 1-9% Ni, <3% Cu, 0.5-5% Mo, 0.05-3% N, 0.01-0.1% Al, <0.004% S, <0.03% P, one or more among (4-20)X[%S] rare earth element, (1-10)X[%S] Ca and 0.0005-0.01% B, and the balance Fe with inevitable impurities.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a two-phase seamless stainless steel pipe and a method for manufacturing the same, and more particularly to a method for manufacturing a two-phase seamless stainless steel pipe with high productivity and low cost.

Duplex stainless steel represented by JIS 5US329J1 has excellent corrosion resistance, stress corrosion cracking resistance, needle groove corrosion resistance, and good weldability, so it is used for various chemical plant piping, oil wells, pipes, geothermal well pipes, and lines. It is attracting attention as a material for pipes, etc.

Seamless stainless steel pipes are generally made using the plug mill method.
It is manufactured using the mandrel method, the pilger mill method, the Nising-Sejourne method, the Erhardt-Butz-pench method, etc.

Since duplex stainless steel has a ferrite-austenite two-phase structure, its hot workability is poor.
In order to manufacture seamless steel pipes of these steel types that have poor hot workability, hot extrusion methods such as the Nigin-Sejourne method have been used.

However, the direct drilling method using hot extrusion method.

When drilling holes, if the length of the billet becomes 5 to 7 times the diameter, the uneven thickness will be large, making it difficult to manufacture long pipes.
In order to completely solve this problem, long tubes are manufactured using the so-called expansion method, in which a hole is made in advance in the center of the billet by machining and then expanded by pressing. However, even in this method, the length of the billet is limited to about 15 times the diameter.

Furthermore, since the Nijing-Sejourne method uses a glass lubricant, there is a step of peeling off the glass lubricant after rolling, which is not easy.

An object of the present invention is to solve the problems of the prior art described above and to provide a long seamless duplex stainless steel pipe with high productivity and a method for manufacturing the same.

The above objects of the present invention are achieved by the following three inventions.

The gist of the first invention is as follows. That is, in a weight ratio of 1, c:o, o is 3% or less.

5l12.00% or less, Mn+2゜00% or less,
Cr:20.0~30.0%, Ni+1.0~9
．． 0%, Cu! 4 3.0% or less, Mo Io, 5
~5.0%, N10.05~0.30%, At! 0.0
1 to 0.10%, SIo, 004% or less, PIo, 03
Contains 0% or less, and further contains rare earth elements: (4 to 20)
×[%S], Cut (1-10)X[%S], B:01
This is a 92-phase seamless stainless steel pipe manufactured by a plug mill method characterized by containing one or more selected from 0.0005 to 0.010%, with the remainder consisting of Fe and unavoidable impurities.

The gist of the second invention is as follows. That is, a piercing process using a piercing mill, an inclined rolling process using an inclined rolling mill, a rolling process using a plug mill, a tube polishing process using a reeler, and an outer diameter reducing process using a sizer for duplex stainless steel having the same composition as the first invention. In the method for manufacturing a two-phase seamless stainless steel pipe using a plug mill method, the hole is punched at a temperature f at the center of the billet in the punching step of 1200 to 1350°C, and a temperature of 11100 to 1350°C at the outer surface of the billet, In the inclined rolling process, rolling is completed at a temperature of 1100°C or higher, and in the plug mill rolling process, tube polishing process, and outer diameter reduction process, the equivalent strain expressed by the following formula (1) is reduced to 0.
．． 5, 0.4, 0.2 or less, and the plug mill rolling process, the tube polishing process, and the total equivalent strain expressed by the following C) formula are 0.6 or less. This is a method for manufacturing stainless steel jointed steel pipes.

ft-ta L t (= t"'/'.

ε = tn p/b.

tol　Do　’Average plate thickness before rolling in each process.

Average outer diameter t, p! Average sheet thickness and average outer diameter after rolling in each process, however, ε't = tn' p/lpo+ Zn t
R/l.

t', = An D, A),. + tn I) II/
I) II.

11+ Average plate thickness after rolling in plug mill, P before rolling, PQ after rolling in tit reeler, trap before rolling, mouse 0 Average plate thickness D D = After rolling in plug mill, ° Before rolling P,
P.

Average outer diameter D D : Flat t after rolling in the reeler and before rolling,
R6 Average outer diameter The gist of the third invention is as follows. That is, in the method for manufacturing a seamless two-phase stainless steel pipe using a plug mill method using two-phase stainless steel having the same components as in the first invention, the temperature at the center of the billet in the drilling step is 112
00-1350℃.

The holes are perforated at the billet outer surface temperature t of 1100 to 1350°C, the rolling is finished at a temperature of 1100°C or higher in the inclined rolling process, and the equivalent strain expressed by the above formula (1) is reduced to 0 in the plug mill rolling process and the polishing process. .5,
Processed at 0.4 or less, 850 to 1200 after the polishing process
A two-phase stainless steel joint characterized in that it is reheated at a temperature of °C for 10 minutes or less, and after said reheating, in the outer diameter reduction step, it is processed at an equivalent strain to of 4 or less as expressed by the above formula (1). This is a method for manufacturing steel pipes.

The present inventors investigated the chemical composition and rolling conditions in order to manufacture duplex seamless stainless steel pipes using the plug mill method, which has various advantages. They discovered that it is possible to manufacture two-phase stainless steel jointed steel pipes without cracking or pipe end cracks using the plug mill method.

The plug mill method is a piercing mill.

(also referred to as the first piercer) - Incline rolling mill (also referred to as the elongator 1 second piercer) - Plug mill - Reeler (also referred to as the polishing machine) - A method of manufacturing in each process of the sizer. However, the productivity and yield are higher than the Nijin-Sejourne method, and the cost is lower.

Next, the reason for limiting the chemical composition of the duplex stainless steel of the present invention will be explained.

C: C is an element that is inevitably included in steel.

If it exceeds 0.03%, corrosion resistance and intergranular corrosion resistance will deteriorate, so it is limited to 0.03% or less.

S is an element used as a deoxidizing agent during melting, but if it exceeds 2.0%, σ phase will occur and cold workability will be significantly deteriorated, so it was limited to 2.0% or less.

Mn: Mn is added to increase strength, but if it exceeds 2.0%, hot workability will be completely degraded, so it is limited to 2.0% or less.

Cr + Cr is an essential element for increasing corrosion resistance and forming a two-phase structure of austenite and ferrite, and has a resistance to pitting corrosion and crevice corrosion of 20.
If it is less than 0%, the effect is small.

On the other hand, as the Cr content increases, the pitting corrosion resistance improves, but if it exceeds 30.0%, the σ phase tends to precipitate and the toughness deteriorates, so the range of Cr is set to 20.0 to 30.0%. Ni is an essential element in terms of increasing resistance to general corrosion and forming a two-phase structure, but if it is less than 1.0%, sufficient corrosion resistance cannot be obtained, and if it exceeds 9.0%. Since the improvement effect is saturated and it is expensive, it is 1.0 to 9. ()% range.

Cu: Although Cu improves corrosion resistance to non-oxidizing acids.

If it exceeds 3.0%, hot workability deteriorates, so 3.0%
% or less.

MO! Mo Ire is an element that improves resistance to localized corrosion that occurs in corrosive environments containing chlorine ions.

If it is less than 0.5%, sufficient tetraphagy cannot be obtained;
Even if it is added in excess of 0.5%, the improvement effect is small and it is also very expensive, so it is limited to a range of 0.5 to 5.0%.

N+ is an important element for forming the N1d2 phase structure and improves corrosion resistance, but if it is less than 0.05%, it will not improve pitting corrosion resistance, while if it exceeds 0.30%, it will significantly impair hot workability. Since it causes deterioration, it is limited to a range of 0.05 to 0.30%.

t j AtdIt has the effect of reducing oxygen that deteriorates the hot workability of the steel type according to the present invention, but if it is less than 0.01%, the oxygen in the steel cannot be reduced to improve the hot workability. On the other hand, if it exceeds 0.10%, surface defects due to alumina clusters will increase, so 0.01 to 0.10%
limited to the range of

S: S is contained in steel as an unavoidable impurity and deteriorates the hot workability of the steel type according to the present invention. Its adverse effects are particularly noticeable during pipe making using the plug mill method, and its content exceeds 0.004%.

Since it is difficult to make pipes without causing defects even when adding Ca, a rare earth element (hereinafter referred to as REM) that is effective in fixing sulfides, it was limited to 0.004% or less.

P: P is also contained as an unavoidable impurity, but 0.030
If it exceeds 0.030%, the hot workability deteriorates and it becomes difficult to manufacture pipes without defects using the plug mill method, so the content was limited to 0.030% or less.

The above c, si%Mn, Cr, Ni,
Cu, Mo, N.

The basic components of the duplex stainless steel of the present invention include limited amounts of At, S, and P, and REM% Ca.

In order to improve hot workability, it is necessary to contain one type or two or more types of B at the same time within the following limited amount range. The reasons for these limitations are as follows.

REV, Cat REV and Ca are both strong sulfide-forming elements;
REM is an effective element for reducing the amount of solid solution S in steel and improving hot workability by forming sulfides.
<4X (%S), Ca((%S), the effect is small, while REM>20X[%s), Ca>xox(
%S], the effect is saturated and there is a risk of an increase in surface defects due to these oxides or sulfides, so RE
M: (4 ~ 20) × [%S], Cat (i ~ 1
0) x [%S].

4 B: B improves hot workability by adding a small amount of B, but 0.
If it is less than 0.005%, it has no effect (addition of more than 0.010% will conversely deteriorate hot workability, so
It was limited to a range of 0.05% to 0.010%.

Next, the reasons for limiting the conditions for making a pipe using the plug mill method for duplex stainless steel having the above-mentioned components will be explained.

First, the temperature at the center of the billet in the perforation step was limited because flaws are likely to occur in the center due to the Mannesmann effect even in the limited components of the present invention.

Although a temperature of 1,200°C or higher is required, since flaws will occur even if the temperature exceeds 1,350°C, the temperature is limited to 1,200 to 1,350°C. Next, the central part of the outer surface is subject to severe surface treatment, but flaws occur at temperatures below 1100'C;
Since scratches will occur even if the temperature exceeds 1,100 to 1,350°C, the temperature is limited to nine.

Next, the reason why rolling is finished at 1100° C. or higher in the inclined rolling step will be explained. 10 at the beginning of research
When inclined rolling is performed at 00C, busy cracks occur and the percentage decreases to 1080%.
No cracks occurred when inclined rolling was carried out at °C, but cracks occurred during plug mill rolling in the next step, despite the small rolling reduction.

From this fact, it is thought that the processing strain during inclined rolling has a large effect on the rolling in the plug mill.

After tilt rolling, reheating may be used to eliminate processing strain, but this is hardly a good idea in these days when energy conservation is being talked about.

The present inventors conducted experiments and research to find a method for rolling without reheating. That is, a simulation experiment was conducted in the laboratory using a Greeple high-temperature, high-speed tensile tester.

A commonly used method is to define i/'i, equivalent stress, and equivalent strain in cold working, and calculate the stress state during various working processes from a uniaxial strain hardening curve.

In hot working, since recrystallization occurs during processing, the concept of equivalent strain is applied to quantitatively evaluate the processing state.

The details of the experimental method will be explained below. Usually, the equivalent strain expressed by equation (1) during inclined rolling is about 0.7. Therefore, the prestrain tn A/A in the Grieple test
Q (However, A, !Cross-sectional area of the specimen before the test.

The cross-sectional area of the specimen after At prestraining was also set to 0.7.

That is, as in the process shown in FIG. 1 showing the processing schedule, the specimen is heated and held at 250° C., then cooled to a predetermined temperature T, held for %t0 seconds, and pre-strained W.

was then cooled, and after reaching a predetermined temperature T, it was held for 17 seconds, and a tensile test Wt was conducted.

An example of the results is shown in FIG. The first one below
Steel having the chemical composition shown in the table was used, and the chemical composition was shown as shown in Table 2 below.

Table 1, Table 2, Figure 2 shows that when pre-strain is applied at a temperature below 1050°C and the tension is applied at a temperature below that, the elongation at break is tnA/A' (where A/ is the cross-sectional area after pre-strain, A: after rupture). cross-sectional area)
It can be seen that the deformability expressed by is significantly lower than when no pre-strain is applied. Even if the material is pre-strained at a temperature of 1100° C. or higher and stretched at a temperature lower than that, the elongation at break is approximately the same as that obtained when the material is pre-strained. Namely.

It can be seen that pre-straining at a temperature of 1100° C. or higher has a significant effect on processing in the 5th step. If inclined rolling is performed at a temperature below 1100°C, the effect will be on plug mills, reelers, etc.
, the temperature remains until the sizer, and the temperature becomes low, making it even more difficult to roll. For this reason, the temperature of inclined rolling was limited to 1100°C or higher. Moreover, from this test result, it can be seen that if the temperature of the hole is 1100° C. or higher, the processing strain at the time of the hole does not affect the inclined rolling.

-Next, the reason why the total equivalent strain of Plugmi is set to be less than 0.5 is that if it exceeds 0.5fi7, cracks will occur in the plug mill, or cracks will occur in the plug mill. Even if a small amount of rolling is performed, flaws will occur. For this reason, the equivalent strain 'i 0 in the plug mill
．． It was limited to 5 or less. If cracks do not occur in the plug mill but do occur in the reeler, this problem can be solved by reheating the product after rolling in the plug mill, but this requires energy for heating, which leads to higher costs. If the equivalent strain is limited to 0.5 or less as in the present invention, there is no need for reheating.

Therefore, pipes can be manufactured at low cost.

Next, the reason why the equivalent strain in the reeler is limited to 0.4 or less will be explained. In order to manufacture pipes with dimensions that meet customer requirements, the plug mill process cannot be omitted. In the present invention, the temperature tl during inclined rolling is
Although the rolling temperature is limited to 10° C. or higher, in a plug mill, rolling must be performed at a temperature lower than that. As is clear from FIG. 2, processing at temperatures below 1100°C has a significant effect on the processing in the next step. to plug mill
Since the amount of processing in step 7 has a significant effect on the rolling of the reeler, the amount of processing in the reeler must be determined by taking into consideration the amount of processing in the plug mill. If a plug mill is used to process the amount necessary to manufacture a pipe with dimensions that meet customer requirements, there is a risk that flaws will occur in the reeler. As a result of experimental research, the present inventors found that cracks occur when rolling with a reeler exceeds the equivalent strain io, 4y.
Limited to 4 or less.

In the second invention, the total strain 'i in the plug mill and reeler is set to 0.6 or less, and the equivalent strain in the sizer is set to 0.2 or less, because if the total strain in the plug mill and reeler exceeds 0.6, Even if no flaws occur on the reeler, flaws occur when processing the sizer. Further, even if the total strain is 0.6 or less, if the equivalent strain in the sizer is rolled in excess of 0.2, flaws will occur in the sizer. Therefore, the total strain 't: o in the plug mill and reeler was set to 6 or less, and the equivalent strain f in the sizer was limited to 0.2 or less.

Next, the amount of reheating and sizer processing in the third invention will be explained. When rolled at a temperature of 850° C. or lower in a sizer, elongation phases tend to occur, making processing in a sizer difficult. Even if the temperature is 850° C. or higher, heating for more than 10 minutes tends to generate a σ phase, making processing with a sizer difficult. There is no need to heat it to a high temperature exceeding 1200°C, and lower temperatures are preferable from the standpoint of energy conservation. Furthermore, if the material is rolled with an equivalent strain exceeding 0.4 in the sizer after reheating, buckling occurs in the circumferential direction. For these reasons, heating was carried out at 850.degree. to 1200.degree. C. for 10 minutes or less after the tube polishing step, and processing was limited so that the equivalent strain in the sizer was 0.4 or less.

Example Pipe production was performed using the chemical composition and manufacturing conditions shown in Table 3, and the results are also shown in Table 3. Note that items that do not satisfy all of the conditions of the present invention are underlined. In sample material A1, which is a comparative example, cracks occurred because R, EM% Ca, and B were not added. In Comparative Examples A2, 3, 4, 5, 6, and 7, cracks occurred because REM, Ca, and B were outside the lower or upper limits of the present invention. Comparative example A8.9,10keS
Cracking occurred because the value exceeded the upper limit. Comparative examples All
Cracks occurred because P exceeded the upper limit. Comparative example A
I 2% 13.14 is due to cracking due to the temperature at the center of the outer surface of the billet being outside the lower or upper limit of the present invention during drilling. In Comparative Examples A15 and A16, cracks occurred during inclined rolling because the tube temperature was below the lower limit of the present invention. In Comparative Examples Al 7, 18, and 19°20, cracks occurred because the equivalent strain in each step exceeded the upper limit of the present invention. In Comparative Example A21, the temperature during reheating was lower than the lower limit, and in Comparative Example A22, the time exceeded the upper limit, so cracks occurred. Comparative Example A23 buckled because the equivalent strain in the Xizer exceeded the upper limit.

On the other hand, inventive examples A; Nos. 24 to 32, which satisfied all the conditions of the present invention, were able to be formed into pipes without cracking in each step.

As is clear from the above embodiments, the present invention limits the components of the duplex stainless steel, limits the billet temperature in the piercing process and the inclined rolling process, and makes the billet temperature suitable for the plug mill rolling process, the tube polishing process, and the outer diameter reduction process. limit strain,
By limiting the reheating temperature if necessary, we can produce two-phase seamless stainless steel pipes with excellent productivity and yield! ll
I was able to create J.

[Brief explanation of the drawing]

Figure 1 is a process diagram showing the specimen processing schedule for the Greeple test in the present invention, and Figure 2 is a diagram showing the specimen processing schedule for the Greeple test in the present invention. FIG. 2 is a diagram showing the relationship between tensile temperature and pre-strain deformability. Agent Takeo Nakaji Figure 1 4 1 hour (sec) Figure 2 Prefecture Onsho (0C)

Claims (1)

[Claims] (1) Weight ratio x e: o, o 3% or less, Si: 2
．． 00% or less, Mn + 2.00% or less, Cr
: 20.0~30.0%, Ni+1.0~9.0
%, Cut 3.0% or less, M. 10.5-5.0%, N! 0.05-0.30%
, At+0.01~0.10%, s:o, oo4% or less, P:0.030% or less, and further contains rare earth elements: (4~20)X(%S:], Ca: (1 ~10
)
and unavoidable impurities. (2) Weight ratio [C: 0.03% or less, Si: 2.00
% or less, Mn + 2.00% or less, Cr: 20.
0-30.0%, Ni 11.0-9.0%, Cu:
3.0% or less, M. 20, 5-5.0%, N: 0.05-0.30%
, At+0.01 to 0.10%, 10.004% or less,
Contains P+0.030% or less, and rare earth elements:
(4-20) x [%S], Cat (1-10) x [%S
), BIo, 0005~o, oio% 1 [Maf! , is a duplex stainless steel containing 281 or more, with the remainder being Fe and unavoidable impurities.The process involves a piercing process using a piercing mill, an inclined rolling process using an inclined rolling mill, a rolling process using a plug mill, and a polishing process using a reeler. In the method for manufacturing a two-phase stainless steel joint or pipe using a plug mill method, which comprises a step of reducing the outer diameter using a sizer, a temperature e of the central part of the billet in the drilling step is 1200 to 1350°C, and a temperature of the outer surface part of the billet. is perforated at 1100 to 1350°C, the rolling is finished at a temperature of 1100°C or higher in the inclined rolling process, and the equivalent strain shown by the following formula (1) is obtained in the plug mill rolling process, tube polishing process, and outer diameter reduction process. 0.5.0 each
．． 4. Processed with 0.2 or less. And in the plug mill rolling process and polishing process, the following (
2) Is it possible to process the total equivalent strain 'i shown in the formula below 0.6? [Method for manufacturing dual-phase stainless steel jointed steel pipes. However, εt=Znt/16 ε, = tn D/D. tol DO' Average plate thickness before rolling in each process. Average outer diameter t, Dj Average plate thickness after rolling in each process. Average outer radius t't=Z"'p/'p0+ Zn tt/
l*. tt, = tn D, /D, 0 + in D
*/”*. t, t, Average plate thickness before and after rolling in the 01 plug mill, 4 0, i, +lJ-, -EoI,, 6E., Mach. Average plate thickness, D, j Average outer diameter DIIDl after rolling in the plug mill and before rolling: Average O outer diameter after rolling and before rolling in the reeler (3) Weight ratio 1c-rC: o, oa% or less, 84:
2.00% or less, Mn: 2.00% or less, Cr:
20.0-30.0%, Ni: 1.0-9.0%
, Cu+3. Q% or less, M. : 0.5-5.0%, N: 0.05-0.30%% A
t+0.01~0.10%, s+o, oo4% or less, P
: Contains 0.030% or less, and further contains rare earth elements = (
4-20)×[%S:], Ca: (1-10)×[%
S), a piercing process using a piercing mill and an inclined rolling mill for duplex stainless steel containing one or more selected from B + 0.0005 to 0.010% and the remainder consisting of Fe and unavoidable impurities. Incline rolling process;
A method for manufacturing a two-phase seamless stainless steel pipe using a plug mill method, which comprises a rolling process using a plug mill, a polishing process using a reeler, and an outer diameter reducing process using a sizer, wherein: The temperature of the billet part is 1200 to 1350'c, the temperature of the outer surface of the billet is 4iiioo to 1°350°C, the rolling is finished at a temperature of 1100°C or higher in the inclined rolling process, and the plug mill rolling process and the tube polishing process are completed. The equivalent strain shown by the following α) formula is 0°5 and 0.4, respectively.
After the polishing process, the tube is reheated at a temperature of 850 to 1200°C for 10 minutes or less, and after the reheating process, the equivalent strain expressed by the formula (1) is 0.
．． A method for producing a two-phase stainless steel jointed steel pipe, characterized in that it is processed at 4 or less. However, t, = An t/l. tB = Zn D/D. +6 + DO' Average plate thickness before rolling in each process, average outer diameter t, D? Average sheet thickness and average outside diameter after rolling in each process