JPH01159321A - Finish rolling method for austenitic stainless seamless steel pipe - Google Patents

Abstract

PURPOSE:To produce a seamless steel pipe having excellent intergranular corrosion resistance with a simple state by subjecting a stock pipe for rolling to finish rolling at a specific temp., then cooling the pipe under specific conditions at the time of producing the austenitic stainless seamless steel pipe. CONSTITUTION:The stock pipe for finish rolling made of an austenitic stainless steel contg., by weight %, <0.08% C, <1.0% Si, <2.0% Mn, 16-26% Cr, and <0.3% N is heated to a 1050-1200%oC range. The stock pipe for finish rolling is then subjected to the finish rolling under the conditions under which the finish temp., i.e., the finished pipe temp. T( deg.C) on the outlet side of a mandrel mill satisfies equation II according to the working strain epsilon at the time of the finish rolling expressed by equation I when the sectional area of the stock pipe for finish rolling is designated as Ao(cm<2>) and the sectional area of the finished pipe as A(cm<2>). The pipe is in succession cooled under the conditions under which the average cooling rate V( deg.C/sec) satisfies V>=C<3>X10<4> (C; the carbon content expressed by weight %) in a 900-500 deg.C region. The seamless steel pipe made of the austenitic stainless steel which has uniformly fine crystal grains and has the extremely low corrosiveness at the crystal grain boundaries is produced even if a heat treatment stage for solutionization is omitted.

Description

Detailed Description of the Invention (Field of Industrial Application) The present invention relates to a method for finish rolling a seamless austenitic stainless steel pipe, and in particular, simplifies the manufacturing process without deteriorating the product characteristics of the seamless stainless steel pipe. This is an attempt to make the world a better place.

(Prior art) Seamless steel pipes are generally manufactured by rolling methods such as the mandrel mill method and plug mill method, or hot extrusion methods such as the Eugene Sejourne method and the Erhard-Busch bench method. Mandrel mill rolling is widely used for pipe manufacturing because of its excellent productivity and dimensional accuracy.

Conventionally, the specific process for producing seamless austenitic stainless steel pipes using the mandrel mill method is to heat a billet of material for seamless steel pipes to a predetermined temperature in a rotary hearth heating furnace, and then use a mandrel mill. First, a hollow tube is produced by piercing and rolling with a piercer or by using a continuous casting machine or the like. Then, the obtained hollow shell tube is stretched to reduce its thickness using a mandrel mill, which is a stretching and rolling mill.

Here, a mandrel mill is a rolling mill that performs elongation rolling with a mandrel bar inserted into a hollow tube, and usually consists of 6 to 8 roll stands, each roll stand is equipped with a pair of grooved rolls, Adjacent roll stands are arranged such that the rotation axes of the grooved rolls are shifted from each other by 90 degrees in a plane perpendicular to the rolling axis. The hollow tube is stretched to a length 2 to 4 times its original length using a mandrel mill.
This becomes the raw tube for finish rolling. Next, the raw pipe for finish rolling is reheated to a predetermined temperature, usually between 900°C and 1000°C, in a reheating furnace as necessary, and then reduced and rolled in a finishing rolling mill, for example, a stretch reducer. The tube is then cooled down to room temperature on a cooling bed to become a finished tube. In a stretch reducer, the outer diameter of the raw pipe is reduced by up to 75%, and it is stretched to more than 40 times the length of the material billet, and the outer surface is as perfect a circle as the last few stands of the stretch reducer. Because it is shaped using grooved rolls, the finished tube has relatively excellent outer diameter dimensional accuracy. Thereafter, the finished tube is heated and maintained at 1010°C to 1150°C again in a heat treatment furnace for solution heat treatment, which is carried out for the purpose of adding corrosion resistance and predetermined mechanical properties, and then cooled to room temperature in a quenching tank. Ru. Finally, after passing through a straightening machine, surface scale is removed in a pickling tank (see Figure 2).

(Problems to be Solved by the Invention) By the way, it is essential for seamless austenitic stainless steel pipes manufactured according to conventional techniques to be subjected to solution heat treatment after finishing them to the desired outer diameter as described above. Therefore, not only does reheating require extra heat treatment costs, but even if solution heat treatment is performed, uniform and fine crystal grains are not necessarily obtained.

Simplification of the manufacturing process, in particular, even if the solution heat treatment process is omitted, it is possible to obtain seamless austenitic stainless steel pipes that are recrystallized, soft, have excellent intergranular corrosion resistance, and have uniform and finer grains. It is an object of this invention to propose a rolling method that can achieve this.

(Means for solving the problem) There is a technique disclosed in JP-A-55-107729 as an attempt to omit solution heat treatment in the production of austenant stainless steel sheets. The manufacturing process was completely different from that of seamless steel pipes, which have complex deformation behavior, so new ideas were needed. Therefore, the inventors reviewed the seamless steel pipe manufacturing process using the Mannesmann-Mandrel mill method in detail, and
After considering that the purpose of solid solution conversion of austenitic stainless steel is to soften recrystallization and to convert carbides into a solid solution, we found that it is possible to recycle austenitic stainless steel in the production of seamless austenitic stainless steel pipes using the Mannesmann-Mandrel mill method. The release of the strain added in the process before the heating furnace and the solid solution of precipitated carbides are completed during the heating of the raw pipe for finish rolling in the reheating furnace, and the re-release during rolling due to the processing strain during the subsequent finish rolling is completed. By making the crystal grains fine and uniform with crystals and cooling at a cooling rate that does not precipitate carbides, the finished tube can be recrystallized and softened in the as-rolled state without having to be heated again and subjected to solution heat treatment. It has become clear that seamless austenitic stainless steel pipes in which carbides have been sufficiently dissolved into solid solution can be obtained. This invention is based on the above knowledge.

That is, this invention has C: 0.08wt% or less (hereinafter simply expressed as %), Si: 1.0% or less, Mn:
2.0% or less, Cr: 16-26%, N: 0.3%
In manufacturing seamless austenitic stainless steel pipes containing the following, 10 pieces of raw pipe for finish rolling containing the above components are used.
After reheating to 50°C or higher and 1200°C or lower, ε= i, n (Ao/
A) The finishing temperature T is determined according to the processing strain ε during finish rolling expressed by the following formula, T≧220・exp(−2・
t) Perform finish rolling under conditions satisfying +900, and 900 to 500°C after this finish rolling.
The average cooling rate V ("C/S) in the temperature range is the following formula, ■≧
C' × 104, where C: carbon content i1 (wt%
This is a finish rolling method for seamless austenitic stainless steel pipe, which is characterized by cooling under conditions that satisfy the following conditions.

First, the reasons for limiting the composition of the seamless austenitic stainless steel pipe that is compatible with the present invention will be explained.

C: C is effective in stabilizing the austenite phase and increasing strength, but if it exceeds 0.08%, Cr carbides are likely to be formed, and in particular the cooling rate in the carbide precipitation region of 900 to 500°C is Since it is necessary to increase the amount of C, the content of C is limited to 0.08% or less.

Si: Si is usually added as a deoxidizing element, but addition of more than 1.0% reduces hot workability.
% or less.

Mn: Mn is added to deoxidize and improve hot workability, but 2
．． Addition of more than 0% impairs corrosion resistance, so Mn is
．． It was limited to 0% or less.

Cr: Cr is an essential element for maintaining the corrosion resistance of stainless steel, and in austenitic stainless steel, corrosion resistance to non-oxidizing acids such as sulfuric acid and hydrochloric acid is insufficient if it is less than 16.0%. However, if the addition exceeds 26.0%, the corrosion resistance tends to be saturated, but it is necessary to increase the amount of expensive Ni in order to maintain the austenitic structure, leading to an increase in cost. For these reasons, Cr was limited to a range of 16.0 to 26.0%.

Ni: Ni has the effect of stabilizing the austenite structure and improves the corrosion resistance against non-oxidizing acids such as sulfuric acid and hydrochloric acid, but if it is less than 6.0%, it is not sufficient. However, 2
If the content exceeds 2.0%, the corrosion resistance tends to be saturated, leading to an increase in cost. For these reasons, Ni is 6.0 to 22.
．． It was limited to a range of 0%.

N: N is an element that is effective in increasing strength and improving corrosion resistance, but adding more than 0.63% leads to a decrease in manufacturability.
was limited to 0.3% or less.

In carrying out this invention, the above components alone may be used, or if necessary, MO of 4% or less, Cu5O of 25% or less, Nb of 8% or less, and Ti of 0.5% or less may be added. You can also do it. Note that the component ranges of these additional elements will be described below.

Collar 0: Mo is an element that has a remarkable effect on improving corrosion resistance, especially pitting corrosion resistance, but it is an expensive element and adding a large amount increases cost, so it is preferably 4% or less.

Cu: Like Mo, Cu is an element that has a remarkable effect on improving corrosion resistance, especially pitting corrosion resistance, but since it is an expensive element, adding a large amount increases cost, so it is preferably 2.5% or less.

Nb: Nb is added to form Nb carbides and suppress the formation of Cr carbides to improve intergranular corrosion resistance and refine crystal grains, but it is added to effectively combine with C. N of
It is sufficient for the amount of b to be C (%) x 10, and since addition of a large amount leads to a decrease in productivity, it is preferable to set the upper limit to 0.8%.

Ti: Same as Nb, Ti is added to form Ti carbides and suppress the formation of Cr carbides to improve intergranular corrosion resistance and refine crystal grains, but it does not combine effectively with C ( TiN for
C (%) x 5 is sufficient, and addition of a large amount leads to a decrease in productivity, so it is preferable to set the upper limit to 0.5%.

(Function) In this invention, firstly, the heating temperature of the raw pipe for finish rolling in the reheating furnace is regulated to 1050'C or higher and 1200°C or lower. It is necessary to release the processing strain added in the process and recrystallize, and to make the precipitated carbide a solid solution. To do this, the heating temperature in the reheating furnace is low (both recrystallization and The absolute condition is 1050°C or higher, which is higher than the solution temperature and can ensure a recrystallized state in the subsequent finish rolling.However, if it exceeds 1200°C,
The crystal grains become coarse in the reheating furnace, and it is difficult to make the crystal grains fine and uniform in the subsequent finish rolling process.

Therefore, the heating temperature of the raw pipe for finish rolling in the reheating furnace was set to 1050°C or more and 1200°C or less.

Next, when the cross-sectional area of the raw pipe for finish rolling is Ao (cm") and the cross-sectional area of the finished pipe is A (cm2), the following formula % formula % (
1) The finishing temperature, that is, the finishing tube temperature T ('C) at the exit side of the mandrel mill, is determined by the following formula according to the working strain ε during finish rolling, which is expressed as: T≧220 ・exp(−2・t ) +900
The reason why finish rolling is performed under conditions that satisfy (2) is as follows.

Figure 1 shows 5US304 steel with an outer diameter of 90 mm and a wall thickness of 3 mm.
~10fflT11, an outer diameter of 146 trtm, and a wall thickness of 5 mm for finish rolling, the reheating temperature was set to 1.
050-1200℃, processing strain ε in reduction rolling
The presence or absence of an unrecrystallized structure remaining after performing finish rolling by changing the and finishing tube temperature T is shown. Here, the processing strain ε can be determined by changing the outside diameter drawing ratio of the finished tube, and the finishing temperature T can be determined by changing the amount of descaling water during finish rolling.
This was changed by adjusting the amount of roll cooling water, etc. The lower the finished tube temperature T and the smaller the processing strain ε, the more unrecrystallized structure remains, and the finished tube temperature T becomes 220 ・exp
If it is (-2.e) +900 ('C) or more, a perfect recrystallized structure can be obtained. If unrecrystallized remains, Y
S, TS increases, elongation decreases, for example, bending of the product,
When pipe expansion, pipe contraction, etc. are carried out, defects such as cracks and breaks are likely to occur, which is undesirable. From the above results, in this invention, in order to obtain a seamless austenitic stainless steel pipe that is recrystallized and softened and has good workability, the finished pipe temperature T is set to 220-exp(-2・e) +900.
C) limited to the above.

Next, the steel is cooled in the temperature range of 900 to 500°C after finish rolling under conditions where the average cooling rate V ("C/5ec) satisfies V2C' x 10' according to the carbon content C (wt%) in the steel. As a result of investigating the effect of the average cooling rate V ('C/5ec) on the precipitation of Cr carbides in the temperature range of 900 to 500'C on austenitic stainless steels with various carbon contents, it was found that carbon itC (wt
%), intergranular corrosion resistance can be maintained if V2CX 10' is satisfied, but if cooling is performed at a slow average cooling rate that does not satisfy the above relational expression, Cr carbides will precipitate and intergranular corrosion resistance will be maintained. It was found that the quality deteriorated. Therefore, in this invention, the average cooling rate in the temperature range of 900 to 500° C. is defined as ■≧C' X 10'. In addition, here 90
Since the cooling rate in the high temperature range exceeding 0°C or the low temperature range below 500°C does not affect the precipitation of Cr carbide, the cooling rate was limited only to the temperature range of 900 to 500°C.

(Example) Table 1 (wt%) Seamless steel pipes were manufactured using the six types of austenitic stainless steel shown in Table 1 above under the manufacturing conditions shown in Table 2, and each steel pipe was obtained. We investigated the mechanical properties, etc. The results are also shown in Table 2.

In addition, the mechanical properties of finished pipes with an outer diameter of 60.5 mm or less are specified by the No. 14C test piece specified in JIS Z2201, and those with an outer diameter exceeding that are specified by the same No. 14B test piece according to JIS Z2241. Measured using the metal material tensile test method, the recrystallized structure was found to have no unrecrystallized structure remaining by microscopic observation of the cross section, and 01 unrecrystallized structure remained for those with a complete recrystallized structure. What you have ×
I wrote down. The intergranular corrosion resistance was determined by the 10% oxalic acid etch test method specified in JIS GO 571, and was evaluated as O for the stepped structure and × for the others. Furthermore, the grain size number was calculated using the austenite grain size test method specified in JISGO551.

As is clear from Table 2, according to the present invention, it is possible to obtain seamless austenitic stainless steel pipes that are softened by recrystallization, have excellent intergranular corrosion resistance, and have uniform and finer grains. It will be done.

For reference, Table 3 shows data for conventional examples in which solution treatment was performed after rolling, but it is clear that according to the present invention, results equivalent to those obtained by solution treatment can be obtained.

(Effects of the Invention) According to the present invention, it is possible to omit the solution heat treatment step in a heat treatment furnace after the rolling step, and the crystal grains are sufficiently softened by recrystallization to have uniform and finer crystal grains with excellent intergranular corrosion resistance. A seamless austenitic stainless steel pipe with the following properties is obtained.

[Brief explanation of the drawing]

Figure 1 is a graph showing the presence or absence of unrecrystallized remains when processing strain and finished pipe temperature are changed. Figure 2 is a schematic diagram showing a conventional example of a seamless steel pipe manufacturing line using the Mannesmann-mandrel mill system. . 1... Billet 2... Rotary hearth heating furnace 3... Mannesmann Piaser 5... Continuous casting machine 4^... Hollow tube 4B.
...Material pipe for finish rolling 6...Mandrel mill 7...
・Mandrel bar 8...grooved roll 9...reheating furnace 10...stretch reducer 11...finishing pipe 12...cooling bed 13・
... Heat treatment furnace 14 ... Quenching tank 15 ... Straightening machine 16 ... Pickling tank Patent applicant Kawasaki Steel Corporation

Claims (1)

[Claims] 1. C: 0.08 wt% or less, Si: 1.0 wt% or less, Mn: 2.0 wt% or less, Cr: 16 to 26 wt%,
In manufacturing seamless austenitic stainless steel pipes containing N: 0.3 wt% or less, the base pipe for finish rolling containing the above components is heated at 1050°C or higher for 1
After reheating to 200℃ or less, the cross-sectional area A_o (cm^2) of the raw pipe for finish rolling and the cross-sectional area A (cm^2) of the finished pipe
2), the finishing temperature T satisfies the following formula, T≧220・exp(−2・ε)+900, according to the processing strain ε during finish rolling expressed as ε=ln(A_o/A) Finish rolling is carried out under the following conditions, and the average cooling rate V (°C/S) in the temperature range of 900 to 500°C after this finish rolling is the following formula, V≧C^3×10
^4. Here, C: Carbon content (wt%). A method for finish rolling a seamless austenitic stainless steel pipe, characterized by cooling under conditions that satisfy the following.