JPH049862B2 - - Google Patents

Description

[Detailed description of the invention]

(Industrial Application Field) This invention relates to austenitic stainless steel used for seamless steel pipes for piping, machine structures, boilers, etc., and particularly to manufacturing seamless steel pipes through a thinning drawing process in a mandrel mill. This relates to an austenitic stainless steel suitable for. (Prior art) Seamless steel pipes are generally manufactured by rolling methods such as the mandrel mill method and plug mill method, or hot tube extrusion methods such as the Eugene Sedziurne method and the Erhardt push bench method. A mandrel mill rolling method is widely used for pipe making because it is excellent in terms of productivity and dimensional accuracy. In the mandrel mill method, for example, as shown in Fig. 1, a billet material 1 is heated in a rotary hearth type heating furnace 2 to a predetermined temperature (generally 1100°C to 1300°C), and then heated by a Mannesmann Piaser 3. It is pierced and rolled to form a hollow tube 4A. Moreover, such a hollow shell 4A may be directly manufactured by the continuous casting machine 5 for producing hollow shells. Since the hollow tube 4A is thick and short, it is stretched to reduce its thickness by a mandrel mill 6, which is a stretching mill. The mandrel mill 6 is a rolling mill that stretches and rolls a hollow tube 4A with a mandrel bar 7 whose surface is coated with hot rolling lubricant, and is usually composed of 6 to 8 roll stands. Each roll stand is provided with a pair of grooved rolls 8, and the rotational axes of the grooved rolls 8 between adjacent roll stands are shifted from each other by 90 degrees in a plane perpendicular to the rolling axis. The temperature in the mandrel mill 6 is 1050℃ to 1200℃ on the rolling mill entry side and 800℃ on the rolling mill exit side.
Generally, the temperature is ~1000℃. Hollow tube 4A
is stretched to a length 2 to 4 times its original length by a mandrel mill 6, and becomes a raw pipe 4B for finish rolling. The raw pipe 4B for finishing rolling is reheated to a predetermined temperature (generally 850 to 1000°C) in a reheating furnace 9 as necessary, and then heated to a finishing rolling mill such as a stretch reduction mill. 10 for finish rolling.
The outer diameter of the raw pipe is reduced by up to 75% by the stretch reducer 10 and stretched to more than 40 times the length of the raw billet. A finished tube 11 having relatively excellent outer diameter dimensional accuracy is obtained because it is shaped by several stands of perfect circular hole type rolls. By the way, in the stretching process using a mandrel mill, if no lubricant is applied to the surface of the mandrel bar, or if a lubricant with poor lubrication performance is applied and rolled, the rolling load will increase, resulting in wear and tear, seizure, etc. of the rolls and mandrel bar. Not only does this increase, but in some cases, rolling becomes impossible due to material being squeezed out to the roll flange. Therefore, it is necessary to apply a hot rolling lubricant with excellent hot lubrication performance to the surface of the mandrel bar. A water-soluble lubricant containing graphite as a main component, as disclosed in Japanese Patent No. 59-37317, is most commonly used. (Problem to be Solved by the Invention) However, when austenitic stainless steels such as SUS 304 and SUS 316 are rolled by a mandrel mill by inserting a mandrel bar coated with graphite-based lubricant, There was a problem in that carburization occurred on the inner surface of the raw pipe during rolling, deteriorating the intergranular corrosion resistance of the inner surface of the rolled seamless steel pipe. (Problems to be solved by the invention) This invention was developed in view of the above-mentioned circumstances, and even if a mandrel bar coated with graphite-based lubricant is inserted and mandrel mill rolling is performed, The purpose of the present invention is to propose an austenitic stainless steel that does not cause carburization on the inner surface of the tube and therefore does not cause deterioration in the intergranular corrosion resistance of the inner surface of the rolled seamless steel tube. (Means for solving the problem) In order to solve the above problem, the present inventors
As a result of repeated research on the carburization mechanism in austenitic stainless steel, it has been found that when Sb and Sn are added to austenitic stainless steel, these elements prevent the addition of C atoms on the steel surface, effectively suppressing carburization. I gained some knowledge. Therefore, after further research on this point, we found that by containing at least one of Sb and Sn at 0.0010 to 0.0097wt%, we performed mandrel mill rolling by inserting a mandrel bar coated with graphite-based lubricant. They discovered that even if they did so, carburization would not occur on the inner surface of the raw pipe and the intergranular corrosion resistance of the inner surface of the rolled seamless steel pipe would not deteriorate, leading to the completion of this invention. That is, this invention contains C: 0.15 wt% or less (hereinafter simply expressed as %), Si: 1.5% or less, Mn: 2.0% or less, Cr: 16.0 to 26.0%, and N: 6.0 to 22.0%, and further contains Sb and Contains a total of one or two selected from Sn: 0.0010 to 0.0097%, and sometimes further Mo: 2.0 to 3.0%, or further Ti: 4 x (C%) to 0.6% and/or
This is an ausnitic stainless steel for seamless steel pipes, containing Nb: 8×(C%) to 1.2%, and the remainder being substantially Fe. This invention will be explained in detail below. This invention applies to SUS 304 series and other SUS
It can be applied to all austenitic stainless steels such as 310 series, 316 series, 321 series, and 347 series. First, the reason why the component composition is limited to the above range in this invention will be explained. C: 0.15% or less C is an austenite phase forming element and is an element necessary to increase strength, but if it exceeds 0.15%, workability deteriorates and it becomes difficult to form a solid solution in solution heat treatment, resulting in poor corrosion resistance. C should be kept at 0.15% or less. Si: 1.5% or less Si is required as a deoxidizing element at a concentration of about 1% for steelmaking work, but if it exceeds 1.5%, hot workability deteriorates, so Si is kept at 1.5% or less. Mn: 2.0% or less Mn acts as a deoxidizing and desulfurizing agent and is also an austenite phase forming element. However, if it is added in an amount exceeding 2%, the corrosion resistance will deteriorate, so the Mn content should be 2.0% or less. Cr: 16.0-26.0% Cr is a basic element that maintains the corrosion resistance of stainless steel. However, if it is less than 16%, sufficient effects will not be produced, so the lower limit is set at 16.0%. From the viewpoint of corrosion resistance, a larger amount is preferable, but if it exceeds 26%, hot workability deteriorates and the material becomes expensive, so the upper limit is set at 26.0%. Ni: 6.0 to 22.0% Ni is a basic element of ausnitic stainless steel, and in order to obtain a stable ausnitite phase in this invention steel, at least 6% or more is required, so the lower limit is set at 6.0%. . but,
If it exceeds 22%, hot workability deteriorates and the material becomes expensive, so the upper limit is set at 22.0%. Sb and/or Sn: 0.0010 to 0.0097% Sb and Sn are particularly important components in this invention, and the addition of such elements can effectively improve carburization resistance. However, if the content is less than 0.0010%, the effect of improving carburization resistance is poor;
Even if the Sb and Sn are added in large amounts, the above effects will not only reach saturation, but also cause deterioration of hot workability. 0.0097
% range. Mo: 2.0 to 3.0% Mo is effective for improving corrosion resistance and is added as necessary. However, since less than 2% is not sufficient to improve corrosion resistance, the lower limit is set at 2.0%. On the other hand, if it exceeds 3%, hot workability deteriorates and the material becomes expensive, so the upper limit is set at 3.0%. Ti: 4 x (C%) ~ 0.6%, Nb: 8 x (C%) ~
Both 1.2% Ti and Nb are effective in improving corrosion resistance, especially intergranular corrosion resistance, and are added as necessary. This corrosion resistance improvement effect is due to the fact that these elements combine with C, reducing the corrosion resistance improvement effect of Cr.
This is due to suppressing the formation of Cr carbides. Therefore, the amount of Ti and Nb added is
You only need the amount necessary to stabilize Ti, and Ti is 4
×(C%) or more, Nb may be 8×(C%) or more. However, excessive addition deteriorates hot workability and causes embrittlement at high temperatures, so Ti is added at 0.6%.
Hereinafter, Nb shall be 1.2% or less. In addition to the above-mentioned components, impurities are inevitably mixed into steel during the steelmaking process, but among these impurities, P and S in particular significantly deteriorate hot workability, so their mixing should be minimized. It is preferable. However, regarding these impurities,
P: 0.04% or less, S: 0.03% or less are acceptable. (Function) The reason why carburization resistance is significantly improved by adding a small amount of Sb and/or Sn to austenitic stainless steel according to the present invention is considered to be due to the following reasons. That is, Sb and Sn segregate at grain boundaries and surfaces in steel, and further prevent chemical adsorption of gas molecules. Therefore, it is thought that the addition of C atoms to the surface of the steel is suppressed and carburization is prevented. (Example) After melting alloy melts having various compositions shown in Table 1 using a vacuum melting furnace with a capacity of 5 tons,
Steel ingots were made, and billets with a diameter of 110 mm and a length of 1300 mm were each made from the obtained steel ingots. After heating each billet to 1240℃ in a rotary hearth type heating furnace, the outer diameter is determined using a Mannesmann piercer.
A mandrel bar made by punch-rolling a hollow tube of 110mm, wall thickness: 11.25mm, length: 3200mm, and then applying a lubricant on the surface with a composition of graphite: 28%, organic binder: 10.4%, and water: 61.6%. Insert and use a mandrel mill to cut outer diameter: 90mm, wall thickness: 3.75mm, length:
The finished rolled raw pipe was 11,700 mm. After that, after reheating to 960℃, use a stretch reducer to measure outer diameter: 60.5mm, wall thickness: 4.0mm, length: 16300.
mm finished tube. Next, solution treatment was carried out by holding at 1080°C for 10 minutes and cooling with water, followed by sensitization heat treatment by holding at 650°C for 2 hours and cooling in air. Table 1 also shows the results of examining the carburization resistance of each of the steel pipes thus obtained. Carburization resistance was determined by performing an oxalic acid electric field etching test on 10% of the tube cross section as specified in JIS G 0571.
As a result, the depth of intergranular corrosion due to carburization compared to the center of the wall thickness on the inner surface side was evaluated as the carburization depth.

[Table] As is clear from the table, according to the present invention, Sb and/or
Those to which Sn was added in the range of 0.001 to 0.0097% showed no carburization phenomenon at all and exhibited excellent carburization resistance. (Effects of the Invention) Thus, according to the present invention, it is possible to easily obtain an austenitic stainless steel with excellent carburization resistance, and further, by inserting a mandrel bar coated with a graphite-based lubricant into this type of steel. Even if mandrel mill rolling is performed, carburization does not occur on the inner surface, and seamless austenitic stainless steel pipes with excellent intergranular corrosion cracking resistance can be manufactured.

[Brief explanation of drawings]

FIG. 1 is a process diagram showing a procedure for manufacturing a seamless steel pipe using a mandrel mill method. 1...Material billet, 2...Rotary hearth type heating furnace, 3...Mannesmann piercer, 4A...Hollow shell tube, 4B...More tube for finish rolling, 5...Continuous casting machine for producing hollow shell tube, 6... Mandrel mill, 7...
... Mandrel bar, 8 ... Groove roll, 9 ... Reheating furnace, 10 ... Stretch reducer, 11 ...
...finished pipe.

Claims (1)

[Claims] 1 Contains C: 0.15 wt% or less, Si: 1.5 wt% or less, Mn: 2.0 wt% or less, Cr: 16.0 to 26.0 wt%, and Ni: 6.0 to 22.0 wt%, and further contains Sb and Sn. An austenitic stainless steel for seamless steel pipes, characterized in that it contains a total of 0.0010 to 0.0097 wt% of one or two selected from among them, with the remainder being substantially Fe. 2 Contains C: 0.15 wt% or less, Si: 1.5 wt% or less, Mn: 2.0 wt% or less, Cr: 16.0 to 26.0 wt%, Ni: 6.0 to 22.0 wt%, and Mo: 2.0 to 3.0 wt%, and further contains Sb and An austenitic stainless steel for seamless steel pipes, which contains a total of 0.0010 to 0.0097 wt% of one or two selected from Sn, and the remainder is essentially Fe. 3 Contains C: 0.15wt% or less, Si: 1.5wt% or less, Mn: 2.0wt% or less, Cr: 16.0 to 26.0wt%, and Ni: 6.0 to 22.0wt%, and at least one of Ti and Nb. Ti: 4 x (C%) to 0.6 wt% Nb: 8 x (C%) to 1.2 wt% depending on the C content, and one or two selected from Sb and Sn. An austenitic stainless steel for seamless steel pipes, characterized in that it contains a total of 0.0010 to 0.0097 wt% of species, and the remainder is essentially Fe.