CN103290188A - Method for controlling martensite/austenite structure in process of straightening WB36 steel pipes - Google Patents

Abstract

The invention relates to a martensite/austenite structure control method in the straightening process of WB36 steel pipes. The WB36 steel is hot-rolled into steel pipes. In order to ensure that the required heating and cooling rates can be achieved, the wall thickness of the pipes is generally required to be no more than 5.42 cm, so as to ensure that the lath martensitic structure is formed after normalizing. After normalizing at 1173K and holding for 30 minutes After treatment, when the steel pipe is cooled to 860-920K, enter the six-roller straightening machine, the straightening speed is 10-15m/min, the straightening force is 5-40MPa, and the steel pipe is straightened and then cooled to room temperature. The smaller the thickness of the elongated martensite/austenite structure in the WB36 steel pipe structure after straightening, the higher the impact performance of the steel pipe. After straightening by the method of the present invention, the room temperature impact energy of the steel pipe is increased from 41J to 96J, improving The amplitude is 234%, which fully shows that temperature and straightening force significantly affect the comprehensive properties of steel pipes, and also shows the versatility and importance of this method in the production process.

Description

Control method of martensite/austenite structure in straightening process of WB36 steel pipe

technical field

The invention belongs to the technical field of Ni-Cu-Mo type low-alloy structural steel production, and particularly relates to a martensite/austenite microstructure control method in the straightening process of a WB36 steel pipe during continuous cooling and straightening at a relatively high speed.

Background technique

WB36 steel (i.e. l5NiCuMoNb5, see Table 1 for chemical composition) is a steel grade in the company standard of German Mannesmann company, which is developed by adding Ni-Cu-Mo alloy on the basis of carbon-manganese steel. Due to its excellent high-temperature mechanical properties and high-temperature oxidation resistance, WB36 is widely used in headers and steam pipes of nuclear power generating units and high-parameter thermal power generating units. The steel is characterized by high strength, the yield limit is 40% higher than that of 20 steel, and the service temperature is 400°C. It can also be used as a high-stress pipe with a pipe wall temperature of 500°C; or as a high-parameter thermal power generation unit, such as supercritical High-temperature and high-pressure components such as boiler drums, steam-water separators, headers, steam generators, steam pipes, and nuclear power equipment in the unit. In the field of nuclear power pipes, the austenitic stainless steel pipes used in the nuclear island primary circuit and a small amount of main cooling pipes and evaporator heat transfer pipes produced by several domestic enterprises with nuclear grade steel pipe production qualifications have long been only imported products. The fill-in-the-miss products do not yet have the ability to supply complete sets. The fundamental reason is: WB36 steel has a complex structure, and the possible structures include ferrite, bainite, martensite/austenite, etc., and lacks a deep understanding of the formation and evolution process of the structure, which leads to the production process. Difficult to achieve fine tissue control. At present, my country's WB36 alloy pipe consumption accounts for only half of the total steel consumption in developed countries. The expansion of the WB36 alloy pipe application field will provide a broad space for the development of the industry. According to the research of WB36 alloy pipe branch of China Special Steel Association, the annual growth rate of demand for high-pressure WB36 alloy pipe in my country will reach 10-12% in the future. Therefore, the development of fine microstructure technology of WB36 hot-rolled seamless steel pipe plays a very important role in improving the core competitiveness of seamless steel pipe products.

Table 1 WB36 steel conventional chemical composition (%)

components Content (wt.%) C ≤0.017 Si 0.25～0.50 mn 0.80～1.20 Cr ≤0.30 Mo 0.25～0.50 Ni 1.00～1.30 Cu 0.50～0.80 Nb 0.015～0.045

N ≤0.020 Al ≤0.050

WB36 seamless steel pipe is generally produced by hot rolling. After hot rolling, there are inevitably longitudinal bending and roundness errors. These defects need to be eliminated by straightening. During straightening, the steel pipe is elastically deformed and plastically deformed to achieve longitudinal straightness and radial roundness requirements. Traditional WB36 seamless steel pipe straightening only considers the improvement of dimensional accuracy and production efficiency, and ignores the influence of straightening force and straightening machine working temperature on the structure and performance of WB36 seamless steel pipe. Studies have shown that: improper straightening process will lead to coarse martensite/austenite structure in the final room temperature structure, thereby significantly deteriorating the impact performance of the steel pipe. Figure 1 shows the relatively coarse black elongated martensite/austenite structure in the room temperature structure after straightening of WB36 seamless steel pipe. If the thickness of the elongated martensite/austenite in the steel pipe structure is reduced by controlling the straightening force and straightening temperature during the straightening process, this will surely optimize the performance of the steel pipe. Considering that the straightening temperature of the steel pipe is generally between 800 and 950K, the appropriate micro compressive stress deformation in this temperature range can promote the precipitation of the ferrite phase and reduce the retained austenite content in the steel. Since the retained austenite will transform into elongated martensite/austenite during the subsequent cooling process, the micro-deformation process in the straightening process can avoid the formation of coarse elongated martensite/austenite, Thereby realizing excellent strength-toughness cooperation, this is also the core of the patent of the present invention.

Contents of the invention

A method for controlling the martensite/austenite structure in the straightening process of WB36 steel pipe according to the present invention aims at ensuring uniformly distributed strips by controlling the straightening force in the straightening process to ensure the straightness and dimensional accuracy of the steel pipe The coarse martensite/austenite structure can reduce the adverse effect of the coarse martensite/austenite structure on the steel properties, thereby improving the comprehensive performance of the steel pipe and promoting its practical application.

In the martensite/austenite structure control method of the WB36 steel pipe straightening process of the present invention, the WB36 steel is hot-rolled into a steel pipe. In order to ensure that the required heating and cooling rates can be achieved, the wall thickness of the pipe is generally required to be no greater than 5.42 cm to ensure After normalizing, it is lath martensitic structure. After normalizing at 1173K and heat preservation for 30 minutes, when the steel pipe is cooled to 860-920K, it enters the six-roller straightening machine. The straightening speed is 10-15m/min, and the straightening force is 5 ~ 40MPa, the steel pipe is straightened and cooled to room temperature.

The refinement method of the elongated martensite/austenite structure in the straightening process of WB36 steel adopted by the present invention is different from the traditional steel pipe straightening process, and the straightening temperature and straightening force significantly affect the length The thickness of the strip martensite/austenite structure, Figure 3 shows the strip martensite in the WB36 steel pipe structure after the 890K straightening force is 5MPa, 10MPa, 15MPa, 20MPa, 25MPa, 30MPa, 35MPa and 40MPa respectively. The thickness of the martensite/austenite structure, the straightening force of 0MPa refers to the traditional straightening process without stress loading direction control, from which it can be found that with the increase of the straightening force, the elongated martensite/austenite The thickness of the austenite structure decreases first and then increases, and the thickness of the martensite/austenite structure is the smallest at 15MPa, which is 80nm. Figure 4 shows the thickness of elongated martensite/austenite structure in WB36 steel pipe structure after straightening at 860K, 870K, 880K, 890K, 900K, 910K and 920K respectively when the straightening force is 15MPa. With the decrease of straightening temperature, the thickness of elongated martensite/austenite structure first decreases and then increases, and the thickness of martensite/austenite structure is the smallest at 890K.

There is a good correspondence between the thickness of the elongated martensite/austenite structure in the WB36 steel pipe structure after straightening and the impact performance of the steel pipe. Figure 5 and Figure 6 respectively show different straightening forces and different straightening temperatures The size of the impact performance of the steel pipe after straightening, generally speaking, the less the thickness of the elongated martensite/austenite structure, the higher the impact performance of the steel pipe. Increased to 96J, the increase rate is 234%, which fully shows that the straightening process parameters, namely temperature and straightening force, significantly affect the comprehensive performance of the steel pipe, and also shows the versatility and importance of this method in the production process.

The room temperature impact energy of foreign WB36 seamless steel pipes is generally above 50J, and the room temperature impact energy of steel pipes produced without the straightening process invented by this patent is 41J, which is obviously lower than that of foreign products. By controlling the temperature and force of the steel pipe straightening, the martensite/austenite structure in the WB36 steel pipe structure is refined, and the impact toughness of the steel pipe is significantly improved, and the product performance index is higher than that of foreign products.

Description of drawings

Fig. 1 Coarse elongated martensite/austenite structure in WB36 seamless steel pipe structure before straightening;

Figure 2 Coarse elongated martensite/austenite structure in the WB36 seamless steel pipe structure after straightening;

Fig. 3 The relationship between the thickness of the elongated martensite/austenite structure and the straightening force in the room temperature structure of WB36 steel pipe after 890K straightening;

Figure 4. The relationship between the thickness of the elongated martensite/austenite structure and the straightening temperature in the room temperature structure of the WB36 steel pipe after straightening at 15 MPa;

Figure 5 The relationship between the impact energy at room temperature and the straightening force after 890K straightening of WB36 steel pipe;

Figure 6. The relationship between the impact energy at room temperature and the straightening temperature after the WB36 steel pipe is straightened at 15 MPa.

Detailed ways

Concrete technical scheme embodiment of the present invention is as follows:

Example 1:

Take WB36 steel pipe with a wall thickness of 2.71 cm. After normalizing at 1173K and holding for 30 minutes, when the steel pipe is cooled to 860K, it enters a six-roller straightening machine; the straightening force is 5MPa, and the steel pipe is straightened and cooled to room temperature.

The thickness of elongated martensite/austenite structure in WB36 steel pipe straightened by this process is reduced from 153nm to 131nm, and the impact energy at room temperature is increased from 32J to 53J.

Example 2:

Take WB36 steel pipe with a wall thickness of 5.42 cm. After normalizing at 1173K and heat preservation for 30 minutes, when the steel pipe is cooled to 920K, enter the six-roller straightening machine; set the parameters of the straightening machine, and the straightening speed is 15m/min. The force is 40MPa, and the steel pipe is straightened and cooled to room temperature.

The thickness of elongated martensite/austenite structure in WB36 steel pipe straightened by this process is reduced from 153nm to 113nm, and the impact energy at room temperature is increased from 32J to 52J.

Example 3:

Take WB36 steel pipe with a wall thickness of 1.36 cm. After normalizing at 1173K and holding for 30 minutes, when the steel pipe is cooled to 900K, enter the six-roller straightening machine; set the parameters of the straightening machine, and the straightening speed is 15m/min. The force is 30MPa, and the steel pipe is straightened and cooled to room temperature.

The thickness of elongated martensite/austenite structure in WB36 steel pipe straightened by this process is reduced from 153nm to 110nm, and the impact energy at room temperature is increased from 32J to 57J.

Example 4:

Take WB36 steel pipe with a wall thickness of 4.07 cm. After normalizing at 1173K and heat preservation for 30 minutes, when the steel pipe is cooled to 890K, enter the six-roller straightening machine; set the parameters of the straightening machine, and the straightening speed is 15m/min. The force is 15MPa, and the steel pipe is straightened and cooled to room temperature.

The thickness of elongated martensite/austenite structure in WB36 steel pipe straightened by this process is reduced from 153nm to 80nm, and the impact energy at room temperature is increased from 32J to 96J.

The invention proposes a method for controlling the martensite/austenite structure in the straightening process of the WB36 steel pipe, which improves the comprehensive performance of the steel pipe by obtaining the elongated martensite/austenite structure. The embodiments have been described, and it is obvious that those skilled in the art can make changes or appropriate changes and combinations to the production methods described herein without departing from the content, spirit and scope of the present invention to realize the technology of the present invention. In particular, it should be pointed out that all similar substitutions and modifications will be obvious to those skilled in the art, and they are all considered to be included in the spirit, scope and content of the present invention.

Claims (1)

1.WB36 steel pipe aligning process martensite/austenite organizational controls method, it is characterized in that with the hot rolling of WB36 steel be steel pipe, tube wall thickness is not more than 5.42 centimetres, after 1173K, 30 minutes normalizing treatment of insulation, when being cooled to 860～920K, steel pipe enters six-roller straightener, aligning speed 10～15m/min, straightening force is 5～40Mpa, is cooled to room temperature behind the steel pipe aligning.

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