JPH01132717A - Production of high-strength austenitic stainless seamless steel pipe - Google Patents

Abstract

PURPOSE:To obtain the title seamless steel pipe which has high yield strength and excellent ductility and corrosion resistance by reheating a steel pipe stock for finish rolling having a prescribed component compsn., then working the stock until the work strain at the time of the finish rolling attains a specified value or above, subjecting the stock to the finish rolling according to said work strain and cooling the pipe under specific conditions. CONSTITUTION:The above-mentioned seamless steel pipe contg., by weight %, 0.01-0.08 C, 0.1-1.0 Si, 0.2-2.0 Mn, 16.0-26.0 Cr, 6.0-22.0 Ni, and <=0.30 N is produced in the following manner: The pipe stock for finish rolling is first reheated to 950-1160 deg.C and is so worked that the work strain (e) at the time of the finish rolling expressed by the equation I (where the sectional area of the pipe stock is designated as A(cm<2>)) attains >=2.0. The pipe stock is then subjected to the finish rolling in such a manner that the temp. T( deg.C) of the finish pipe on the outlet side satisfies the equation II according to the (e). The pipe is thereafter cooled under the conditions under which the average cooling rate v( deg.C/s) in the temp. region of 900-500 deg.C after the finish rolling satisfies the equation III according to the C (weight%) in the steel. As a result, the austentic stainless seamless steel pipe having the yield point extremely higher than the yield point of the conventional seamless steel pipe is obtd.

Description

[Detailed Description of the Invention] <Industrial Application Field> The present invention relates to a method for manufacturing high-strength austenitic seamless stainless steel pipes, and in particular, eliminates the offline solution heat treatment after hot pipe forming, thereby eliminating the need for conventional solution heat treatment. The present invention relates to a method for manufacturing seamless austenitic stainless steel pipes that have corrosion resistance equivalent to that of solution heat-treated materials and have significantly higher yield strength than conventional ones.

<Prior art> Seamed steel pipes are manufactured by rolling methods such as the -191 mandrel mill method and plug mill method, or by hot extrusion methods such as the Eugene Sejerne method and the Erhard-Wunschbench method. Mandrel mill rolling is widely used for manufacturing small-diameter pipes because of its excellent productivity and dimensional accuracy.

When manufacturing seamless austenitic stainless steel pipes using a mandrel mill method, for example, as shown in FIG. The tube is pierced and rolled by the saw 6 to become a hollow tube 8A. Alternatively, the hollow shell tube 8A may be directly manufactured by the continuous casting machine 5 for producing hollow tubes. Since this hollow tube 8A has a thick wall and a short length, it is stretched to reduce its thickness by a mandrel mill IO, which is a stretching mill. Mandrel mill lO is a hollow tube 8
It is a rolling mill that performs elongation rolling with a mandrel bar 12 inserted in A, and is usually composed of 6 to 8 roll stands, and each roll stand has two grooved rolls! 4
The rotary axes of the grooved rolls 14 are arranged between adjacent roll stands so as to be shifted by 90 degrees from each other in a plane perpendicular to the rolling axis. The hollow blank tube 8A is stretched to a length 2 to 4 times its original length by a mandrel mill 10, and becomes a blank tube 8B for finish rolling. After the raw pipe 8B for finish rolling is reheated to a predetermined temperature in the reheating furnace 16 as necessary,
Finish rolling is performed by the stretch reducer 18, which is a finish rolling mill, and the material is cooled to room temperature on a cooling bed. The stretch reducer 18 reduces the outer diameter of the raw pipe by up to 75% and stretches it to more than 40 times the length of the material billet.
Further, since its outer surface is shaped by several stands of perfect circular hole rolls on the final side of the stretch reducer 18, a finished tube 20 that is relatively hard and has an accurate outer diameter can be obtained. Thereafter, the finished tube 20 is heated and maintained at a temperature of 10°C to 1150°C in a heat treatment furnace for solution heat treatment for the purpose of adding corrosion resistance and predetermined mechanical property values, and then cooled to room temperature by water cooling. cooled down. After straightening, scale on the surface is removed by pickling or the like to produce the final product.

<Problems to be Solved by the Invention> However, since the above-mentioned reheating solution heat treatment is performed off-line, the equipment cost increases and it is not preferable in terms of processing efficiency. In addition, from the viewpoint of materials, materials that have been subjected to conventional solution heat treatment generally have low yield strength, and for applications that require high strength, elements such as C and N can be added to improve the composition. I had no choice but to do it.

In recent years, there has been a strong demand for higher strength stainless steel pipes (higher strength is being considered for all uses such as piping, heat transfer, and structural use.) If this can be achieved by a method that reduces manufacturing costs by omitting heat treatment, it would be a great advantage to reduce costs and improve characteristics at the same time.

By the way, a method of manufacturing an austenitic stainless steel sheet by omitting the conventional general solution heat treatment is disclosed in, for example, Japanese Patent Laid-Open No. 107729/1983. However, this content is related to the manufacturing method of steel plates, and high strength cannot be achieved.
26619, but since the manufacturing process was completely different from that of seamless steel pipes, a new idea was required. Although high strength can be obtained with the former, there are problems such as a significant decrease in ductility and toughness and inferior corrosion resistance to reheated solution heat treated materials, and the latter has poor corrosion resistance.

The present invention has been made in view of the above circumstances, and provides online seamless steel pipes made of austenitic stainless steel with corrosion resistance equivalent to that of conventional reheating solution treatment materials, and with high strength. The purpose of the present invention is to provide a manufacturing method that exhibits unique characteristics.

Means for Solving Problems〉 The present inventors reviewed the manufacturing process in detail when manufacturing seamless austenitic stainless steel pipes using the Mannesmann mandrel mill method, and determined the purpose of solid solution treatment of stainless steel. The present invention was achieved as a result of intensive study of the fact that recrystallization and solid solution formation of carbides, and the manufacturing conditions for achieving strength while satisfying this condition in the case of seamless steel pipes.

That is, in the present invention, C: 0.01 to 0.01% by weight.
08%, Si: 0.1-1.0%, Mn:
0.2-2.0%.

Cr: 16.0-26.0%, Ni: 6.0
~ 22.0%, N: 0.30% or less, in manufacturing an austenitic stainless steel seamless steel pipe, after reheating the raw pipe for finish rolling to 950 to 1160 ° C.
When the cross-sectional area of the raw pipe for finish rolling is AO (cd) and the cross-sectional area of the finished pipe is A (cd), the processing strain ε during finish rolling, expressed by the following formula (%), is 0.2 or more. The finished tube temperature T(”
The step of finishing rolling under conditions where C) satisfies the following formula % formula % () and the temperature range of 900 to 500'C after this finishing rolling are determined by the average cooling rate v (
A high-strength austenitic stainless steel seamless steel sheet, characterized by having a step of cooling under conditions where 'C/s) satisfies the following formula % formula %) according to the carbon content l1C (weight %) in the steel. This is a method for manufacturing steel pipes.

Function> Next, the reason for limiting the ingredients in the present invention will be explained. Note that all percentages of the components shown below are percentages by weight.

C2C is effective in stabilizing the austenite phase and increasing its strength, and therefore 0.01% or more is required, but as the amount increases, Cr carbides are more likely to be formed, and 900%
Since it is necessary to increase the cooling rate in the carbide precipitation region of ~soo'c, C was limited to 0.08% or less.

Si: Si is normally required as a deoxidizing element in an amount of 0.1% or more, but
Addition of more than 1.0% will reduce hot workability.
It was limited to 1.0% or less.

Mn=Mn is required to be 0.2% or more to improve deoxidation and hot workability, but addition of more than 2.0% impairs corrosion resistance, so it is limited to 2.0% 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, while 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 is 16.0~ 26.0%
limited to the range of

Ni: Ni has the effect of stabilizing the austenite structure and improves 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.
．． If the addition exceeds 0%, the corrosion resistance tends to be saturated and the cost increases. Therefore, the upper limit is set at 22.0%, and the
It was limited to a range of 2.0%.

N: N is an element that is effective in increasing strength and improving corrosion resistance, but addition of more than 0.30% will reduce manufacturability, so it should not be added.
．． It was set to 30% or less.

In carrying out the present invention, only the above-mentioned present invention components may be used, or in addition, Mo of 4% or less, Cu of 2.5% or less,
The effect is the same even if 0.8% or less of Nb and 0.5% or less of Ti are added. The component range of the added elements will be described below.

hO: Mo is an element that has a remarkable lay effect in improving corrosion resistance, especially pitting corrosion resistance, but since it is an expensive element, adding a large amount will increase the cost, so it was limited to 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 will increase the cost, so it was limited to 2.5% or less.

Nb: Nb forms Nb carbide, suppresses the formation of Cr carbide, and is added to improve intergranular corrosion resistance and refine crystal grains. %)
×lO is sufficient, and addition of a large amount leads to a decrease in productivity, so the upper limit was limited to 0.8%.

Ti: Same as Nb, Ti forms Ti carbide and is added to suppress the formation of Cr carbide to improve intergranular corrosion resistance and refine crystal grains, but it is also added to improve growth effect with C. It is sufficient for the amount of Ti to be C (%) x 5, and since addition of a large amount leads to a decrease in productivity, the upper limit was limited to 0.5%.

Next, the reason for limiting the manufacturing conditions will be explained.

Reheating temperature in the reheating furnace: 950-1160°C: 2nd
The figure shows the hot stretch redeaser (HSR
As can be seen from Figure 0, which shows the mechanical properties of the rolled online solution treated material based on the present invention, when the reheating temperature drops below 1160°C, 0.2%PS,
TS begins to rise and increases with decreasing reheat temperature. This tendency has also been observed in other austenitic stainless steels; strength increases below approximately 1160°C, and high strength cannot be obtained at temperatures higher than that; therefore, the reheating temperature is 1160°C. ' Must be below C.

On the other hand, if the reheating temperature is lower than 950°C, it will be insufficient to achieve the recrystallization state and the solid solution state of the carbide, which are the objectives of reheating. As a result, mechanical properties such as ductility and toughness are reduced, and corrosion resistance is significantly reduced due to grain boundary carbides.

For the above reasons, the reheating temperature in the reheating furnace is 950℃.
It must be at least 1160°C.

Process so that the working strain ε during finish rolling is 0.2 or more: If the cross-sectional area of the raw tube for finish rolling is Ao (cm2), and the cross-sectional area of the finished tube after H3R rolling is A (cd), The processing strain during finish rolling is shown below (represented by 112).

ε=1! ,,l(Ao/A)------
−・−−−−−−−・−・・・・・−・・−−−−−−
--・-... (1) When rolling a raw tube for finish rolling into a finished tube by HSR, in order to obtain a high strength state, at least 0.
It is necessary to add a processing strain ε of 2 or more.

Finish rolling is carried out under the condition that the finish tube temperature T on the HSR outlet side satisfies the following formula % according to the processing strain ε: Figure 3 shows 5US304N, outer diameter: 90am.
, wall thickness 3-10m, and outer diameter: 146aa, wall thickness 5m
Using a raw pipe for finish rolling, the reheating temperature was set to 950 to 11
60° C., and shows the presence or absence of unrecrystallized structure remaining after finish rolling was performed while changing the work strain ε and finish tube temperature T in HSR rolling. Here, the processing strain ε is determined by changing the outside diameter drawing ratio of the finished tube, and the finishing temperature T is determined by the amount of descaling water during finish rolling.

This was changed by adjusting the amount of roll cooling water, etc.

In this figure, the boundary line between the recrystallized structure (marked with O) and the residual unrecrystallized structure (marked with *) is approximately expressed by the following equation (2).

T=22(lexp(2・ε) +900 ('C
) ---(2) As is clear from the figure, the higher the finished tube temperature T and the greater the working strain ε, the more recrystallized structure is obtained, and even if the finished tube temperature T is in the region larger than the expression (2), However, in the region smaller than (2) 2, unrecrystallized Mi ridges remain.

By the way, if sufficient recrystallization is performed, the strength decreases, so in order to increase the strength, it is necessary to leave an appropriate amount of unrecrystallized structure. Therefore, in order to obtain a high-strength austenitic stainless steel seamless steel pipe, it is necessary for the finished pipe temperature T to satisfy the condition of equation (3) below.

T<220-exp(-2・g) +900 (”
C) ---43) In the temperature range of 900 to 500°C, the average cooling rate V ("C/s) is 1 t of carbon content in steel.
Cooling under conditions that satisfy V≧C' As a result of investigating austenitic stainless steels with various carbon contents, the following (4
) formula % If formula (4) is satisfied, C is a slow average cooling rate V that does not cause intergranular corrosion due to carbide precipitation and does not satisfy formula (4) above.
It has been found that when cooled at C, carbides precipitate and intergranular corrosion occurs.
The average cooling rate in a temperature range of 500°C was defined by equation (4).

In addition, here, high temperature range exceeding 900°C or 500°C
Since the cooling rate in a low temperature range below .degree. C. does not affect the precipitation of Cr carbides, the cooling rate was therefore limited only to the temperature range of 900 to 500.degree.

<Example> Using test materials having five types of austenitic stainless steel components A to H shown in Table 1, seamless stainless steel pipes were manufactured under the manufacturing conditions shown in Table 2. Under the manufacturing conditions in Table 2, the experiment Ntxl -1° is the invention example, Hidden 11
-16 is a comparative example, and No. 17.18 is a conventional example.

Note that the reference temperature T in the table is a value determined using the above equation (2).

Regarding the comparative example, items that do not satisfy the conditions of the present invention are marked with an *, but for kll, the raw pipe reheating temperature is 11
An example where the finished tube temperature is 1 tot at 70°C,
3 is an example where the average cooling rate of 900 to 500°C does not satisfy the above formula (4), Na14 is an example where the raw tube reheating temperature is below the lower limit, and Na15 is an example where the finished tube temperature is 1070°C and exceeds the reference temperature Tt. Example, '&16 is the processing strain ε in H3R rolling
is 0.14, which is less than the lower limit value of 0.2.

The conventional example k17.18 is a case where solution treatment by reheating, which is currently commonly performed, is performed.

The mechanical properties and corrosion resistance of each of the experimental materials under the various manufacturing conditions described above were investigated, and the test results are also shown in Table 2. In the corrosion resistance test results, an O mark indicates good corrosion resistance, and an X mark indicates good corrosion resistance.
The mark indicates that it is significantly inferior.

As is clear from Table 2, all of the examples of the present invention have higher strength, with a 0.2% yield strength of 35 kgf/am'' or more, compared to the conventional example Nα17゜18, which is a reheated solution heat-treated material. , it can be seen that the balance with ductility is excellent and the corrosion resistance is also good.On the other hand, all of the comparative examples that do not satisfy the conditions of the present invention are inferior in mechanical properties or corrosion resistance compared to the inventive examples. .

<Effects of the Invention> As explained above, according to the present invention, as is clear from the above examples, when producing austenitic seamless stainless steel pipe, the raw pipe for finish rolling is heated to 950°C or higher. And after reheating to 1160 ° C or less to achieve a recrystallized state and a re-solid solution state of carbides, true stretch reducer rolling is performed, and the working strain ε at that time is 0.2 or more and according to the working strain ε to specify the rolling end temperature, and further 9
By limiting the average cooling rate in the temperature range of 00 to 500 degrees Celsius according to the carbon content, solution treatment can be carried out online, eliminating the need for reheating solution treatment and a heat treatment furnace for it, and reducing yield strength. It is possible to obtain seamless austenitic stainless steel pipes that have high ductility and excellent ductility and corrosion resistance.

In addition, conventional austenitic stainless steel pipes have a
Although it was difficult to use it as a structural member due to its low % yield strength, it can be used widely as a structural material because it can obtain high strength according to the present invention.

[Brief explanation of the drawing]

Fig. 1 is a schematic diagram showing an example of a seamless steel pipe production line using the mandrel mill method, Fig. 2 is a characteristic diagram showing the influence of reheating temperature on the closing characteristics of finished pipe after hot stretch reducer rolling, FIG. 3 is a characteristic diagram showing the influence of processing strain and finished tube temperature T on unrecrystallized &lIl residue in the finished tube after hot stretch reducer rolling. 2...Material billet, 4...Rotary hearth type heating furnace. 5... Continuous casting machine for manufacturing hollow tubes. 6... Mannesmann Piasa. 8A: Hollow tube, 8B: Base tube for finish rolling. DESCRIPTION OF SYMBOLS 10... Mandrel mill, 12... Mandrel bar 114... Hole roll, 16... Reheating furnace 118... Real stretch reducer (H3P
). 20... Finished pipe. Patent applicant: Kawasaki Steel Co., Ltd. Figure 2: Heating temperature 11 (°C) before heating
) Figure 3 0.2 0.6 1.0 1
．． 4 1.8 Processing strain ε

Claims (1)

[Claims] In weight %, C: 0.01 to 0.08%, Si: 0.1 to 0.08%
1.0%, Mn: 0.2-2.0%, Cr: 16.0-
26.0%, Ni: 6.0-22.0%, N: 0.30
In manufacturing seamless austenitic stainless steel pipes containing 950 to 11%
After reheating to 60℃, the cross-sectional area of the raw tube for finish rolling was changed to A_
o (cm^2), and the cross-sectional area of the finished tube is A (cm^2), the working strain ε during finish rolling, expressed by the following formula ε=ln(A_o/A), is 0.2 or more. The finished tube temperature T (°C
) satisfies the following formula T<220・exp(-2・ε)+900(℃) /s)
A high-strength austenite characterized by having a step of cooling under conditions that satisfy the following formula v≧C＾3×10＾4 (°C/s) depending on the carbon content C (wt%) in the steel. A method for manufacturing seamless stainless steel pipes.