WO2014098521A1

WO2014098521A1 - Stainless steel pipe with excellent erosion resistance and manufacturing method thereof

Info

Publication number: WO2014098521A1
Application number: PCT/KR2013/011959
Authority: WO
Inventors: Kwang Yuk Kim; Deok Chan Ahn; Dong Chul Chae
Original assignee: Posco
Priority date: 2012-12-21
Filing date: 2013-12-20
Publication date: 2014-06-26
Also published as: CA2895697A1; CA2895697C; KR101463313B1; KR20140081474A

Abstract

The present invention relates to a stainless steel pipe with excellent erosion resistance, which can improve erosion resistance in the inner area of the pipe while maintaining low-temperature toughness, and a manufacturing method thereof. The stainless steel pipe with excellent erosion resistance according to one embodiment of the present invention is a steel pipe including Cr at 11.0 to 14.0 wt% and having a ferrite factor (FF) of 6 to 9 as expressed by the following formula 1; and based on a thickness of the steel pipe, the inner area has a mixed phase structure of martensite and ferrite, the outer area has a single phase structure of ferrite or a mixed phase structure of ferrite and martensite, and a martensite fraction of the inner area is greater than a martensite fraction of the outer area: [Formula 1] FF = [Cr + 6Si +8Ti + 4Mo + 2Al +4Nb] - [2Mn + 4Ni + 40(C+N)] wherein Cr, Si, Ti, Mo, Al, Nb, Mn, Ni, C and N represent contents (wt%) of respective ingredients.

Description

STAINLESS STEEL PIPE WITH EXCELLENT EROSION RESISTANCE AND MANUFACTURING METHOD THEREOF

The present invention is related to a stainless steel pipe with excellent erosion corrosion resistance and a manufacturing method thereof, and more specifically, to a stainless steel pipe with excellent erosion resistance, which improves erosion resistance in the inner area of the pipe while maintaining low temperature toughness, and a manufacturing method thereof.

Generally, stainless steels are classified according to chemical components or metal structures thereof. According to the metal structures, the stainless steels are classified into an austenitic type (300 series), a ferritic type (400 series), a martensitic type and a duplex type.

Of these stainless steels, the ferritic type stainless steels are broadly used as kitchen utensils, electric equipments, materials for automobiles, internal and external materials for construction due to their excellent formability, corrosion resistance and its relatively low cost.

However, since the ferritic type stainless steels have ferritic phase structures at room temperature, they have limits in using as a structual materials since they do not show neither a high strengthening phenomena caused by phase transformation, which can be observed at the austenitic type stainless steel, nor a strengthening phenomena between two phases, which can be observed at duplex stainless steels. Furthermore, ferritic type stainless steels also exhibit a phenomenon of showing brittleness at a low temperature (a ductile brittle transition temperature (DBTT)) and its effect becomes more critical in the cases of heavy gauges (referred to as a thick steel plate). On the other hand, grain growth upon welding operation might also affect the deterioration of ductility and strength of the material.

Due to the characteristics, the ferritic type stainless steels have been highly restrictively used as a structural material.

However, the ferritic stainless steels have inherent cost benefit since they contain relatively low contents of expensive elements such as nickel, molybdenum and the like compared to other stainless grades.

Consequently, if the ferritic type stainless steel can be used as a structural material, users can widen the choice of material selection as well as saving the global material resources.

For the ferritic type stainless steels, in order to be used as a structural material having strength, weldability and DBTT features, the optimum alloy design as well as manufacturing process control has to be controlled.

In the case of the alloy design, only ferritic single phase is not enough to get high strength as well as problems of grain size coarsening upon welding process. Accordingly, to be used as a structural material, microstructural changes such as mixed phases has to be followed to overcome the handicap.

Also, likewise carbon steels, the strength of ferritic type stainless steel is proportionly increased upon the increase of reduction ratio. The behavior is not much exception even though mixed phase concept is introduced in the ferritic type stainless steels. However, the change of heat treatment condition for mixed phase alloy might be more tricky than the single phase alloy that the quality features can be affected much. Consequently, to be a competitive structural material having both inherent strength and cost benefits, both alloy design and microstructural features has to be optimized in this ferritic type stainless steels.

On the other hand, pipes for oil and gas treatment facilities face lots of wear issues. Particularly, when producing heavy oil such as oil sand, wear issue of transport pipes becomes critical due to diverse slurry movements. In other words, in the process of separating the bitumen from rock or sand ores, the movements of hot water mixed slurries normally accelerate the erosion corrosion damages of pipes that it should be minimized to save the maintenance cost. Accordingly, for these pipes, the erosion corrosion characteristics of pipe inner side becomes a hot issue.

Therefore, generally, as the pipes for transferring the oil sand slurry, steel pipes are widely used. Particularly, the steel pipe itself has good abrasion resistance, but a technique for hardening the inner circumferential surface of the steel pipe by heat treatment has been suggested and used according to needs for higher abrasion resistance than that of the inner side of the steel pipe.

As for an example, hardening technique of the inner surface of the steel pipe by heat treatment is disclosed in detail in "Method and apparatus for quenching inner surface of steel pipe (Japanese Patent Laid-Open Publication No. 2002-60834; Patent Reference 1)." Patent Reference 1 discloses a technique for heating a steel pipe by induction heating from outside of the steel pipe till the entire thickness reaches a target temperature, and rapidly quenching the pipe inner surface by spraying waters, which hardens the inner surface of the pipe but not much notable hardening at the outer surface.

The method disclosed in Patent Reference 1 is a technique applied to carbon steel pipe. Here, the amount of water and quenching rate at the pipe inner surface has to be well controlled to get the uniform microstructures.

On the other hand, since the heavy oil normally forms a toxic condition due to the containing of chloride ions or organic acids in the slurries, the material for these pipe also requires good corrosion resistance in this environment. It is one of the critical reason that economical stainless steel is needed in this industrial field. Also, since the environment is harsh in the sense of temperature and wear condition, both toughness at low temperature and good erosion resistance of the material is required.

Accordingly, the present inventors have completed the present invention based on the fact that dual phase utility ferritic stainless steel pipe can improve erosion corrosion resistance by way of induction heating method.

The description provided above as a related art of the present invention is just for helping to understand the background of the present invention and should not be construed as being included in the related art known by those skilled in the art.

The present invention has been made in an effort to solve the above-described problems associated with the prior art. The present invention is objected to provide a stainless steel pipe with excellent erosion resistance, wherein erosion resistance of the inner area of the pipe is improved while maintaining corrosion resistance and low temperature toughness, and a manufacturing method thereof.

Particularly, in order to improve the erosion resistance of the inner area of the pipe, the microstructure of pipe material has to be composed of dual phases combined with martensite and ferrite phases rather than ferritic single phase. The present invention provides a stainless steel pipe with excellent erosion resistance, wherein a mixed phase structure partially containing a martensite phase rather than a ferrite single phase is formed, and a manufacturing method thereof.

In order to accomplish the above-described objects, the stainless steel pipe with excellent erosion resistance upon the embodiment of the present invention is characterized in that it is a steel pipe including Cr at 11.0 to 14.0 wt% and having a ferrite factor (FF) of 6 to 9 as expressed by the following formula 1, wherein, based on a thickness of the steel pipe, the inner area has a mixed phase structure of martensite and ferrite, the outer area has either ferrite phase structure or a mixed phase structure of ferrite and martensite, and a martensite fraction of the inner area is greater than a martensite fraction of the outer area:

[Formula 1]

FF = [Cr + 6Si +8Ti + 4Mo + 2Al +4Nb] - [2Mn + 4Ni + 40(C+N)]

wherein Cr, Si, Ti, Mo, Al, Nb, Mn, Ni, C and N represent contents (wt%) of respective components.

The stainless steel pipe is characterized in that it includes C at 0.01 to 0.05 wt%, N at 0.01 to 0.05 wt%, Ti at 0.25 wt% or less, and Si at 0.5 wt% or less.

The stainless steel pipe is characterized in that it further includes Nb at 0.25 wt% or less, wherein the sum of the contents of Ti and Nb is 0.25 wt% or less.

The stainless steel pipe is characterized in that it has a thickness of 8 to 30 mm.

On the other hand, a manufacturing method of the stainless steel pipe with excellent abrasion resistance according to one embodiment of the present invention is characterized in that it includes rapidly heating the inner area of the stainless steel pipe, which comprises Cr: 11.0 to 14.0 wt% and has a ferrite factor (FF) of 6 to 9 as expressed by the following formula 1, to a temperature of the inner area higher than a Ac1 transformation temperature and heating the outer area of the stainless steel pipe to a temperature of the outer area lower than the Ac1 transformation temperature; and cooling the stainless steel pipe to form a mixed phase structure of martensite and ferrite in the inner area of the stainless steel pipe:

[Formula 1]

FF = [Cr + 6Si +8Ti + 4Mo + 2Al +4Nb] - [2Mn + 4Ni + 40(C+N)]

wherein, Cr, Si, Ti, Mo, Al, Nb, Mn, Ni, C and N represent contents (wt%) of respective components.

The stainless steel pipe is characterized in that it includes C at 0.01 to 0.05 wt%, N at 0.01 to 0.05 wt%, Ti at 0.25 wt% or less, Nb at 0.25 wt% or less, Si at 0.5 wt% or less, Ni at 2.0 wt% or less, Mo at 2.0 wt% or less, Al at 0.1 wt% or less, Mn at 2.0 wt% or less, the balance of Fe and inevitable impurities, and the sum of the contents of Ti and Nb is 0.25 wt% or less.

The inner area of the stainless steel pipe is characterized in that it is rapidly heated by induction heating.

The inner area of the stainless steel pipe is characterized in that it is heated by induction heating to a temperature of 100 to 300℃ higher than the Ac1 transformation temperature.

The temperature of the outer area of the stainless steel pipe heated by induction heating is characterized in that it maintained at a temperature lower than the Ac1 transformation temperature, and a difference in temperature between the inner area and the outer area of the stainless steel pipe has to be satisfied in a range of 100 to 300℃.

The cooling of the stainless steel pipe is characterized in that it includes the cooling of the stainless steel pipe includes air-cooling or water spray cooling the outer area of the stainless steel pipe.

On the other aspect, a manufacturing method of the stainless steel pipe with excellent erosion resistance upon the embodiment of the present invention is characterized in that it includes entering an induction heating device into an inner hollow portion of the stainless steel pipe, which has an area in which a ferrite phase and an austenite phase coexist, at a temperature higher than a Ac1 transformation temperature; rapidly heating the inner area of the stainless steel pipe based on the thickness thereof by induction heating by applying electricity to the induction heating device; cooling the inner area of the stainless steel pipe to form a mixed phase structure of martensite phase and ferrite phase in the inner area of the stainless steel pipe.

The stainless steel pipe is characterized in that when the stainless steel pipe is heated by the induction heating, the stainless steel pipe has a temperature gradient at which the temperature gradually decreases from the inner area to the outer area based on the thickness thereof, and the temperature of the inner area of the stainless steel pipe reaches above the Ac1 transformation temperature and the temperature of the outer area of the stainless steel pipe does not reach the Ac1 temperature.

The inner area of the stainless steel pipe is characterized in that it is heated by induction heating to a temperature of 100 to 300℃ higher than Ac1 transformation temperature, and the outer area of the stainless steel pipe is maintained at a temperature lower than the Ac1 transformation temperature.

The cooling of the stainless steel pipe is characterized in that it includes air-cooling or water spray cooling the outer area of the stainless steel pipe.

According to the embodiments of the present invention, a low chromium-containing ferritic type stainless steel pipe with excellent corrosion resistance and low-temperature toughness can be used to ensure erosion resistance required upon transport of slurries

FIG. 1 is a phase diagram of a low-carbon ferritic type stainless steel as a material of the present invention according to the Cr content;

FIG. 2 is a graph showing changes in temperature of the materials according to heat treatment temperature and heat treatment time;

FIG. 3 is an image of a microstructure of the low carbon ferritic type stainless steel as the material of the present invention before heat treatment;

FIG. 4 is an image of a microstructure of the low carbon ferritic type stainless steel as the material of the present invention after heat treatment at 720℃;

FIG. 5 is an image of a microstructure of the low carbon ferritic type stainless steel as the material of the present invention after heat treatment at 780℃;

FIG. 6 is an image of a microstructure of the low carbon ferritic type stainless steel as the material of the present invention after heat treatment at 800℃;

FIG. 7 is an image of a microstructure of the low carbon ferritic type stainless steel as the material of the present invention after heat treatment at 870℃;

FIG. 8 is a graph showing the relationship of the erosion resistance depending upon the change of materials hardness; and

FIG. 9 is a graph showing the relationship of low-temperature impact toughness depending upon changes in heat treatment temperatures.

Hereinafter, the embodiments of the present invention will be described in detail with reference to the accompanying drawings.

FIG. 1 is a phase diagram of a low-carbon ferritic type stainless steel as a material of the present invention according to the Cr content, FIG. 2 is a graph showing changes in temperature of the materials according to heat treatment temperature and its time, FIG. 3 is an image of a microstructure of the low carbon ferritic type stainless steel as the material of the present invention before heat treatment, FIG. 4 is an image of a microstructure of the low carbon ferritic type stainless steel as the material of the present invention after heat treatment at 720℃, FIG. 5 is an image of a microstructure of the low carbon ferritic type stainless steel as the material of the present invention after heat treatment at 780°, FIG. 6 is an image of a microstructure of the low carbon ferritic type stainless steel as the material of the present invention after heat treatment at 800℃, FIG. 7 is an image of a microstructure of the low carbon ferritic type stainless steel as the material of the present invention after heat treatment at 870℃, FIG. 8 is a graph showing the relationship of the erosion resistance according to changes in hardness of the materials, and FIG. 9 is a graph showing the relationship of low-temperature impact toughness depending upon a change in temperature.

In order to improve the erosion resistance of an inner part (inner area based on thickness) of a steel pipe manufactured with a ferritic type stainless steel, which has a dual phase microstructure and excellent corrosion resistance and low-temperature toughness characteristics, in the present invention, the inner area of the stainless steel pipe is induction heated to form enough volume of martensite phase at the inner area.

The material of the present invention is a low Cr ferritic type stainless steel containing Cr at 11 to 14 wt%, and a steel pipe type steel, which has a dual phase structure (ferrite + tempered martensite) at room temperature to ensure strength and ductility. Generally, this type of steel is called a "utility ferritic stainless steel (hereinafter, referred to as a 'UF stainless steel')" is used.

As shown in FIG. 1, the UF stainless steel has an area in which a ferrite phase and an austenite phase coexist at a temperature of 800℃ or higher. The austenite phase can increase the hardness of the material by transforming into the martensite phase if cooled to the room temperature except a very slow cooling condition.

In order to confirm this fact, FIG. 2 represents real heat treatment conditions at various temperatures, and FIGS. 3 to 7 are images showing microstructures before and after heat treatment at 720℃, 780℃, 800℃ and 870℃, respectively. As shown in FIG. 6, it could be confirmed that the martensite phase appeared at a temperature of 800℃ or more, and thereby the hardness increased. On the other hand, as shown in FIG. 8, it could be confirmed that the erosion resistance of the UF stainless steel is increased in proportion to the material hardness.

Further, FIG. 9 represents the low-temperature impact toughness according to a change in temperature. In FIG. 9, it could be confirmed that impact toughness was rapidly decreased when the martensite phase was produced at a high temperature, higher than the Ac1 transformation temperature. This deterioration of the impact toughness is a general phenomenon as long as the martensite phase is produced all through the thickness of the steel pipe material. However, if the microstructure is formed differently depending on the thickness, the situation may be changed. This is related with the fact that generally overall ductility of the plate tend to be more influenced by the ductile phase part rather than hard phase part, maintaining similar ductiltiy of the ductile phase only feature.

The UF steel is a steel characterized in that the ferrite phase and the austenite phase coexist at a temperature of 800℃ or higher in terms of stability, and the ferritic type stainless steel pipe according to one embodiment of the present invention includes Cr at 11.0 to 14.0 wt%, C at 0.01 to 0.05 wt%, N at 0.01 to 0.05 wt%, Ti at 0.25 wt% or less (exclusive of 0 wt%), Nb at 0.25 wt% or less (exclusive of 0 wt%), Si at 0.5 wt% or less (exclusive of 0 wt%), Ni at 2.0 wt% or less (exclusive of 0 wt%), Mo at 2.0 wt% or less (exclusive of 0 wt%), Al at 0.1 wt% or less (exclusive of 0 wt%), Mn at 2.0 wt% or less (exclusive of 0 wt%), the balance of Fe and inevitable impurities, and the sum of the contents of Ti and Nb is in a range of 0.25 wt% or less.

The reasons of restricting the compositional range of each element as described above will be described in detail.

It is preferred that the Cr content is in a range of 11.0 to 14.0 wt%. When the Cr content is less than 11 wt%, it may be difficult to form an autogenous protecting film due to degraded corrosion resistance and oxidation resistance, and when it excesses 14 wt%, the ductility may be lowered and the production cost may increase.

It is preferred that the C content is in a range of 0.01 to 0.05 wt%. When the C content is less than 0.01 wt%, the hardness and the strength of the material may be lowered, and when it excesses 0.05 wt%, the probabilities of deterioration of the low-temperature toughness and the occurrence of intergranular corrosion may increase.

It is preferred that the N content is in a range of 0.01 to 0.05 wt%. When the N content is less than 0.01 wt%, the hardness and the strength of the material may be lowered, and when it excesses 0.05 wt%, the deterioration of the low-temperature toughness may be intensified.

It is preferred that the Ti content is in a range of 0.25 wt% or less (exclusive of 0 wt%). When the Ti content excesses 0.25 wt%, surface defects may be caused and the probability of lowering the low-temperature toughness may be increased due to formation of TiN precipitates in the material.

It is preferred that the Nb content is in a range of 0.25 wt% or less (exclusive of 0 wt%). When the Nb content excesses 0.25 wt%, the material cost may be increased, and the probabilities of lowering the low-temperature toughness may be increased due to Nb(C, N) precipitates.

It is preferred that the Si content is in a range of 0.5 wt% or less (exclusive of 0 wt%). When the Si content excesses 0.5 wt%, the ferrite factor (FF) may increase so as to soften the material.

It is preferred that the Ni content is in a range of 2.0 wt% or less (exclusive of 0 wt%). When the Ni excesses 2.0 wt%, the production cost of the material may increase and the probability of the low-temperature toughness feature may be affected much by reducing the ferrite factor (FF), which will be described later.

It is preferred that the Mo content is in a range of 0.3 wt% or less (exclusive of 0 wt%). When the Mo content excesses 0.3 wt%, the production cost of the material may be increased and the low-temperature toughness may be degraded due to formation of Mo precipitates.

It is preferred that the Al content is in a range of 0.1 wt% or less (exclusive of 0 wt%). When the Al content excesses 0.1 wt%, the probability of nozzle clogging may be increased during continuous casting process and also surface defects may be issued due to AlN precipitates.

It is preferred that the Mn content is in a range of 2.0 wt% or less (exclusive of 0 wt%). When the Mn content excesses 2.0 wt%, the quality feature of welding area may be degraded due to increased formation of MnS precipitates.

In the ferritic type stainless steel of the present invention, the Ti and the Nb, which have higher affinity to C and N than Cr, were added as a dual stabilization purpose, in order to reduce the sensitivity of intergranular corrosion caused by precipitation of chrome carbides at the grain boundary upon welding. The Ti can be used alone to obtain the effect of improving the intergranular corrosion, but when adding Ti only to a large quantity, large inclusions such as TiN, TiC and the like may be easily formed, thereby causing the surface defects due to the presence of Ti stripes upon rolling. Accordingly, it is preferred to add both Ti and the Nb as for a dual stabilization. Here, the sum of the Ti and Nb contents is recommended not to be higher than 0.25 wt%. Summerizing it, the reason of limiting Ti and Nb contents upto 0.25 wt% or less is that the content is enough to prevent the intergranular corrosion of the material considering the carbon contents, and too much Ti and Nb addition may cause the occurrence of surface defects and degeneration of the low-temperature toughness.

On the other hand, it is preferred to control the compositional range of the stainless steel pipe according to the present invention, having the ferrite factor (FF) of 6 to 9, as expressed by the following Formula 1 .

[Formula 1]

FF = [Cr + 6Si +8Ti + 4Mo + 2Al +4Nb] - [2Mn + 4Ni + 40(C+N)]

wherein Cr, Si, Ti, Mo, Al, Nb, Mn, Ni, C and N represent contents (wt%) of respective composition.

The term ferrite factor (FF) refers to a ratio of ferrite stabilizing elements and austenite stabilizing elements of a material, the ferrite stabilizing elements such as Cr, Si and the like and the austenite stabilizing elements such as Mn, Ni and the like. If the contents of the austenite stabilizing elements are increased, the ferrite factor (FF) value become smaller and the martensite content is increased, which increases the material hardness under the control of the heat treatment conditions.

Accordingly, it is preferred to control the ferrite factor (FF) to 6 to 9, and it is more preferred to satisfy the range of 8 to 9. The reason is that when the ferrite factor (FF) is less than 6, the low-temperature toughness may not be maintained to the desired level because the amount of the martensite increases after the heat treatment, and when the ferrite factor (FF) is over 9, the erosion resistance may not be maintained to the desired level because the amount of the martensite decreases even after the heat treatment.

As seen from the above-described ferrite factor (FF)-related formula, the UF stainless steel is characterized that it can control the microstructure at high temperature range of heat treatment reflecting a proper combination of a ferrite stabilizing element group such as Cr and an austenite stabilizing element group such as Ni. Namely, when the material is heated to the higher temperature, the austenite phase starts to be stabilized at a certain temperature, and this temperature is called as a Ac1 transformation temperature. When cooled in the air after being kept at a temperature higher than this temperature for a certain period, the produced austenite phase is characterized in that it is transformed to a hard martensite phase. As such, the formation range of austenite phase is called Gamma loop in phase diagram, and if the material temperature reaches in this range and cooled either air or water atmosphere, hardness of the material is increased due to the formation of martensite phase.

However, there is a tendency that as the hardness of the material increases, the toughness of the material may be deteriorated in inverse proportion to an increase in hardness of the material. Accordingly, for the steel material used for a steel pipe for transporting oil sand slurry, the erosion resistance is important, but the hardness may not be blindly increased because the low-temperature toughness should be considered.

Therefore, in this embodiment, an induction heating method was applied in order to improve the erosion resistance by increasing the hardness of the inner area through which the slurry is transported, based on the thickness of the steel pipe, and to secure the low-temperature toughness by controlling the outer area relatively softly. Here, it is also because the UF stainless steel is a steel whose metallurgical characteristic is very suitable for the application of the induction heating method as mentioned above.

In other words, in order to improve the erosion resistance of the inner area based on the thickness of the stainless steel pipe having enough ductility through the previous primary heat treatment at a temperature lower than the Ac1 transformation temperature, an additional heat treatment is needed on the inner area based on the thickness of the stainless steel pipe. This heat treatment can be embodied by various methods, but here the solution was proposed by the induction heating method. In further detail, induction heating is directly applied to the inner area, whose hardness is needed to increase, at a temperature higher than the Ac1 transformation temperature, but in order to maintain the ductility of the material as it is, when induction heating the inner area, the stainless steel pipe has to have a temperature gradient at which the temperature gradually decreases from the inner area to the outer area based on the thickness thereof, and the temperature of the outer area based on the thickness of the stainless steel pipe is kept at a temperature lower than the Ac1 transformation temperature.

Preferably, the temperature of the inner area of the stainless steel pipe heated by the induction heating is rapidly heated to a temperature of 100 to 300℃ higher than the transformation point Ac1, and then cooled in the air or water spray condition. The temperature of the outer area of the stainless steel pipe is kept at a temperature lower than the Ac1 transformation temperature having a difference in temperature of 100 to 300℃ or more between the outer area and the inner area of the stainless steel pipe.

However, in the case of the thin material, the difference in temperature between the inner and outer faces may not be easily realized by the heat treatment method, but in the case of thick plate having a thickness of 8 mm or higher, it is more easily realized by the heat transfer gradient in the thickness. For this reason, in this embodiment, it is preferred to limit the thickness of the stainless steel pipe to a range of 8 mm or more. However, in order to have the preferable temperature gradient between the inner area (a part whose temperature reaches the Ac1 transformation temperature) and the outer area (a part whose temperature does not reach the Ac1 transformation temperature) of the stainless steel pipe, it is preferred that the minimum thickness of the stainless steel pipe is 8 mm or more and the maximum thickness thereof is 30 mm or less.

In this embodiment, in order to induction heating the inner area of the stainless steel pipe, an induction heating device is inserted into an internal hollow portion of the stainless steel pipe, and electricity is applied to the induction heating device followed by rapidly heating the inner area of the stainless steel.

To control the temperature between inner and outer area of the pipe, with a time gap of 0~10 second, air or water spray cooling might be followed along either pipe inside or pipe outside just after the narrow band area of the indution heating. In this case, a mixed phase of martensite and ferrite is formed in the inner area of the stainless steel pipe, while a ferrite single phase structure or a mixed phase structure of the ferrite and the martensite is formed in the outer area. Following this step, to be sure, the martensite fraction of the inner area will be higher than the martensite fraction of the outer area.

When the stainless steel pipe is being heated by the induction heating, the stainless steel pipe has a temperature gradient at which the temperature gradually decreases from the inner area to the outer area based on the thickness thereof, and the temperature of the inner area of the stainless steel pipe reaches above the Ac1 transformation temperature but the outer area of the stainless steel pipe should not much higher(less than 100℃) than the Ac1 transformation temperature.

In the case of the UF stainless steel, when the temperature is raised to a temperature higher than the Ac1 transformation temperature as described above, the austenite phase is observed in the steel structure, and the amount of formation is correlated with the heat treatment temperature and holding time. Namely, when the holding time is as short as several seconds, the phase may be slightly transformed at a temperature higher than the Ac1 transformation temperature, and only if when the temperature and the holding time satisfy at least certain optimum conditions, sufficient martensite phase formation becomes satisfied upon cooling. When the formation of the martensite phase is activated as described above, the hardness of the material should be increased, thereby enhancing the erosion resistance of the material.

Of course, as described in the suggested embodiments, the method is not limited to induction-heating the inner area by inserting the induction heating device into the internal hollow portion of the stainless steel pipe, but various method, which can partially heat only the inner area of the stainless steel pipe, can be embodied with changes and modifications

On the other hand, the emplementation activity of UF stainless steel induction heating can be embodied as the following. The Ac1 transformation temperature of the UF stainless steel at quasi-static condition having the above-described components(FF: 6~9) is around 800℃. Considering that the induction heating method is more dynamic condition, the temperature control of induction heating method has to be differently applied. Namely, Table 1 shows the hardness variation after sufficient soaking time but it is difficult to obtain the same microstructure after dynamic induction heating method.

Table 1

Soaking temp.	Y.S(Mpa)	T.S(Mpa)	E`l(%)	Hardness(HRB)
25℃(As-rolled)	814.4	926.4	17.3	99.7
720℃	591.4	662.2	28.0	91.2
740℃	414.5	538.6	33.9	79.6
760℃	323.9	479.1	39.3	79.0
780℃	350.2	503.3	37.6	81.9
800℃	404.1	541.6	34.6	86.9
830℃	467.2	599.1	30.4	94.4
850℃	508.5	635.4	27.8	96.6
870℃	548.2	671.2	25.4	98.5

Thus, considering that the induction heating method is a rapid heating, the optimum temperature ranges to get enough phase transformation are to be conducted in the range of 100 to 300℃ higher than the Ac1 transformation temperature, and then the cooling is conducted either in the air or water spray condition. Consequently, the difference in temperature between the inner area, to which the induction heating is applied, and the outer area of the stainless steel which is in an opposite direction of the inner area based on the thickness, should be 100 to 300℃ or higher, as described above.

Hereinafter, the effects of maintaining the low-temperature toughness while improving the hardness of inner pipe will be proved using the UF stainless steel by way of induction heating method as suggested above.

(Execution Example)

The current investigation was conducted to see the changes of hardness and mechanical characteristics depending on heat treatment temperatures with a UF steel plate including Cr at 11.0 to 14 wt%(gauge is 12mm).

The Ac1 transformation temperature of the corresponding material having a ferrite factor of 6 to 9 was around 800℃. In this Execution Example, in order to increase the hardness of the inner part of the stainless steel, the induction heating was conducted on the inner area of the stainless steel pipe, heating the inner part to a temperature higher than the Ac1 transformation temperature. Then, the outer area of the stainless steel pipe was kept at a temperature lower than the Ac1 transformation temperature. In this case, the outer area of the stainless steel pipe showed an initial soft microstructure, thereby maintaining the similar low-temperature toughness compared to the material tested before the induction heating.

The induction heating used for the inner area of the stainless steel pipe was a rapid heating method, and the heated area had such a characteristic that it was relatively rapidly cooled after the rapid heating. Therefore, compared to the quasi-static heating condition, the effective temperature for phase transformation in this condition has to be quite higher than the Ac1 transformation temperature. Accordingly, the actual induction heating condition should be at least 100℃ higher than the Ac1 transformation temperature.

Then, the formation of martensite phase in the inner area of the stainless steel pipe was activated, and an example of hardness variation is shown in the following Table 2.

Table 2

Induction heating temp.	Hardness(HRB)
800℃	190
850℃	200
900℃	220
950℃	250
1000℃	270
1050℃	> 270

Compared with the hardness of Table 1, which was obtained by normal heat treatment in a furnace, it could be confirmed that there was a difference in temperature of approximately 100 to 300℃.

The following Table 3 shows low-temperature toughness data of the materials used in induction heating treatment. Here, it could be confirmed that the difference in low-temperature toughness values was not distinct compared to the materials used in normal soaking heat treatment condition at 750℃ as shown in FIG. 9.

Table 3

Usage temp.	Toughness(CVN, J)
20℃	> 60
0℃	> 40
-20℃	> 30
-40℃	> 20

The invention has been described in detail with reference to preferred embodiments thereof. However, it will be appreciated by those skilled in the art that changes or modifications may be made in these embodiments without departing from the principles and spirit of the invention, the scope of which is defined in the appended claims and their equivalents.

Claims

A stainless steel pipe with excellent erosion resistance,

which is a steel pipe comprising Cr at 11.0 to 14.0 wt% and having a ferrite factor (FF) of 6 to 9 as , expressed by the following formula 1,

wherein, based on a thickness of the steel pipe, the inner area has a mixed phase structure of martensite and ferrite, the outer area has either ferrite phase structure or a mixed phase structure of ferrite and martensite, and a martensite fraction of the inner area is greater than a martensite fraction of the outer area:

[Formula 1]

FF = [Cr + 6Si +8Ti + 4Mo + 2Al +4Nb] - [2Mn + 4Ni + 40(C+N)]

wherein, Cr, Si, Ti, Mo, Al, Nb, Mn, Ni, C and N represent contents (wt%) of respective components.
The stainless steel pipe with excellent erosion resistance according to claim 1, which comprises C at 0.01 to 0.05 wt%, N at 0.01 to 0.05 wt%, Ti at 0.25 wt% or less, and Si at 0.5 wt% or less.
The stainless steel pipe with excellent erosion resistance according to claim 2, further comprising Nb at 0.25 wt% or less, wherein the sum of the contents of Ti and Nb is 0.25 wt% or less.
The stainless steel pipe with excellent erosion resistance according to claim 1, wherein the stainless steel pipe has a thickness of 8 to 30 mm.
A manufacturing method of a stainless steel pipe with excellent erosion resistance, comprising:

rapidly heating the inner area of the stainless steel pipe, which comprises Cr: 11.0 to 14.0 wt% and has a ferrite factor (FF) of 6 to 9 as expressed by the following formula 1, to a temperature of the inner area higher than a Ac1 transformation temperature and heating the outer area of the stainless steel pipe to a temperature of the outer area lower than the Ac1 transformation temperature; and cooling the stainless steel pipe to form a mixed phase structure of martensite and ferrite in the inner area of the stainless steel pipe:

[Formula 1]

FF = [Cr + 6Si +8Ti + 4Mo + 2Al +4Nb] - [2Mn + 4Ni + 40(C+N)]

wherein, Cr, Si, Ti, Mo, Al, Nb, Mn, Ni, C and N represent contents (wt%) of respective components.
The manufacturing method of a stainless steel pipe with excellent erosion resistance according to claim 5, wherein the stainless steel pipe comprises C at 0.01 to 0.05 wt%, N at 0.01 to 0.05 wt%, Ti at 0.25 wt% or less, Nb at 0.25 wt% or less, Si at 0.5 wt% or less, Ni at 2.0 wt% or less, Mo at 2.0 wt% or less, Al at 0.1 wt% or less, Mn at 2.0 wt% or less, the balance of Fe and inevitable impurities, and the sum of the contents of Ti and Nb is 0.25 wt% or less.
The manufacturing method of a stainless steel pipe with excellent erosion resistance according to claim 5, wherein the inner area of the stainless steel pipe is rapidly heated by induction heating.
The manufacturing method of a stainless steel pipe with excellent erosoin resistance according to claim 7, the inner area of the stainless steel pipe is heated by induction heating to a temperature of 100 to 300℃ higher than the Ac1 transformation temperature.
The manufacturing method of a stainless steel pipe with excellent erosion resistance according to claim 7, the temperature of the outer area of the stainless steel pipe heated by induction heating is maintained at a temperature lower than the Ac1 transformation temperature, and a difference in temperature between the inner area and the outer area of the stainless steel pipe has to be satisfied in a range of 100 to 300℃.
The manufacturing method of a stainless steel pipe with excellent erosion resistance according to claim 5, the cooling of the stainless steel pipe includes air-cooling or water spray cooling the outer area of the stainless steel pipe.
A manufacturing method of a stainless steel pipe with excellent erosion resistance, comprising;

entering an induction heating device into an inner hollow portion of the stainless steel pipe, which has an area in which a ferrite phase and an austenite phase coexist, at a temperature higher than a Ac1 transformation temperature;

rapidly heating the inner area of the stainless steel pipe based on the thickness thereof by induction heating by applying electricity to the induction heating device;

cooling the inner area of the stainless steel pipe to form a mixed phase structure of martensite phase and ferrite phase in the inner area of the stainless steel pipe.
The manufacturing method of a stainless steel pipe with excellent erosion resistance according to claim 11, wherein, when the stainless steel pipe is heated by the induction heating, the stainless steel pipe has a temperature gradient at which the temperature gradually decreases from the inner area to the outer area based on the thickness thereof, and the temperature of the inner area of the stainless steel pipe reaches above the Ac1 transformation temperature and the temperature of the outer area of the stainless steel pipe does not reach the Ac1 temperature.
The manufacturing method of a stainless steel pipe with excellent erosion resistance according to claim 12, wherein the inner area of the stainless steel pipe is heated by induction heating to a temperature of 100 to 300℃ higher than Ac1 transformation temperature, and the outer area of the stainless steel pipe is maintained at a temperature lower than the Ac1 transformation temperature.
The manufacturing method of a stainless steel pipe with excellent erosion resistance according to claim 11, wherein the cooling of the stainless steel pipe includes air-cooling or water spray cooling the outer area of the stainless steel pipe.