JPH09287024A - Production of ferritic stainless steel seamless pipe - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for efficiently producing a ferritic stainless steel seamless pipe having small unevenness of the characteristic, such as excellent strength and corrosin resistance, in a low cost. SOLUTION: The seamless steel pipe is produced by using the ferritic stainless steel and executing the following (1)-(3) processes in order: (1) elongating work and finish work applied to a hollow base stock are executed at >=40% the total working ratio in both works and at 800-1,100 deg.C finish temp. to make the seamless steel pipe; (2) the seamless steel pipe is charged into a supplemental heating furnace and held at the temp. T deg.C regulated with the formula, 695+4×(%Cr)+20×(%Mo)+12×(%W)<=T deg.C<=1150, for 10sec to 30min; and (3) the seamless steel pipe taken out from the supplemental heating furnace is cooled at 1-100 deg.C/sec cooling velocity to 600 deg.C and successively, at >=20 deg.C/min at <=350 deg.C.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a seamless steel pipe having high strength and excellent toughness and corrosion resistance by using a so-called ferritic stainless steel which has a substantially ferritic structure at room temperature. It is a thing.

[0002]

2. Description of the Related Art In the past, a seamless steel pipe made of ferritic stainless steel has been manufactured by subjecting it to a solution treatment after the pipe has been manufactured. However, in ferritic stainless steels, carbonitrides and intermetallic compounds such as σ phase tend to precipitate during cooling after hot working, and 475 ° C embrittlement susceptibility is high.
There are many handling problems such as brittleness and lack of required product properties, and pipes colliding and cracking during transportation during the manufacture of seamless steel pipes.

Further, the seamless steel pipe as hot-rolled cannot be said to have a sufficiently high yield stress in terms of strength, and it must be further cold worked in order to obtain high strength. In addition, the process of performing re-heating and solution heat treatment after pipe making is not preferable from the viewpoint of productivity because the manufacturing cost becomes high.

As a method of omitting the solution treatment of ferritic stainless steel, for example, as disclosed in Japanese Patent Laid-Open No. 61-69917, a steel having a limited content of C and N is used and hot rolling is performed. A method of accelerating cooling at a specific cooling rate afterwards has been proposed. Although it is possible to prevent the precipitation of carbonitrides by this method, the effect of suppressing the precipitation of intermetallic compounds, which is a major cause of embrittlement, is unknown, and it requires severe and complicated processing. In addition, it is completely unclear whether this technology can be applied as it is to the production of seamless steel pipes that have severe requirements regarding product characteristics.

[0005]

As proposed in the above-mentioned Japanese Patent Laid-Open No. 61-69917, even if it is possible to carry out quenching treatment after the final processing to omit solution treatment, a seamless steel pipe It is difficult to apply this method to the actual production line of.

[0006] In the process of manufacturing a seamless steel pipe, the hollow shell or the steel pipe after rolling does not come into uniform contact with the entire surface of the rolling roll, the beam of the conveying line or the like. Therefore, the cooling condition differs depending on the part of the one steel pipe (position in the longitudinal direction and the circumferential direction), and a considerable temperature fluctuation occurs. If such a steel pipe is rapidly cooled as it is, it will be rapidly cooled from different temperatures depending on the parts, and as a result, parts having different mechanical properties and corrosion resistance will be generated in one steel pipe. That is, the product steel pipe becomes unusable for practical use with many variations in characteristics. Further, when a steel pipe immediately after rolling, in which strain is excessively introduced, is rapidly cooled, intermetallic compounds such as σ phase and Laves phase are likely to precipitate, and sufficient performance cannot be obtained.

An object of the present invention is to suppress the precipitation of carbonitrides and intermetallic compounds by a simple process which omits the conventional solution treatment which is performed by reheating in a separate line after pipe making, and the strength is improved. Another object of the present invention is to provide a method for producing a seamless steel pipe having excellent corrosion resistance, particularly a steel pipe having a small variation in characteristics within one steel pipe.

[0008]

The present invention is a method for producing a seamless steel pipe using stainless steel which has a substantially ferritic structure at room temperature, characterized in that the following steps (1) to (2) are sequentially performed. The manufacturing method is the main point.

[0009] A step of performing a drawing process and a finishing process on a hollow shell at a total working ratio of 40% or more and a finishing temperature of 800 to 1100 ° C to obtain a seamless steel pipe, the seamless steel pipe being a supplementary heating furnace. The process of charging the steel tube to a temperature of T ℃ specified by the following formula (a) for 10 seconds to 30 minutes, the seamless steel pipe taken out from the auxiliary heating furnace at a cooling rate of 1 to 100 ℃ / sec. The process of cooling to 600 ° C and then to a temperature of 350 ° C or less at a cooling rate of 20 ° C / min or more.

695 + 4 × (% Cr) + 20 × (% Mo) + 12 × (% W) ≦ T (° C.) ≦ 1150 (a) “substantially ferrite structure at room temperature” which is the target of the method of the present invention The "stainless steel that becomes" is a stainless steel that has a structure mainly composed of ferrite (may contain martensite of 50% or less) at room temperature. There are no particular restrictions on the chemical composition, but typical components and their contents are as follows (% is% by weight).

C: 0.15% or less, Si: 0 to 4%, Mn: 0 to
2%, Cr: 10-40%, Mo: 0-5%, W: 0-5%, N
i: 0 to 2%, Co: 0 to 4%, Al: 0 to 4%, Nb: 0 to
1.0%, Ti: 0 to 1.0%. In addition to these alloy elements, an appropriate amount of another alloy element may be contained.

[0012]

BEST MODE FOR CARRYING OUT THE INVENTION Each step of the method of the present invention will be sequentially described below. The pipe-making material (billet) may be a billet manufactured by slab rolling or forging from an ingot or a continuously cast slab, bloom, or the like.
If a round billet is cast by continuous casting, it can be directly subjected to the punching process.

Hollow shell for drawing and rolling (hollow shell)
Can be manufactured (drilled) by any method. For example, a so-called piercer such as an inclined roll mill can be used. The perforation conditions may be basically the same as in the case of manufacturing a normal ferritic stainless steel seamless steel pipe.

Stretching and finishing processes: There are various systems for performing this process, such as the Mannesmann-mandrel mill system and the Mannesmann-plug mill system. Either method can be adopted in the method of the present invention. For example, in the mandrel mill system, a drawing process is performed by a mandrel mill and a finishing process is performed by a sizer or reducer.

Stretching and finishing are low-temperature working as compared with perforating, and are important working steps for refining crystal grains. If the finishing temperature in these processes is lower than 800 ° C, the toughness of the product steel pipe deteriorates because intermetallic compounds precipitate. On the other hand, if the temperature exceeds 1100 ° C, the crystal grains become coarse and the toughness decreases. Therefore, the finishing temperature must be 800-1100 ℃. It should be noted that if the finishing temperature is lowered and the temperature is raised in the next auxiliary heating furnace, the effect of grain refinement becomes greater, so it is desirable to set the finishing temperature to about 800 to 950 ° C for improving toughness. .

The total degree of processing of stretching and finishing is
If it is less than 40%, the grain refinement is not sufficient. The upper limit of the workability is not particularly limited, but if it exceeds 90%, the load on the tool will be heavy, so it is preferably in the range of 40 to 90%.

From the viewpoint of grain refinement, it is preferable that the interval between the drawing step and the finishing step is as short as possible.
That is, the effect is great if the finishing process is performed before the dislocations introduced during the stretching process are recovered, and the strain is sufficiently accumulated and then the grain size is reduced by recrystallization.

In the above-mentioned processing, the distance between the rolling mill (for example, a mandrel mill) for stretching and the rolling machine (for example, sizer or reducer) for finishing is determined by
It can be carried out using equipment installed at intervals shorter than the length of the hollow shell (hollow shell) processed by the former. For example, a rolling process is preferred in which the finish rolling is immediately performed and the bar is pulled out from the hollow shell at the same time by using the extracting sizer.

Supplementary heating step: Supplementary heating is performed by charging the seamless steel pipe produced in a supplemental heating furnace provided in a pipe production line. This supplementary heat is carried out for the purpose of re-dissolving the intermetallic compound precipitated after the rolling process and for the purpose of soaking the steel pipe sufficiently to reduce the above-mentioned variation. By using the supplementary heating furnace, the temperature of the entire tube can be made uniform, and the temperature can be accurately adjusted, and the heat treatment conditions can be selected according to the characteristics desired for the product, which is a great advantage.

The appropriate temperature for supplementary heat depends on the content of alloy components of the raw material stainless steel. That is, the temperature specified by the formula (a) must be set according to the contents of Cr, Mo and W in the raw steel.

The above equation (a) was derived as follows. That is, using ferritic stainless steels of various chemical compositions, pipes were made by the Mannesmann pipe making method, and the post-heating temperature after pipe making was changed in the range of 800 to 1200 ° C and cooled at 30 ° C / sec. A test material was prepared. Corrosion resistance was investigated using the test material, and regression analysis was performed based on the result. FIG. 1 shows an example of the result (when stainless steel having the composition shown in Table 1 described later is targeted). As is clear from FIG. 1, the region having good corrosion resistance is the region above the straight line L. Here, "good corrosion resistance" means that the pitting corrosion potential does not decrease or is less than 0.1 V compared to the steel pipe obtained by the conventional manufacturing method in which solution treatment is performed offline. The term "poor corrosion resistance" means that the pitting corrosion potential is decreased by 0.1 V or more.

The straight line L in FIG. 1 is expressed as T (° C.) = 695 + 4 × (% Cr) + 20 × (% Mo) + 12 × (% W). Therefore, in order to secure good corrosion resistance, if the temperature of supplementary heat T (℃) is T (℃) ≧ 695 + 4 × (% Cr) + 20 × (% Mo) + 12 × (% W) It will be good. The meaning of this formula is that as Cr, Mo and W of ferritic stainless steel increase, the supplementary heat temperature for ensuring corrosion resistance must be increased, which means that if these components increase It is considered that intermetallic compounds such as σ phase are likely to precipitate.

On the other hand, if the temperature of the supplementary heat is raised too much, the previous process
The working strain accumulated in (step) is completely released, the strength of the steel pipe after the next cooling treatment does not increase, and the coarsening of crystal grains begins to cause deterioration of toughness. The upper limit of the temperature of supplementary heat without these adverse effects is 1150 ° C.

If the finishing temperature in the step is within the range satisfying the expression (a), the supplementary heat may be performed at the finishing temperature. Of course, a heat pattern may be used in which the finishing temperature is set to a low temperature within the range of 800 to 1100 ° C. and the supplemental heat is heated (raised) to a higher temperature within the range satisfying the expression (a). Furthermore, during supplementary heating, the steel pipe does not have to be kept at a constant temperature, and a heat pattern in which the steel pipe is gradually cooled within a temperature range satisfying the expression (a) can be adopted. However, in any case, it is important to consider that the temperature of one steel pipe is uniform.

The time for supplementary heating depends on the size of the steel pipe to be treated,
Depending on the wall thickness, in particular, at least 10 seconds are required to bring the entire temperature of the tube to a uniform temperature and to dissolve the carbonitride and the intermetallic compound sufficiently. On the other hand, when supplementing heat for a long time of more than 30 minutes, coarsening of crystal grains progresses, resulting in a decrease in toughness of the product steel pipe.

Cooling step: In the cooling step of the steel pipe exiting the auxiliary heating furnace, it is important to adjust the cooling rate for each temperature range. First, in the region where carbonitrides and intermetallic compounds precipitate in the temperature range of 600 ℃ or higher, cool at 1 to 100 ℃ / sec, and then in the 475 ℃ brittle temperature range from 600 ℃ to 350 ℃, Cool at a rate of 20 ° C / min or more that can avoid aging.

The temperature range from the supplemental heating temperature to 600 ° C. is a temperature range where precipitation of intermetallic compounds and carbonitrides easily occurs. 1 ℃
If the cooling rate is lower than / sec, precipitation of intermetallic compounds occurs, and toughness and corrosion resistance decrease. Precipitation of carbonitrides, especially in ferritic stainless steels, is unavoidable even with a considerably high cooling rate. Along with the precipitation of carbonitride, a chromium deficient layer is formed in the vicinity of the precipitate, and so-called sensitization (deterioration of corrosion resistance) occurs.

However, if the cooling rate is 100 ° C./sec or less, the chromium deficient layer can be repaired by the diffusion of Cr and the corrosion resistance can be recovered. In the present invention, the upper limit of the cooling rate is 100
The reason why the temperature is limited to ° C / sec is to obtain the effect of repairing the chromium-deficient layer.

The temperature range from 600 ° C. to 350 ° C. is a region where brittleness is likely to occur at 475 ° C. when heated for a long time. In the present invention, the cooling rate in this region is set to 20 ° C./minute or more (a cooling rate as high as possible is desirable) in order to pass through this region as quickly as possible. If the cooling rate is less than 20 ° C / min, embrittlement may occur. There are no particular restrictions on cooling in the temperature range lower than 350 ° C.

[0030]

EXAMPLE Steels A to G shown in Table 1 were melted to prepare a billet having an outer diameter of 225 mm, which was rolled using a Mannesmann-mandrel mill to form a steel pipe having an outer diameter of 273.1 mm and a wall thickness of 9.3 mm. Manufactured. The manufacturing conditions are as shown in Table 2. The conventional example in Table 2 is a conventional manufacturing method in which a steel pipe after finishing is cooled to room temperature, and then reheated to a solution temperature at a heat treatment line different from the pipe making line and rapidly cooled.

[0031]

[Table 1]

[0032]

[Table 2]

With respect to the obtained steel pipe, a tensile test piece and a Charpy impact in the pipe axial direction from a total of 12 places, three places at intervals of 3 m in the longitudinal direction from the pipe end and four equally divided positions in each of these positions in the circumferential direction. A test piece and a corrosion resistance test piece were sampled and subjected to the following tests to examine mechanical properties and corrosion resistance.

The tensile test was carried out using a round bar test piece having a diameter of 4 mm and a parallel portion of 34 mm. Charpy impact test is 5mm
The impact value at 0 ° C. was evaluated using a 2 mm V notch test piece of × 10 mm × 55 mm. Corrosion test according to JIS G 0577, 80 ℃
It was evaluated by the pitting potential in artificial seawater.

Table 2 shows the average value of the test results of the above 12 test pieces. As shown in Table 2, mechanical properties and corrosion resistance vary greatly depending on the alloy component system. Therefore, the effect of manufacturing conditions cannot be clarified unless a comparative study is conducted for each steel type. First, trial number 15 ~
On the basis of the performance of 21 conventional examples, the yield strengths of trial Nos. 1 to 7 which are examples of the present invention are high in all steel types, and the impact value is also good. Corrosion resistance is almost the same as the conventional example.

On the other hand, of the comparative examples of trial numbers 8 to 14, trial number 8,
Nos. 9 and 11 are those in which the workability of the stretching process and the finishing process are too small, the finishing temperature is too high, and the supplementary heat temperature is too high and the time is too long.
All of these have inferior impact values due to coarsening of crystal grains. Test Nos. 10 and 12 are examples in which the supplementary heat temperature is below the lower limit temperature obtained by equation (a) and the cooling rate up to 600 ° C is too low. Although improved, the toughness and corrosion resistance are greatly reduced.

Test No. 13 is an example in which the cooling rate up to 600 ° C. is too high, and although there is no problem in mechanical properties, sensitization is observed and corrosion resistance is reduced. Sample No. 14 is an example in which the brittle region at 475 ° C. is gradually cooled, and although the strength is high, the toughness and corrosion resistance are greatly reduced.

Table 3 shows the test results of the steel pipes of trial Nos. 1 to 7 in Table 1 by the above 12 test pieces in the longitudinal direction and the circumferential direction of the pipe. The yield point, impact value, and corrosion resistance value at each position are almost the same, and it is clear that their variations are extremely small.

[0039]

[Table 3]

[0040]

As is clear from the examples, when a ferritic stainless steel seamless steel pipe is produced by the method of the present invention, a steel pipe having high strength and high toughness is obtained as compared with the steel produced by the conventional process. To be Moreover, its corrosion resistance is not inferior to that of a steel pipe which has been subjected to solution heat treatment by a conventional process. Also, the variation in characteristics due to the position of the tube is extremely small.

Since the method of the present invention can be continuously carried out on-line from pipe production to heat treatment, it greatly contributes to improvement of productivity and reduction of production cost in production of seamless steel pipe.

[Brief description of drawings]

[Fig. 1] Formula obtained by regression analysis for each steel type, namely 695
FIG. 4 is a diagram showing changes in corrosion resistance when the actual supplementary heat temperature is changed, with the horizontal axis being + 4 × (% Cr) + 20 × (% Mo) + 12 × (% W).

Claims (1)

[Claims] 1. A method for producing a seamless steel pipe using stainless steel which has a substantially ferritic structure at room temperature,
A method for producing a seamless steel pipe, which comprises sequentially performing the following steps. 40% or more of the total working ratio of both drawing and finishing processes applied to hollow shells, finishing temperature 800 ~
1100 ℃ to make a seamless steel pipe, the above seamless steel pipe is charged into the auxiliary heating furnace, and the temperature T ℃ specified by the following equation (a) is maintained for 10 seconds to 30 minutes, A step of cooling the seamless steel pipe taken out of the furnace to 600 ° C at a cooling rate of 1 to 100 ° C / sec, and then to a temperature of 350 ° C or less at a cooling rate of 20 ° C / min or more. 695 + 4 x (% Cr) + 20 x (% Mo) + 12 x (% W) ≤ T (° C) ≤ 1150 ・ ・ ・ (a)