JP6831254B2 - Welded steel pipe with excellent acid dew point corrosion resistance, its manufacturing method, and heat exchanger - Google Patents

Description

On the surface of a member that comes into contact with a gas containing sulfur oxides or hydrogen chloride, so-called "sulfuric acid condensation" or "hydrochloric acid condensation" occurs at a temperature lower than the dew point of the gas. If the member is a metal, corrosion may proceed due to condensed water containing sulfuric acid or hydrochloric acid, which may cause a problem. Such acid corrosion in the condensed water is referred to herein as "acid dew point corrosion". The present invention relates to a welded steel pipe using steel imparted with resistance to acid dew point corrosion, particularly one having improved sulfuric acid resistance of a welded portion, and a method for producing the same. It also relates to a heat exchanger using the steel pipe.

Combustion exhaust gas from thermal power plants and waste incineration facilities is mainly composed of water, sulfur oxides (sulfur dioxide, sulfur trioxide), hydrogen chloride, nitrogen oxides, carbon dioxide, nitrogen, oxygen, and the like. In particular, if sulfur trioxide is contained in the exhaust gas even at 1 ppm, the dew point of the exhaust gas often reaches 100 ° C. or higher, and sulfuric acid condensation is likely to occur. In addition, the exhaust gas from coal-fired power plants and the exhaust gas from waste incineration facilities (urban waste incineration facilities and industrial waste incineration facilities) contain a considerable amount of hydrogen chloride, and hydrochloric acid condensation is likely to occur.

The temperature at which sulfuric acid condensation occurs (sulfuric acid dew point) and the temperature at which hydrochloric acid condensation occurs (hydrochloric acid dew point) vary depending on the composition of the combustion exhaust gas. Generally, the sulfuric acid dew point is about 100 to 150 ° C., and the hydrochloric acid dew point is about 50 to 80 ° C. Even in the exhaust gas flow path of the same combustion equipment, the sulfuric acid dew point corrosion-dominated part and the hydrochloric acid dew point corrosion-dominated part are. It can occur. For this reason, metal members that have a relatively low temperature in the exhaust gas flow path (for example, members that make up duct walls and chimneys of flues, dust collector members, pipes for utilizing the heat of exhaust gas, heat exchange members such as fins, etc.) For example), it is necessary to apply a material that is excellent in both sulfuric acid dew point corrosion resistance and hydrochloric acid dew point corrosion resistance.

Sb-added steel is known as a steel having improved acid dew point corrosion resistance (Patent Documents 1 and 2). In particular, in order to improve both sulfuric acid dew point corrosion resistance and hydrochloric acid dew point corrosion resistance, it is said that the combined addition of Sb and Cu or Mo is effective (Patent Document 2). However, Sb is an expensive element and causes an increase in the cost of steel materials, and when a large amount of Sb is consumed as a raw material for steel materials, there is concern in terms of raw material procurement. In addition, the addition of Sb reduces the hot workability of steel.

Stainless steel is a material having excellent acid resistance, but depending on the acid concentration and temperature, corrosion may proceed more easily than Sb-added steel. Stainless steel is expensive and cannot be said to be a perfect material for acid dew point corrosion.

On the other hand, various methods for improving sulfuric acid dew point corrosion resistance and hydrochloric acid dew point corrosion resistance have been studied without relying on the addition of Sb. For example, Patent Document 3 discloses a technique for improving sulfuric acid dew point corrosion resistance by strictly controlling the contents of P, S, Cu, and Mo. Patent Document 4 discloses a technique for improving the properties of both sulfuric acid dew point corrosion resistance and hydrochloric acid dew point corrosion resistance by strictly controlling the amount of Cr and Mo added. Patent Document 5 describes that controlling the S content to a certain level or higher and suppressing the Mo content is effective in further improving the sulfuric acid dew point corrosion resistance. Patent Document 6 describes Cu, Cr, which is necessary for improving both sulfuric acid dew point corrosion resistance and hydrochloric acid dew point corrosion resistance by refining the ferrite crystal grain size in steel containing Cu, Cr, and Mo. , It is described that the allowable range of Mo can be expanded.

Special Publication No. 43-14585 Japanese Unexamined Patent Publication No. 2003-213367 Japanese Unexamined Patent Publication No. 2012-180546 Japanese Unexamined Patent Publication No. 2012-57221 Japanese Unexamined Patent Publication No. 2013-194251 International Publication No. 2015/147166 JP-A-2009-173995 Japanese Unexamined Patent Publication No. 6-24014

According to the technique of Patent Document 6, it has become easy to industrially produce a steel sheet having improved sulfuric acid dew point corrosion resistance and hydrochloric acid dew point corrosion resistance at the same time without relying on the addition of Sb with good yield. However, in the "steel pipe" obtained by welding such a steel plate, there is a problem that the sulfuric acid resistance at the welded portion is lowered. This is a problem peculiar to steels containing S to improve sulfuric acid resistance, and the same applies to the above-mentioned Sb-added steels. That is, it is considered that in the welded portion, an S-enriched region is formed in the matrix around the MnS-based inclusions in the quenching process after welding, and corrosion proceeds due to the potential difference between the base iron and the S-enriched region. Therefore, conventionally, after welding pipe making, the steel pipe is heat-treated in the furnace to make the structure uniform to the extent that the metal structure of the welded portion is almost indistinguishable from the base metal portion. Generally, in order to manufacture a steel pipe that exhibits excellent sulfuric acid dew point corrosion resistance including welded parts, high temperature heating is applied for a relatively long time after controlling the non-oxidizing atmosphere using a continuous annealing furnace or the like. Therefore, the current situation is that high cost is required.

On the other hand, a technique (seam annealing) for improving the workability and toughness of a welded portion by reheating the welded portion of the steel pipe with a high-frequency induction heating device is known (for example, Patent Document 7). Further, there is known a quenching treatment technique (induction hardening) in which a steel pipe having an enhanced hardenability is heated and cooled by an induction heating device (for example, Patent Document 8). However, it is difficult to achieve the above-mentioned uniformization of the structure by such a short-time treatment.

The present invention is a low-cost welded steel pipe made of a steel plate having excellent sulfuric acid dew point corrosion resistance and hydrochloric acid dew point corrosion resistance, in which the sulfuric acid resistance of the welded portion is improved, and in particular, the base material is excessively used. It is an object of the present invention to provide a product which is not softened and has sufficiently high strength.

As described above, applying careful heat treatment to the welded steel pipe to make the structure uniform is a major factor in increasing the cost. As a result of various studies, the inventors have made it almost impossible to distinguish the metal structure of the welded part from the base metal in the case of a welded steel pipe using a type of acid-resistant dew point corrosion steel containing S and compoundly adding Cu, Cr and Mo. The sulfuric acid resistance of the weld can be significantly improved by short-time heat treatment of the weld using the seam annealing or high-frequency quenching technology described above, without the need for careful homogenization heat treatment. It was found that the sulfuric acid resistance of the weld can be raised to the same level as the base metal depending on the condition control. That is, the hardness of the welded portion hardened in the cooling process during welding does not necessarily decrease to the same level as the hardness of the base metal (in other words, there is a certain difference in the hardness of the welded portion and the base metal. It is possible to remarkably improve the sulfuric acid resistance of the welded portion even in the steel pipe in the generated microstructure state). In particular, if short-time heating conditions are applied such that the hardness of the weld is close to the hardness of the base metal (the difference between the two is, for example, within 20 HV), the sulfuric acid resistance of the weld is at the same level as that of the base metal. It turned out to be improved. The present invention is based on such findings.

Specifically, in terms of mass%, C: 0.001 to 0.15%, Si: 0.005 to 0.80%, Mn: 0.1 to 1.50%, P: 0.002 to 0. 025%, S: 0.010 to 0.030%, Cu: 0.08 to 1.20%, Ni: 0.005 to 0.50%, Cr: 0.04 to 0.25%, Mo: 0 .010 to 0.085%, Al: 0.005 to 0.10%, N: 0.001 to 0.015%, Ti, Nb, V: Total 0 to 0.20%, B: 0 to 0. 010%, Sb, Sn: A steel pipe having a chemical composition of 0 to 0.10% in total, the balance Fe and unavoidable impurities, having a welded portion extending in the longitudinal direction of the pipe, and perpendicular to the longitudinal direction of the pipe. In the center of the wall thickness of the cross section, the difference Hw-Hp between the hardness Hw (HV) of the welded portion and the average hardness Hp (HV) of the base metal portion is 70 HV or less. Achieved. The wall thickness of the base metal portion is, for example, 1.0 to 5.0 mm. The average hardness Hp of the base metal portion is, for example, 135 HV or more. The Hw-Hp can be 20 HV or less.

Here, the "base metal" is the material to be welded, the "welded metal" is the metal melted and solidified during welding, and the "welded portion" is the portion including the "welded metal" and the "heat-affected zone (HAZ)". Means (see JIS Z3001-1: 2013). The "base material portion" in the material after welding means a portion derived from the base metal excluding the welded portion. The hardness of the cross section perpendicular to the longitudinal direction of the tube can be measured at HV0.03 (test force 0.2942N) in the Micro Vickers hardness test according to JIS Z2244: 2009. The average hardness Hp of the base metal portion can be determined as follows.

[How to find the average hardness Hp of the base metal part]
An annular line (hereinafter referred to as "annular line") is prepared by preparing a cross section perpendicular to the longitudinal direction of the pipe as a hardness measuring surface and connecting a position having a depth of 1/4 of the wall thickness from the outer surface side of the pipe in the measuring surface. ) Is assumed. Cyclic average thickness of the base metal t (mm), the weld center position on the annular line and P _0, the point advanced from P ₀ on the annular line in one direction by a distance t P _A, from P ₀ Let P _B be a point on the line that is advanced by a distance t in the direction opposite to P _A. If whether the P _A or P _B is in a position out of the heat affected zone in doubt, P _A further upper annular line t advanced by points, respectively, and P _B. In the route from P _A to P _B on the annular line in the direction not passing through the weld, set 9 points on the annular line at approximately equal intervals including P _A and P _B , and Vickers hardness at these 9 points. The value obtained by dividing the sum of these measured values by 9 is defined as the average hardness Hp (HV) of the base metal portion. Here, "almost equidistant" means that the maximum value of the distance (8 points in total) on the circular line between two adjacent points does not exceed twice the minimum value.

As a method for manufacturing the steel pipe, a steel pipe having the above chemical composition is welded into a tubular shape to form a welded portion extending in the longitudinal direction of the pipe, and heat is applied by high-frequency induced heating to the welded portion. Alternatively, there is provided a manufacturing method in which the entire circumference of the pipe is heated to a temperature of 650 ° C. or higher and 1000 ° C. or lower, or 700 ° C. or higher and 1000 ° C. or lower, and then cooled. In this heat treatment, the time for which the welded portion is held at a temperature of 650 ° C. or higher and 1000 ° C. or lower, or 700 ° C. or higher and 1000 ° C. or lower can be set to 10 seconds or shorter. When the cooling is water cooling, for example, the maximum temperature of the material can be set in the range of 650 ° C. or higher and 800 ° C. or lower. A heat pattern may be adopted in which the maximum temperature reached of the material is in the range of 700 ° C. or higher and 1000 ° C. or lower, and the temperature range of at least 550 ° C. is air-cooled for cooling. In this case, air cooling can be continued to room temperature, or water cooling can be switched to after the temperature drops below 550 ° C.

The steel pipe thus obtained can be applied to, for example, a heat exchange member exposed to the combustion exhaust gas of a coal-fired power plant or the combustion exhaust gas of a waste incineration facility. In particular, it is suitable for a member whose surface is condensed when exposed to the exhaust gas.

According to the present invention, in a welded steel pipe made of a steel plate having excellent sulfuric acid dew point corrosion resistance and hydrochloric acid dew point corrosion resistance, a low-cost welded steel pipe having improved sulfuric acid resistance can be realized. In the conventional method of improving the sulfuric acid resistance of the welded portion by careful heat treatment for homogenizing the metal structure including the welded portion, the strength level of the base metal portion is lowered by softening. It is even more costly to increase the strength level by post-treatment such as roll forming or cold drawing. Since the strength decrease of the base metal portion is small in the steel pipe according to the present invention, it can be adjusted to an appropriate strength level by processing performed before welding pipe making (cold rolling on a steel plate, etc.). For example, a steel pipe having an average hardness Hp of 135 HV or more in the final base metal portion has a higher strength level than a soft steel pipe that has undergone a careful heat treatment for homogenizing the structure. Such high strength can cope with thinning of members and can contribute to high efficiency of heat exchangers.

A photograph illustrating a cross section of a welded steel pipe A after a sulfuric acid immersion test, cut perpendicularly to the longitudinal direction of the pipe. An enlarged photograph of the vicinity of the outer surface of the pipe in FIG. A photograph illustrating a cross section of a welded steel pipe B after a sulfuric acid immersion test, which is cut perpendicular to the longitudinal direction of the pipe. An enlarged photograph of the vicinity of the outer surface of the pipe in FIG. A photograph exemplifying a cross section of a test piece after a sulfuric acid immersion test of a conventional welded steel pipe, which is cut perpendicular to the longitudinal direction of the pipe.

[Chemical composition]
The constituent elements of steel applied to the present invention will be described. Unless otherwise specified, "%" for a component element means mass%.

C does not have a great influence on the acid dew point corrosion resistance, but a large amount of C content causes a decrease in workability, so the C content is set to 0.15% or less. On the other hand, excessively low C causes an increase in manufacturing cost. Here, those having a C content of 0.001% or more are targeted.

Si is effective not only for deoxidation during steelmaking, but also for improving acid corrosion resistance and ensuring strength as a structural material. It is more effective to secure a Si content of 0.005% or more. Excessive addition of Si reduces the descalability during hot spreading and causes an increase in scale flaws. Furthermore, it becomes a factor that lowers the weldability. The Si content is limited to 0.80% or less.

Mn is effective in adjusting the strength of steel and has an effect of preventing hot brittleness due to S. It is more effective that the Mn content is 0.10% or more, and the Mn content may be controlled to 0.30% or more or 0.50% or more. However, Mn is a factor that lowers the hydrochloric acid corrosion resistance. The Mn content is allowed up to 1.50% and may be controlled in the range of 1.20% or less, or 1.00% or less.

P is limited to 0.025% or less because it deteriorates hot workability and weldability. In order to further improve the sulfuric acid dew point corrosion resistance and the hydrochloric acid dew point corrosion resistance, it is effective to reduce the P content, but an excessive reduction becomes a factor of increasing the steelmaking load and pushing up the cost. The P content may be adjusted in the range of 0.002 to 0.025%. More preferably, it is 0.005 to 0.015%.

S is an element effective for improving the sulfuric acid dew point corrosion resistance. In order to fully exert its action, the S content is set to 0.010% or more. It may be managed in the range exceeding 0.010%. However, as the S content increases, the sulfuric acid resistance at the welded portion decreases, and even if the heat treatment described later is performed, the original effect of improving the sulfuric acid dew point corrosion resistance due to the S content cannot be utilized as a whole steel pipe. As a result of various studies, the S content is limited to 0.030% or less.

Cu is effective for improving sulfuric acid dew point corrosion resistance and hydrochloric acid dew point corrosion resistance, and in the present invention, it is necessary to secure a Cu content of 0.08% or more. However, since an excessive Cu content causes a decrease in hot workability, it is desirable to set the content to 1.20% or less.

Ni does not directly affect the improvement of acid dew point corrosion resistance, but is an element that exerts an effect of suppressing the decrease in hot workability due to the addition of Cu, and the content may be 0.005% or more. desirable. When importance is attached to hot workability, it is more effective to secure a Ni content of 0.05% or more, and it is more effective to set it to 0.10% or more. However, if it exceeds 0.50%, the effect is saturated and the cost becomes high. The Ni content is set in the range of 0.50% or less.

Cr and Mo are important elements for simultaneously improving the sulfuric acid dew point corrosion resistance and the hydrochloric acid dew point corrosion resistance without relying on special elements such as Sb. As a result of various studies, simultaneous improvement of sulfuric acid dew point corrosion resistance and hydrochloric acid dew point corrosion resistance by compound addition of Cr in the range of 0.04 to 0.25% and Mo in the range of 0.01 to 0.085%. Is possible. It is more effective to set the Cr content to 0.1 to 0.25%. Further, it is more effective to set the Mo content to 0.03 to 0.07%.

Al is an element required for deoxidation during steelmaking. It is effective to adjust the Al content to 0.005% or more, and it is more effective to adjust the Al content to 0.0010% or more. However, Al becomes a factor that lowers the hot workability. As a result of various studies, the Al content is limited to 0.10% or less, and may be controlled to 0.050% or less.

Like C, N does not have a large effect on acid dew point corrosion resistance, but a large amount of N content causes a decrease in processability, so the N content is set to 0.015% or less. On the other hand, excessively low N causes an increase in manufacturing cost.

Ti, Nb, and V have the effect of refining the ferrite crystal grain size and are effective in improving the acid dew point corrosion resistance. Therefore, one or more of these can be added as needed. In that case, it is more effective to set the total content of one or more of Ti, Nb, and V to 0.005% or more. However, even if it is added in an excessive amount, the above action is saturated and the manufacturing cost increases. When one or more of Ti, Nb, and V are added, it is desirable that the total content thereof is 0.20% or less.

Since B is an element that can exhibit the effect of refining the ferrite crystal particle size with a small amount of addition, it can be added as needed. In that case, it is more effective that the content of B is 0.0005% or more. However, even if B is added in excess, the above action is saturated and the manufacturing cost increases. When B is added, it is desirable to add it in a content range of 0.010% or less.

Similar to Cr and Mo, Sb and Sn are elements effective for improving acid dew point corrosion resistance through the action of slowing the electrochemical anode-cathode reaction. In the present invention, excellent acid dew point corrosion resistance is imparted by a method in which the S content is strictly controlled in a narrow range and Cu, Cr, and Mo are added in combination. Therefore, addition of Sb and Sn is not always necessary. However, when Sb and Sn are added, the acid dew point corrosion resistance can be further improved. Therefore, when it is important to increase the level of acid dew point corrosion resistance, one or more of Sb and Sn can be added as needed. In order to fully exert the effect of adding these elements, it is desirable to contain one or more of them so that the total content of Sb and Sn is 0.005% or more. However, even if it is added in an excessive amount, the above action is saturated and the manufacturing cost increases. When one or more of Sb and Sn are added, it is desirable that the total content thereof is 0.10% or less.

[Metal structure]
The base metal portion of the welded steel pipe targeted in the present invention has, for example, a metal structure mainly composed of a recrystallized ferrite phase. In addition to the case of a ferrite single-phase structure, a structure may be a structure in which one or more of cementite and pearlite are contained in a total range of 10% by volume or less and the balance is a ferrite phase. When the entire circumference of the tube is heated to the austenite region and rapidly cooled using an induction hardening device, the base metal part may have a martensite single phase or a metal structure containing one or more of ferrite, martensite, and bainite. is there. In this specification, cementite and pearlite may be referred to as the second phase. Of these, pearlite is a layered structure composed of a thin ferrite phase and a cementite phase, but the ferrite phase described as the rest of the second phase in the present specification, that is, the ferrite phase whose ferrite crystal grain size is to be measured, is included in the ferrite phase. The ferrite phase that constitutes pearlite is not included. Similarly, cementite described in the same row as pearlite as a component of the second phase does not include cementite constituting pearlite. The average hardness Hp of the base metal portion is preferably adjusted to 135 HV or more, and more preferably 145 HV or more.

On the other hand, the structure of the welded portion changes before and after the heat treatment described later. When welding is performed during pipe formation, a hard structure in which the martensite phase is mixed is formed in the welded portion. In the steel pipe according to the present invention obtained by subjecting the steel pipe in that state to the heat treatment described later, a transformation phase derived from the martensite phase remains in the welded portion depending on the heating conditions, but the softening has progressed to some extent. However, the hardness Hw of the welded portion may be in a harder state than the average hardness Hp of the base metal portion. In this case, it can be said that the welded portion of the welded steel pipe is in a textured state in which there is room for further softening when further heat energy is applied. Even in such a welded portion having a structure that has not been completely made uniform with the base metal portion, the sulfuric acid resistance is significantly improved as compared with that immediately after welding. The cause is that the unstable and easily soluble Mn / Fe-based sulfide generated from the MnS-based inclusions during the rapid heating / cooling process during welding returns to MnS with a stable structure by the heat treatment described later. It is thought that it may be. That is, even for a short time, once heated to a temperature range of 650 ° C. or higher and 1000 ° C. or lower, preferably 700 ° C. or higher and 1000 ° C. or lower, and cooled by air cooling or the like, unstable Mn / Fe sulfide becomes a mother. It changes to a stable MnS like the material part, and as a result, it becomes possible to impart the same sulfuric acid corrosion resistance to the welded part as the base material part.

The structural state of the welded portion with improved sulfuric acid resistance as described above can be specified by the difference Hw-Hp between the hardness Hw of the welded portion and the average hardness Hp of the base metal portion. Specifically, in the case of a steel type satisfying the above-mentioned chemical composition, at the center of the wall thickness of the cross section perpendicular to the longitudinal direction of the pipe (specifically, a position at a depth of 1/4 of the wall thickness from the outer surface side of the pipe). It was found that the sulfuric acid resistance of the welded portion was significantly improved when the difference Hw-Hp between the hardness Hw (HV) of the welded portion and the average hardness Hp (HV) of the base metal portion was 70 HV or less. .. In particular, when Hw-Hp is 20 HV or less, the sulfuric acid resistance of the welded portion is restored to almost the same level as that of the base metal.

〔Production method〕
The welded steel pipe having the above-mentioned structural state can be manufactured by subjecting a raw steel sheet to a welded pipe to produce a steel pipe, and then subjecting the steel pipe to a short-time heat treatment.
The material steel sheet may be either a hot-rolled steel sheet or a cold-rolled steel sheet, and can be manufactured by a general steel sheet manufacturing process having casting, hot rolling, cold rolling, and annealing in the above order. If necessary, temper rolling can be performed. The plate thickness can be, for example, 1.0 to 5.0 mm depending on the target wall thickness of the steel pipe.

Welded pipe forming can be performed by a known high-frequency welded pipe forming method in which a steel plate is formed into a tubular shape and the edges of the steel plate width direction ends are welded and joined to each other. The welded steel pipe has a welded portion extending in the longitudinal direction of the pipe. The weld bead portion can be cut using a cutting tool or the like. In the present invention, the steel pipe having the welded structure is not heat-treated so that the entire outer peripheral surface of the pipe is exposed to a high temperature atmosphere in the furnace, but heat is applied by high frequency induction heating. Heat treatment is performed for a short time by the method described above. This short-time heat treatment can be carried out by using a heating facility that performs heat treatment (so-called seam annealing), which is conventionally performed as a means for improving the workability and toughness of the weld heat-affected zone, or an induction hardening apparatus. is there. When heating using an induction hardening device, the entire circumference of the pipe is heated for a short time. In particular, if the entire circumference of the pipe including the welded portion is heated to 800 ° C. or higher and then cooled, the metal structures of the welded portion and the base metal portion become the same, and the sulfuric acid corrosion resistance becomes the same. In this case, if water cooling is applied immediately after high-frequency heating, it is possible to obtain a high-strength steel pipe having a metal structure mainly composed of martensite in both the welded portion and the base metal portion. Regardless of seam annealing or all-around heating, if the average cooling rate from the maximum temperature reached to 550 ° C is controlled to 50 ° C / s or less, the metal structure will be mainly ferrite and pearlite, and the workability of steel pipes will be improved. In addition to being advantageous, stable MnS can be sufficiently formed, and the effect of improving sulfuric acid corrosion resistance is also increased. For example, the cooling rate can be controlled by air-cooling until the temperature reaches at least 550 ° C. In this case, water cooling may be performed in a temperature range of 550 ° C. or lower. For example, a heat pattern such as air cooling up to at least 550 ° C. and water cooling from a temperature of 450 ° C. or higher can be adopted. On the other hand, even if the maximum reached temperature is set to 650 ° C. or higher and 800 ° C. or lower, preferably 700 ° C. or higher and 800 ° C. or lower, and water cooling is performed immediately after high-frequency heating, the metal structure becomes a ferrite-based structure, which is advantageous for workability. Steel pipes can be manufactured with high productivity.

The above heat treatment can be carried out by carrying out the heat treatment while advancing the steel pipe in the longitudinal direction on the heat treatment line in which the high frequency induction heating coil is arranged. In the cross section perpendicular to the longitudinal direction of the pipe, the welded part (that is, weld metal + heat affected zone) or the entire circumference is heated to a temperature range of 650 ° C or higher and 1000 ° C or lower, or 700 ° C or higher and 1000 ° C or lower, and then cooled. As a result, the amount of heat input and the moving speed of the pipe are controlled. If there is a part of the weld that does not rise to the above temperature range by short-time heating such as high-frequency heating, unstable and easily soluble Mn / Fe-based sulfide tends to remain in that part, and the resistance of the weld It is difficult to stabilize and sufficiently improve the sulfate property. If the heating temperature exceeds 1000 ° C., the energy required for heating becomes very large, which leads to an increase in manufacturing cost. Therefore, the maximum temperature reached is preferably 1000 ° C. or lower, and more preferably 800 ° C. or higher and 950 ° C. or lower.

It is desirable that the time for staying in the range where the material temperature of the welded portion is 650 ° C. or higher and 1000 ° C. or lower, or 700 ° C. or higher and 1000 ° C. or lower is 10 seconds or less. Heating for a longer time is uneconomical because it raises the cost. The inventors conducted heat treatment experiments on various steels in the chemical composition range using salt baths to obtain various temperature rise curves depending on a sample to which a thermocouple was attached. As a result, when the residence time in the temperature range of 650 ° C. or higher and 1000 ° C. or lower, or 700 ° C. or higher and 1000 ° C. or lower was set to 0 seconds or longer, a clear improvement in sulfuric acid resistance of the welded portion was observed. Here, the residence time of 0 seconds corresponds to the case where cooling is started immediately when the temperature reaches 650 ° C. or 700 ° C. It is more preferable that the residence time in the temperature range of 650 ° C. or higher and 1000 ° C. or lower, or 700 ° C. or higher and 1000 ° C. or lower is in the range of 1 second or more and 10 seconds or less.

When the maximum temperature reached is 800 ° C. or higher and 1000 ° C. or lower, it is more preferable that the staying time in the temperature range of 800 ° C. or higher and 1000 ° C. or lower is in the range of 1 second or longer and 10 seconds or lower.

The heat curve (heating / cooling curve) of a material in a production line equipped with a high-frequency induction heating device uses software programmed based on equipment specifications and preliminary experimental data, and uses tube diameter, wall thickness, line speed, and high-frequency output. It can be simulated by inputting manufacturing conditions such as.

After the above heating, it can be cooled by, for example, "air cooling", "water cooling", or "air cooling and water cooling". The heat pattern in which water cooling is performed immediately after heating is preferably applied when the maximum temperature reached is 650 ° C. or higher and 800 ° C. or lower. In the case of a heat pattern of cooling by air cooling and water cooling, it is preferable to switch to water cooling after air cooling to a temperature of at least 550 ° C. or lower.

<< Characteristics survey of steel pipes as welded pipes >>
A hot-rolled steel sheet having the following chemical composition and having a thickness of 3 mm was prepared.
By mass%, C: 0.05%, Si: 0.31%, Mn: 0.93, P: 0.011%, S: 0.014%, Cu: 0.34%, Ni: 0.16 %, Cr: 0.22%, Mo: 0.05%, Al: 0.028%, N: 0.026%, balance Fe.
Using this steel plate as a material, a welded steel pipe having a circular cross section with an outer diameter of 34.0 mm was manufactured by a high-frequency welded pipe making line. After welding pipe formation, weld bead grinding of the outer and inner surfaces was performed in the line. In this way, a welded steel pipe A having a metal structure as welded was obtained.

For the welded steel pipe A, a sample was prepared with the cross section perpendicular to the longitudinal direction of the pipe as the observation surface, and the hardness of the cross section was measured at HV0.03 (test force 0.2942N) according to JIS Z2244: 2009. The hardness Hw of the welded portion at the center of the thickness and the average hardness Hp of the base metal portion were determined. The average hardness Hp of the base metal portion was determined according to the above-mentioned "How to obtain the average hardness Hp of the base metal portion". The number of measurement test pieces was n = 3, and the values obtained by dividing the total Hw and the total Hp obtained from each test piece by the number of tests 3 were adopted as the values of Hw and Hp of the welded steel pipe, respectively.
As a result, the hardness Hw of the welded portion of the welded steel pipe A was 271 HV, and the average hardness Hp of the base metal portion was 165 HV. Their difference Hw-Hp is 106 HV.

A corrosion test piece (width 25 mm, length 30 mm) containing the welded portion in the center was cut out from the welded steel pipe A and immersed in a sulfuric acid aqueous solution having a sulfuric acid concentration of 40% by mass and a temperature of 60 ° C. for 4 hours. The weight change before and after the sulfuric acid immersion test was measured with an electronic balance, and the corrosion loss per unit area of the test piece including the weld was calculated.
As a result, the corrosion weight loss of the test piece (base material reference) of the base metal part not including the welded part of the welded steel pipe A was 65 mg / cm ² , and that of the test piece (reference as it was welded) including the welded part of the welded steel pipe A. The corrosion weight loss was 162 mg / cm ² .

FIG. 1 exemplifies a photograph of a cross section of a welded steel pipe A after a sulfuric acid immersion test, cut perpendicularly to the longitudinal direction of the pipe. The cut surface is etched so that crystal grains appear. Groove-shaped wall thinning can be seen on both the inner and outer surfaces of the pipe at the weld. The above corrosion test conditions are a very severe sulfuric acid corrosion environment for steel materials, but since this steel has inherently high sulfuric acid resistance, the amount of dissolution (thickness reduction) of the base material is small. However, it can be seen that its excellent sulfuric acid resistance is reduced at the weld. FIG. 2 shows an enlarged photograph of the vicinity of the outer surface side surface of the pipe in FIG. (A) is the left side of the welded portion, (b) is the center of the welded portion, and (c) is the right side of the welded portion. Significant wall thinning near the end of the weld heat affected zone.

<< Example of heat treatment by seam annealing >>
The welded portion of the welded steel pipe manufactured under the same conditions as the welded steel pipe A was subjected to seam annealing in-line to apply heat by high-frequency induction heating. By simulating the heat curve in this heat treatment line from the conditions such as pipe diameter, wall thickness, line speed, high frequency output, etc., the maximum temperature reached at the weld is 700 ° C, and the stay from 650 ° C to the maximum temperature is reached. The heating was performed under a condition that the time was about 1 second and the temperature of the entire welded portion was raised to 650 ° C. or higher. After reaching the maximum temperature, it was immediately cooled by water cooling treatment by spraying water. In this way, the welded steel pipe B after the heat treatment was obtained.

Further, the welded portion of the welded steel pipe manufactured under the same conditions as the welded steel pipe A was subjected to seam annealing in-line to apply heat by high-frequency induction heating. By simulating the heat curve in this heat treatment line from the conditions such as pipe diameter, wall thickness, line speed, high frequency output, etc., the maximum temperature reached at the weld is 750 ° C, and the stay from 700 ° C to the maximum temperature is reached. The heating was performed under a condition that the time was about 1 second and the temperature of the entire welded portion was raised to 700 ° C. or higher. After reaching the maximum temperature reached, heating was stopped and air-cooled, and after the temperature of the welded portion reached 550 ° C., it was cooled by water cooling treatment by spraying water. In this way, the welded steel pipe G after the heat treatment was obtained.

For the welded steel pipes B and G, the hardness Hw of the welded portion at the center of the wall thickness and the average hardness Hp of the base metal portion were determined by the same method as in the case of the welded steel pipe A.
As a result, the hardness Hw of the welded portion of the welded steel pipe B was 227 HV, the average hardness Hp of the base metal portion was 169 HV, and the difference Hw-Hp between them was 58 HV. The hardness Hw of the welded portion of the welded steel pipe G was 219 HV, the average hardness Hp of the base metal portion was 169 HV, and the difference Hw-Hp between them was 50 HV. In each case, the welded portion was softened by the heat treatment after the welded pipe formation, but the softening to the same level as the base metal portion was not observed.

A corrosion test piece (width 25 mm, length 30 mm) containing the welded portion in the center is cut out from the welded steel pipes B and G, and in a sulfuric acid aqueous solution having a sulfuric acid concentration of 40% by mass and a temperature of 60 ° C. as in the case of the welded steel pipe A described above. A sulfuric acid immersion test was carried out.
As a result, the corrosion loss of the test piece including the welded portion of the welded steel pipes B and G was 85 mg / cm ² and 73 mg / cm ² , respectively, which is much higher than the corrosion loss of 162 mg / cm ² in the above-mentioned as-welded reference. It had good sulfuric acid corrosion resistance.

FIG. 3 exemplifies a photograph of a cross section of the test piece after the sulfuric acid immersion test of the welded steel pipe B, which is cut perpendicular to the longitudinal direction of the pipe. The cut surface is etched so that crystal grains appear. FIG. 4 shows an enlarged photograph of the vicinity of the outer surface side surface of the pipe in FIG. (A) is the left side of the welded portion, (b) is the center of the welded portion, and (c) is the right side of the welded portion. The welded part has a slightly larger amount of dissolution than the base material part. However, the erosion of the welded portion is clearly less than that of the welded steel pipe A as it is welded. In the welded steel pipe having the chemical composition specified in the present invention, the sulfuric acid resistance of the welded portion was remarkably improved by subjecting the welded portion to a heat treatment for a short time after the welded pipe was formed.

<< Example of heat treatment by heating all around >>
A welded steel pipe manufactured under the same conditions as the welded steel pipe A was subjected to a heat treatment using an induction hardening device to apply heat input by high frequency induction heating to the entire circumference of the pipe. The heating temperature was measured using a radiation thermometer. Welded steel tube C (580 ° C / water cooling) after heat treatment by heat treatment that heats to 580 ° C or 680 ° C and then water-cools within 1 second, or heat treatment that heats to 800 ° C or 950 ° C and then air-cools. ), D (680 ° C / water cooling), E (800 ° C / air cooling), and F (950 ° C / air cooling) were obtained. Further, by performing a heat treatment of heating to 850 ° C. or 950 ° C., then air-cooling to 500 ° C., and then water-cooling, the welded steel pipe H (850 ° C./air-cooled → water-cooled) and I (950 ° C./air-cooled →) after the heat treatment. Water-cooled) was obtained.

For the welded steel pipes C, D, E, F, H, and I after these heat treatments, the hardness Hw of the welded portion at the center of the wall thickness and the average hardness Hp of the base metal portion are obtained by the same method as in the case of the welded steel pipe A. I asked.
As a result, the average hardness Hp of the base metal portion of each of these heat-treated welded steel pipes is 135 HV or more, which is higher than that of the commercially available products of the above-mentioned conventional examples. The difference Hw-Hp between the hardness Hw of the welded portion and the average hardness Hp of the base metal portion was within 20 HV in E, F, H, and I at which the maximum temperature reached was 800 ° C. or higher.

A corrosion test piece (width 25 mm, length 30 mm) containing the welded portion in the center is cut out from the welded steel pipes C, D, E, F, H, and I, and the sulfuric acid concentration is 40% by mass as in the case of the welded steel pipe A described above. , A sulfuric acid immersion test was carried out in which the mixture was immersed in a sulfuric acid aqueous solution at a temperature of 60 ° C. for 4 hours.
As a result, the corrosion weight loss of the test piece including the welded part was C (580 ° C / water-cooled): 104 mg / cm ² , D (680 ° C / water-cooled): 86 mg / cm ² , E (800 ° C / air-cooled): 68 mg, respectively. ^{/ cm 2, F (950 ℃} / air ^{cooling): 64mg / cm 2, H} (850 ℃ / air cooling → ^{water-cooled): 70mg / cm 2, I} (950 ℃ / air cooling → water-cooled): was 68mg / cm ^2. Even when heated at 580 ° C, the sulfuric acid corrosion resistance is improved as compared with the welded steel pipe A (reference as it is welded) which has not been heat-treated, but it can be significantly improved when the temperature is raised to a temperature range of 650 ° C or higher. Understand. Further, it was confirmed that E, F, H, and I, which were heated to a temperature range of 800 ° C. or higher, had substantially the same sulfuric acid corrosion resistance as the base metal portion even though the heating was performed for a short time. ..

The above results are summarized in Table 1.

<< Conventional example >>
A sulfuric acid immersion test similar to the above was carried out on a commercially available welded steel pipe having excellent acid dew point corrosion resistance. This steel pipe uses steel having improved acid dew point corrosion resistance by containing about 0.1% by mass of Sb. As a result of the sulfuric acid immersion test, the corrosion weight loss of the test piece including the welded portion of this steel pipe was 66 mg / cm ² .

FIG. 5 exemplifies a photograph of a cross section of the test piece after the sulfuric acid immersion test of the welded steel pipe, which is cut perpendicular to the longitudinal direction of the pipe. The cut surface is etched so that crystal grains appear. Although the welded portion is located in the center of FIG. 5, it can be seen that the texture is carefully uniformed to the extent that the metal structure of the welded portion is almost indistinguishable from the base metal portion. When the heat treatment for uniformization is performed in this way, there is almost no difference in sulfuric acid resistance between the portion derived from the welded portion and the base metal portion. However, in general heat treatment of steel pipes, it is necessary to keep the entire pipe at a high temperature, which increases the manufacturing cost. The cross-sectional hardness of this steel pipe is about 120 HV in both the welded portion and the base metal portion, and it is considered that the softening is progressing by the heat treatment. When applied to a member that requires strength, it is desirable to select a thick steel pipe.

Claims (9)

In terms of mass%, C: 0.001 to 0.15%, Si: 0.005 to 0.80%, Mn: 0.1 to 1.50%, P: 0.002 to 0.025%, S: 0.010 to 0.030%, Cu: 0.08 to 1.20%, Ni: 0.005 to 0.50%, Cr: 0.04 to 0.25%, Mo: 0.001 to 0. 085%, Al: 0.005 to 0.10%, N: 0.001 to 0.015%, Ti, Nb, V: total 0 to 0.20%, B: 0 to 0.010%, Sb, Sn: A steel tube having a total of 0 to 0.10%, a chemical composition consisting of the balance Fe and unavoidable impurities, having a welded portion extending in the longitudinal direction of the tube, and a wall thickness center of a cross section perpendicular to the longitudinal direction of the tube. The difference Hw-Hp between the hardness Hw (HV) of the welded portion and the average hardness Hp (HV) of the base metal portion is 70 HV or less , the average hardness Hp of the base metal portion is 135 HV or more, and the base metal portion is A steel tube having excellent acid dew point corrosion resistance , which has a recrystallized ferrite single-phase structure or a structure in which one or more of cementite and pearlite are contained in a total range of 10% by volume or less and the balance is a recrystallized ferrite phase . The steel pipe according to claim 1 , wherein the Hw-Hp is 20 HV or less. The steel pipe according to claim 1 or 2 , wherein the base metal portion has a wall thickness of 1.0 to 5.0 mm. In terms of mass%, C: 0.001 to 0.15%, Si: 0.005 to 0.80%, Mn: 0.1 to 1.50%, P: 0.002 to 0.025%, S: 0.010 to 0.030%, Cu: 0.08 to 1.20%, Ni: 0.005 to 0.50%, Cr: 0.04 to 0.25%, Mo: 0.001 to 0. 085%, Al: 0.005 to 0.10%, N: 0.001 to 0.015%, Ti, Nb, V: total 0 to 0.20%, B: 0 to 0.010%, Sb, Sn: High frequency induction for a steel pipe in which a steel plate having a chemical composition consisting of 0 to 0.10% in total, the balance Fe and unavoidable impurities is welded into a tubular shape to form a welded portion extending in the longitudinal direction of the pipe. The heat input by heating was applied to raise the temperature of the welded portion or the entire circumference of the pipe to a temperature of 650 ° C. or higher and 1000 ° C. or lower, and the time for which the welded portion was maintained at a temperature of 650 ° C. or higher and 1000 ° C. or lower was set to 10 seconds or less . A method for producing a steel pipe having excellent acid dew point corrosion resistance, in which the difference Hw-Hp between the hardness Hw (HV) of the welded portion and the average hardness Hp (HV) of the base metal portion is 70 HV or less by applying a heat treatment for cooling the welded portion. .. The method for manufacturing a steel pipe according to claim 4 , wherein the maximum temperature reached is 650 ° C. or higher and 800 ° C. or lower, and the cooling is water cooling. In terms of mass%, C: 0.001 to 0.15%, Si: 0.005 to 0.80%, Mn: 0.1 to 1.50%, P: 0.002 to 0.025%, S: 0.010 to 0.030%, Cu: 0.08 to 1.20%, Ni: 0.005 to 0.50%, Cr: 0.04 to 0.25%, Mo: 0.001 to 0. 085%, Al: 0.005 to 0.10%, N: 0.001 to 0.015%, Ti, Nb, V: total 0 to 0.20%, B: 0 to 0.010%, Sb, Sn: High frequency induction for a steel pipe in which a steel plate having a chemical composition consisting of 0 to 0.10% in total, the balance Fe and unavoidable impurities is welded into a tubular shape to form a welded portion extending in the longitudinal direction of the pipe. The heat input by heating was applied to raise the temperature of the welded part or the entire circumference of the pipe to 700 ° C. or more and 1000 ° C. or less, and the time for the welded part to be maintained at the temperature of 700 ° C. or more and 1000 ° C. or less was set to 10 seconds or less . A method for producing a steel pipe having excellent acid dew point corrosion resistance, in which the difference Hw-Hp between the hardness Hw (HV) of the welded portion and the average hardness Hp (HV) of the base metal portion is 70 HV or less by applying a heat treatment for cooling the welded portion. .. The method for manufacturing a steel pipe according to claim 6 , wherein in the cooling, a temperature range up to at least 550 ° C. is air-cooled. The method for manufacturing a steel pipe according to any one of claims 4 to 7 , wherein the thickness of the steel plate to be used for welded pipe making is 1.0 to 5.0 mm. A heat exchanger using the steel pipe according to any one of claims 1 to 3 for a heat exchange member exposed to the combustion exhaust gas of a coal-fired power plant or the combustion exhaust gas of a waste incineration facility.