JPH03275211A - Manufacture of hot extruding stainless steel pipe - Google Patents

Abstract

PURPOSE:To obtain the stainless steel pipe of high quality by executing the soaking treatment on the continuous steel material for the specific period of time at over the specific temp., then lowering the temperature while keeping the uniformly heated state, executing the hot working of over the specific section area reduction ratio in the specific temp. range and executing the hot pipe extruding manufacture. CONSTITUTION:The soaking treatment is executed to the material to be continuously forged at >=1250 deg.C for >=2 hours and <3 hours, and is cooled while keeping the uniformly heated state, is subjected to the hot working of >=20% the section area reduction ratio at <=1150 deg.C and >=950 deg.C. After that, as it is or after machining the outer surface, the hot extruding pipe manufacture working is executed. Therefore, the hot extruding stainless steel pipe is manufactured with high quality without the slit flaw over all length with the soaking treatment of comparatively short time. Accordingly, the heating cost required for the soaking treatment can be reduced and the larger the diameter of the material to be continuously forged is, the larger, the effect is.

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Field of Application) The present invention relates to a method of manufacturing a hot extruded alloy steel pipe made of continuous casting material.

[Conventional technology]

With the recent development of continuous casting technology, it has become possible to manufacture high-quality continuous cast materials with a circular cross section for stainless steel.With this background, hot extruded pipes can be directly manufactured from continuous cast stainless steel materials. Things started to happen. When continuous casting is used as the raw material for hot extruded pipes, the hot blooming process becomes unnecessary, and the manufacturing cost of hot extruded pipes is significantly reduced.

However, when the continuously cast material is stainless steel, it is known that streak-like defects occur on the surface of the hot extruded tube, significantly reducing the product value.
For example, Japanese Patent Publication No. 61-49570 discloses a measure in which a hot extrusion pipe G is first subjected to a soaking treatment at a temperature of 1200° C. or more to a continuously cast material, and then subjected to light processing.

When a continuous casting material used as a hot extruded pipe material is subjected to a soaking treatment, the amount of δ ferrite is reduced and micro-segregation is prevented, and by applying light processing to this material, a finely textured layer is formed on the surface layer, and the As a result, the occurrence of streak defects can be suppressed in the same way as when hot extrusion pipe-making is performed from a steel billet obtained by hot blooming.

[Problems to be Solved by the Invention] However, in this countermeasure, the soaking time is determined by the diameter of the continuous casting material, and the larger the diameter of the continuous casting material, the longer the soaking process is required. As a result, in the case of a continuously cast material having a diameter of 300 mm, a soaking treatment lasting at least 6 hours is required, resulting in a significant deterioration of thermoeconomic efficiency.

Further, even if such a long-time soaking treatment is performed, there is a possibility that the structure may not be completely refined down to the flesh quality depending on the degree of processing in light processing.

If the structure is not refined down to the internal quality, streak-like defects will occur at the tip of the hot extruded tube, where the tip surface of the continuously cast material becomes the outer surface.

The present invention was made in consideration of such circumstances, Kr = 7j,
The purpose is to provide a method that can produce a hot extruded stainless steel pipe free of streaks over its entire length at a low cost, regardless of the material diameter.

[Means for solving the problem] For suppressing streak-like defects that occur on the surface of hot extruded stainless steel pipes, it is effective to perform diameter reduction processing by die punching on continuous casting material prior to hot extrusion pipe manufacturing. The present applicant previously proposed this (Japanese Patent Application Laid-open No. 6゜221121). According to die punching and diameter reduction processing, it is relatively easy to ensure a degree of processing sufficient to refine the crystal grains to a fleshy quality due to the heating during hot extrusion pipe making, and the roundness of the continuously cast material is increased. Another advantage is that the cutting allowance on the outer surface can be reduced in the billet collection process for preparing billets for hot extrusion.

The present inventors have obtained the following knowledge in the process of investigating the crystal grain refinement effect when die punching and diameter reduction processing is performed on a continuously cast material that has undergone soaking treatment.

By performing hot processing on continuous cast materials, such as die punching and diameter reduction processing, processing strain reaches the flesh quality with a relatively small reduction blade. When applying soaking to a continuous cast material undergoing hot working, the longer the soaking time is, the better. is 300mm
Even if the continuous casting material has a large diameter exceeding , the soaking treatment may be performed for 2 to 3 hours, and the soaking treatment for more than 6 hours is not necessary.

In other words, for continuous casting materials with a large diameter exceeding 300 mm,
When soaking for 3 hours, δ after treatment
Although a large amount of ferrite remains, even in such a state, if the steel is transferred to hot working without being allowed to cool, the amount of δ ferrite will be sufficiently reduced at the end of hot working.

The present invention was made based on this knowledge, and after subjecting a continuously cast material made of stainless steel to a soaking treatment at 1250°C or higher for 2 hours or more and less than 3 hours, the temperature was lowered to 1150°C while maintaining a soaking state. The gist of the following is a method for producing a hot extruded stainless steel pipe, which is characterized by hot working at a temperature of 950°C or higher with a reduction in area of 20% or more, and then hot extruding the pipe as it is or by cutting the outer surface. .

[For production]

FIG. 1 is a graph showing heat patterns before hot extrusion tube manufacturing for the method of the present invention and the conventional method.

When the diameter of continuous casting material is 300 mm, JP-A-63-4
In the conventional method disclosed in Japanese Patent No. 9570, the soaking time is 6 to 8 hours. On the other hand, in the method of the present invention, the soaking time is 2 hours or more and less than 3 hours, and the soaking transition time for lowering the temperature to the hot processing temperature while maintaining the soaking state is also usually 1 to 2 hours. As a result, the total heating time for 11 times is shortened to 3 to 5 hours. Moreover, the heating temperature in the soaking transition treatment is 100 to 300° C. lower than the soaking temperature. Therefore, in the method of the present invention, heating costs are significantly reduced, and this effect becomes more pronounced as the diameter of the continuous casting material increases.

Figure 2 shows continuous casting material with a diameter of 200 mm at 1250℃
2. After soaking for 75 hours, the temperature was lowered while maintaining the soaking state, and hot working was performed at 1000°C. This is a chart showing a comparison with the case (marked with a *). The hot working was 20-rule rolling (section reduction rate: 20%). By lowering the temperature while maintaining the soaking state after soaking and adding hot working, even if the soaking time is 2.75 hours, the amount of δ ferrite after hot working can be reduced by 4 to 6 hours after soaking. It will decrease to about the same level as if you continued.

The reason why the soaking temperature in the method of the present invention is set to 250°C or higher is that if the temperature is lower than 1250°C, a large amount of δ ferrite remains after the soaking process, and even if it is carried over to hot processing by soaking transfer, streak-like This is because surface defects occur. The upper limit of the soaking temperature is not particularly limited, but from the viewpoint of thermoeconomics, it is desirable that the soaking temperature does not significantly exceed 1250°C.

If the soaking time is less than 2 hours, streak-like surface defects can be prevented as in the case where the soaking temperature is less than 1250°C, and if the soaking time is more than 3 hours, the continuous casting material may fail. Since heating is necessary, the heating time was set to 2 hours or more and less than 3 hours.

FIG. 3 is a chart showing the degree of influence of temperature and area reduction rate on grain size during hot working.

The crystal grain size represents the grain size in the central structure of a continuously cast material that has undergone hot working. According to the same figure, the crystal grain size is 1
．． It can be seen that in order to obtain fine grains of less than 0 mm, a low reduction in area is required when the processing temperature is low, but as the processing temperature increases, a higher reduction in area is required.

As for the hot working temperature, if it exceeds 1150℃, a high area reduction rate is required, so the reduction blade during hot working becomes large, and if it is below 950℃, the necessary area reduction rate is low, but the slab deforms. As a result, the reduction edge during hot working increases. Therefore, the hot working temperature is 11
The temperature was 50°C or lower and 950°C or higher.

The reason why the area reduction rate during hot working was set to 20% or more was because
This is because if the processing strain is less than 20% in the temperature range of 1150° C. or lower and 950° C. or higher, the processing strain will not reach the flesh quality, and streak-like surface defects will occur at the tip of the hot extruded alloy steel pipe.

The soaking transition time from the soaking process to the hot working temperature while still in the soaking state is 0.25 to 0.0,000,000,000
The soaking time is about 5 hours, and the soaking time is about 0°5 to 1.5 hours (see Figure 1).

Examples of hot working include rolling, forging, spinning, die punching, and diameter reduction.

The material of continuous casting material is 5US304, 5US32.
1,5US310,5US347,5tJ3410.5
Stainless steel such as US430 can be mentioned.

(Example) Examples of the present invention will be described below in comparison with comparative examples and conventional examples.

Made of 5US304.5us321 with a diameter of 188mm
, 193 mm, 199 mm, and 206 mm were subjected to soaking treatment, then cooled to the hot working temperature in a soaking state, and hot worked to a diameter of 178 mm with an area reduction rate of 10%, 15%, 20%, and 25%, respectively. Processed (20-mill rolling). Outer diameter 173mm, inner diameter 35m from each hot-processed continuous casting material
Billets with a length of 66.9 mm and a length of 66.9 mm were taken, and each billet was hot extruded at a tube forming temperature of 1250°C to form a tube with an outer diameter of 40 mm.
A stainless steel tube with a diameter of 5 mm, an inner diameter of 32 mm, and a wall thickness of 4.25 mm was used.

Table 1 shows the results of investigating the δ ferrite 1-ft after soaking treatment, the δ ferrite amount after hot working, and the crystal grain size for each manufactured stainless steel pipe. For comparison, Table 1 also shows the results obtained when light processing was performed after long soaking and cooling.

As is clear from Table 1, the conventional method can suppress streaks on hot extruded pipes by soaking for 5 hours, but the method of the present invention achieves the same results with soaking for 2 to 2.15 hours. The heat treatment time is only 3.05 to 44 hours, including soaking transition.

Table 2 shows that the diameters of continuous casting materials are 366, 377, and 38.
The results are shown for cases of 8 mm, 400 mm, and 415 mm. These are area reduction rates of 10%, 15%, 20%, and 2, respectively.
The diameter was unified to 347 mm by 5% and 30% hot working (20-roll rolling).

The hot extrusion tube manufacturing conditions are fin and throat dimensions of 338 mm outside diameter.
X inner diameter 80mm, length 802mm, pipe manufacturing temperature 1250
℃, pipe dimensions are outer diameter 124.24 mm, inner diameter '77 m
m, and the wall thickness was 23.62 mm. According to the method of the present invention, 4
200mm even in the case of continuous casting material of 00mm diameter class.
Heat treatment can be completed in about the same time as for continuously cast materials of diameter class, and the energy saving effect increases as the diameter of the continuous cast materials increases.

[Effects of the Invention] As is clear from the above description, according to the method of the present invention, a high quality hot extruded stainless steel pipe without streaks over its entire length can be manufactured by a relatively short soaking treatment. Therefore, the heating cost required for the soaking process is reduced, and the effect is greater as the diameter of the continuously cast material becomes larger.

[Brief explanation of drawings]

Fig. 1 is a graph showing the heat pattern before hot extrusion pipe production for the method of the present invention and the conventional method, Fig. 2 is a chart showing the influence of hot processing following soaking treatment on the amount of δ ferrite 1, FIG. 3 is a chart showing the relationship between processing conditions in hot working following soaking treatment and grain size.

Claims (1)

[Claims] (1) After subjecting a continuous cast material made of stainless steel to a soaking treatment at 1250°C or higher for 2 hours or more but less than 3 hours, the temperature is lowered to 1150°C or lower while maintaining a soaking state.
Hot working with a cross-section reduction rate of 20% or more at 50℃ or higher,
A method for producing a hot extruded stainless steel pipe, which is characterized in that the pipe is then formed by hot extrusion either as it is or by cutting the outer surface.