JP4751603B2 - Stainless steel pipe manufacturing method - Google Patents

Description

  The present invention relates to a method for producing stainless steel and a method for producing a stainless steel pipe, which are widely used in the fields of semiconductor manufacturing, chemical industry, food industry, thermal power, nuclear power generation equipment, etc. and excellent in surface cleanliness and corrosion resistance. The “stainless steel” here mainly refers to a steel material (stainless steel material) made of stainless steel, but is not limited to this, and includes a cold product including an intermediate product before being processed into the stainless steel material. “Stainless steel” as a material.

  Stainless steel materials such as steel pipes and steel plates have high strength and excellent corrosion resistance. Therefore, in the industrial fields such as semiconductor manufacturing, chemical industry, food industry, thermal power or nuclear power generation equipment, Widely used as a component. In particular, stainless steel pipes (hereinafter referred to as “stainless steel pipes”) are widely used, for example, as heat transfer pipes for feed water heaters installed in nuclear power generation facilities, etc., and are usually used as small diameter and long pipes. It is often done.

  However, even in the case of stainless steel having high corrosion resistance, if halogen compounds such as fluoride and chloride, phosphates, sulfates, etc. adhere to the surface, the surface cleanliness decreases and the corrosion resistance is not always sufficient. . In particular, in the case of a stainless steel pipe, deposits on the inner surface of the pipe are difficult to remove, and the portion may corrode and metal ions may be eluted.

  Also, for example, during cold working of stainless steel pipes, carbon and carbon compounds (lubricating oil) applied as a lubricant on the inner and outer surfaces are not completely removed by the subsequent cleaning (degreasing) treatment, and a part remains. In this case, if the heat treatment is performed as it is, the carbon concentration may increase due to intrusion diffusion of carbon from the surface of the tube, and carburization may occur.

  Steel pipes that have carburized on the surface are susceptible to sensitization due to thermal effects such as welding, and may cause intergranular corrosion. In other words, Cr carbide precipitates at the grain boundary due to the thermal effect during welding or due to the long-term use incorporated in the equipment, and a Cr-deficient layer appears in the vicinity (adjacent to the grain boundary), resulting in corrosion resistance. Is significantly deteriorated, and corrosion preferentially proceeds in that portion.

  In order to prevent the development of the excellent corrosion resistance inherent in such stainless steel, conventionally, when a stainless steel pipe is taken as an example, there is a method of removing the deposit on the pipe surface by washing. It has been mainly taken. That is, management of chemical components used for cleaning, frequent renewal of cleaning liquid, increase of cleaning processing time and number of times, confirmation of removal of deposits, analysis of deposits, grasp of amount of deposit, implementation of corrosion test, etc. However, this method (cleaning and removal of deposits) has variations in work and missing (oversight) in the confirmation work, which is not only costly and labor intensive but also uneasy in terms of reliability.

  Although it is conceivable to increase the surface smoothness and cleanliness by cutting and grinding to ensure corrosion resistance, cutting and grinding are inefficient and the yield is low because chips are discharged. Cost increases.

  As a method for improving the corrosion resistance of stainless steel, for example, Patent Document 1 proposes a surface treatment method for a stainless steel material. In this method, (i) a cold-worked stainless steel material is prepared, (ii) immersed in an aqueous solution containing a water-soluble inorganic silicate, or subjected to electrolytic cleaning by immersion, and (iii) then in an oxidizing atmosphere. Silicate (partially deposited as an oxide) attached to the surface of stainless steel in step (ii) in a method that includes annealing and (iv) then pickling or mild cold working after pickling. However, it is said that the growth of oxide scale caused by annealing is suppressed to reduce the degree of decrease in Cr concentration on the surface of the stainless steel material and grain boundaries, and the corrosion resistance of the stainless steel material is improved.

  This method seems to be effective for suppressing the decrease in Cr concentration on the stainless steel surface and grain boundaries. However, no proposal has been made for measures against the deterioration of the excellent corrosion resistance inherent in stainless steel due to the above-described decrease in surface cleanliness.

JP 7-150399 A

  The present invention prevents the excellent corrosion resistance inherent in stainless steel or stainless steel pipes from being deteriorated by the reduction in surface cleanliness as described above, and provides semiconductor manufacturing, chemical industry, food industry, thermal power or nuclear power generation. A method of manufacturing stainless steel or stainless steel pipe having excellent surface cleanliness and corrosion resistance, which can be widely applied as a component of equipment and devices in industrial fields such as equipment, particularly for feed water heaters installed in nuclear power generation equipment, etc. It aims at providing the manufacturing method of a stainless steel pipe suitable as a small diameter and long pipe, such as a heat exchanger tube.

  In order to solve the above problems, the present inventor has studied a method for improving the cleanliness of the surface by subjecting the surface of the stainless steel or stainless steel pipe to mechanical grinding or electrolytic polishing. As a result, for stainless steel pipes, in addition to the cleaning work of immersing the steel pipe in degreasing liquid (using alkaline degreasing liquid), deposits such as chloride and sulfide on the surface, or lubricating oil applied as a lubricant, It has been found that the surface can be removed by performing mechanical grinding or electropolishing, followed by heat treatment to improve the cleanliness of the tube surface and improve the corrosion resistance.

The present invention has been made based on this finding, and the gist of the method for manufacturing a stainless steel tube under SL (2). The following (1) stainless steel production method is a reference invention.

(1) A stainless steel manufacturing method in which stainless steel is cold worked, degreased and then heat treated, wherein the degreasing step comprises:
(A) a first degreasing step by immersion (b) method for producing a stainless steel and a second degreasing step by cutting machine Lab.

  In the method for producing stainless steel according to (1), it is desirable that the cold working is performed by oil lubrication so that the surface roughness does not deteriorate.

  Here, as described above, “stainless steel” mainly refers to a stainless steel material including a steel pipe and a steel plate, but is not limited to this, including intermediate products before being processed into the stainless steel material, "Stainless steel" as a cold material. The “stainless steel” is a material that is obtained in a hot process and that is subjected to cold working.

(2) A stainless steel pipe manufacturing method in which a stainless steel base pipe is cold worked and degreased, followed by heat treatment, wherein the degreasing step comprises:
(A) The 1st degreasing process by immersion, (b) The manufacturing method of the stainless steel pipe which consists of the 2nd degreasing process which grinds the surface of a pipe | tube by the blast process which uses an abrasive grain .

  In the method for producing a stainless steel pipe according to (2), it is preferable that the cold working is performed by oil lubrication so that the surface roughness does not deteriorate.

  The above-mentioned “stainless steel pipe” is a material obtained by hot pipe making and used for cold working.

  The method of the present invention can be applied to stainless steel containing chromium such as austenitic stainless steel and ferritic stainless steel, and can also be applied to other Ni-based alloys.

  According to the manufacturing method of a stainless steel or the manufacturing method of a stainless steel pipe of the present invention, a stainless steel or a stainless steel pipe excellent in surface cleanliness and corrosion resistance, in particular, a small diameter and long length whose manufacturing target is difficult to clean the inner surface of the pipe. Even if it is a heat exchanger tube, the heat exchanger tube which has the outstanding surface cleanliness and corrosion resistance can be manufactured.

The method for producing stainless steel according to the present invention (method (1)) is “a method for producing stainless steel in which a stainless steel is cold worked and degreased, followed by heat treatment, wherein the degreasing step comprises: ,
(A) A first degreasing step by dipping and (b) a stainless steel manufacturing method comprising a second degreasing step by mechanical grinding or electrolytic polishing ”. Here, the degreasing process is referred to as the “first degreasing process” and the “second degreasing process” to distinguish the degreasing processes in this order (that is, after performing the first degreasing process, the second degreasing process). This is to clearly show that

On the other hand, the method for producing a stainless steel pipe of the above (2) is a method for producing “stainless steel pipe” in particular among stainless steels, and “after cold working and degreasing a stainless steel base pipe, A method of manufacturing a stainless steel pipe for performing heat treatment, wherein the degreasing step includes:
(A) A first degreasing step by dipping and (b) a method for producing a stainless steel pipe comprising a second degreasing step by mechanical grinding or electrolytic polishing ”.

  Here, the general manufacturing process of stainless steel is outlined below.

[Melting and refining process]
In this process, molten steel melted in an electric furnace or molten iron from a blast furnace is put into a refining furnace, desulfurized, dephosphorized, and decarburized, and then alloying elements are added so as to be within the product component regulations. And deoxidation treatment. In the refining of stainless steel and Ni-base alloy, generally, processing is performed by using an AOD furnace, a VOD furnace alone, or a VOD furnace after an AOD furnace. In particular, a steel type that needs to keep the nitrogen content as low as possible, such as high purity ferritic stainless steel, is processed in a VOD furnace. In addition, there are some which combine secondary dissolution for the purpose of countermeasures against cavities, segregation and precipitate reduction.

  As a raw material for melting in this electric furnace or blast furnace, there are a method using scrap, a method using molten pig iron obtained using iron ore as a raw material, and a method using a combination thereof.

  In the method using scrap, by using a scrap having a composition relatively close to that of a product as a raw material, there is an advantage that the amount of additional alloy raw material to be added can be reduced and the component adjustment is easy. However, in general, it is inevitable that impurities are mixed into scrap, and it is difficult to remove the impurities in the refining process, and it is necessary to dissolve the scrap using high-priced and expensive alloy raw material to reduce the amount of impurities in the product.

  On the other hand, in the method using hot metal, since hot metal is iron ore reduced at a high temperature, the content of alloying elements and impurities is high but the purity of the product is high. Therefore, the content of impurities such as P, S, and Cu, which are generally undesirable in terms of material properties such as corrosion resistance and hot workability, can be kept low. Further, in the production of high purity ferritic stainless steel, it is possible to prevent Ni from being mixed from scrap. For steel materials used in nuclear power, Co, which is harmful to radiation contamination, can be reduced to a minimum, so this hot metal method is effective.

  Furthermore, compared with melting in an electric furnace, a large amount can be melted in a blast furnace, so that melting at a low cost is possible.

  However, when hot metal is used, the amount of alloy raw material used for adjusting the components increases, so that it is not suitable for the production of high alloys having a high content of alloy elements other than iron.

[Ingot making, material manufacturing process]
There are a steel ingot method and a continuous casting method as a method of ingoting molten steel produced in a refining process into a stainless steel slab. The slab manufactured by the steel ingot method or the continuous casting method is processed into a material for manufacturing a steel pipe, a steel plate, or the like of the next process such as a slab, a bloom, or a billet by rolling or forging. In the continuous casting method, a slab having a rectangular cross section (slab, bloom) or a round slab (such as a curved continuous caster, a vertical continuous caster or a horizontal continuous caster) Billet). Moreover, you may use the slab (slab and billet) obtained by continuous casting as a raw material for manufacture of a steel plate or a steel pipe.

The material obtained by rolling or forging the slab is broken down into giant crystals that are the cast structure of the slab, resulting in a fine rolled structure. There are few troubles. In addition, the continuous casting method has a higher yield than the steel ingot method, and when the continuous casting material is used as it is as a material for manufacturing a steel plate or a steel pipe, the rolling and forging processes are omitted, so that the economic effect is greater.

[Steel plate manufacturing process]
The material obtained in the ingot-making and material manufacturing process is hot-rolled using a thick plate rolling machine or a hot strip mill to obtain a plate-shaped or coil-shaped stainless steel plate. The obtained hot-rolled steel sheet is then heat-treated, pickled, and in some cases, further cold-rolled.

[Steel pipe manufacturing process]
As a method for producing a stainless steel pipe using the material obtained in the agglomeration and material production process, the center portion of the material using a hot press piercing mill and an inclined rolling perforation mill is known as a seamless pipe production method. After that, the tube is stretched with a mandrel mill, plug mill, assel mill, and pilga mill, and the inclined pipe type hot pipe manufacturing method, air hull push bench pipe manufacturing method, Eugene Examples thereof include a hot pipe manufacturing method of a press method such as a Jurney tube manufacturing method and mandrel forging. A cold-finished stainless steel pipe may be obtained by combining cold working such as drawing and rolling after hot pipe making.

  In addition to the seamless pipe manufacturing method, a stainless steel tube may be obtained by pipe forming and welding using a hot rolled coil or a cold rolled coil obtained by manufacturing a steel plate.

[Heat treatment, finishing, inspection process]
Stainless steel plates and steel pipes manufactured through the above steps are subjected to heat treatment for softening and solidification, as well as finishing treatments such as pickling, shot blasting, surface polishing, etc. It becomes a product through inspection processes such as dimensional inspection.

  Below, the manufacturing method of the stainless steel of said (1) after hot processing which is the characterizing part of this invention is mainly demonstrated, and the manufacturing method of the stainless steel pipe of (2) is described as needed.

  It is as follows when the manufacturing method of the stainless steel of (1) is arranged in order of a process. In addition, this process order is the same also in the manufacturing method of the stainless steel pipe of said (2) except using a stainless steel base pipe as a stainless steel base in (A).

(B) Cold working stainless steel (b) First degreasing by dipping (c) Second degreasing by mechanical grinding or electrolytic polishing (d) Heat treatment is performed.

  That is, this stainless steel manufacturing method is [cold working] → [dip degreasing] → [degreasing by mechanical grinding] → [heat treatment] or [cold working] → [immersion degreasing] → [degreasing by electrolytic polishing] → This is a method for producing stainless steel having a general process of [heat treatment]. The “immersion degreasing” is degreasing performed by immersing the workpiece in a degreasing liquid such as an alkaline liquid.

  FIG. 1 is a diagram showing an example of a general stainless steel pipe manufacturing process to which the stainless steel pipe manufacturing method of (2) is applied, particularly when the stainless steel is a stainless steel pipe. (A) of the figure is a process example of the conventional method, (b) is a process example for comparison, and (c) is a process example to which the manufacturing method of the present invention is applied.

  In FIG.1 (c), the part enclosed with the broken line corresponds to the process of said (A)-(D) prescribed | regulated by this invention. That is, the steps (a), (b), (c) and (d) shown in the broken line in the figure are respectively the above-mentioned (a), (b), (c) and (d) defined in the present invention. ). In addition, “degreasing” in the step (b) shown in the figure means immersion degreasing.

  As shown in FIG. 1 (c), the stainless steel base tube obtained by hot pipe making is subjected to a chemical conversion film lubrication treatment or an oil lubrication treatment and then cooled in the steps (a) to (d). After being subjected to hot working, degreasing and heat treatment and, if necessary, "pickling" treatment, in each process of "preparation" and "inspection" Adjustment "and" inspection "are performed. Furthermore, U-bending processing and finishing inspection are performed as necessary.

  On the other hand, in the process example of the conventional method in FIG. 1A, the process corresponding to the above-mentioned provision of the present invention is [cold working] → [degreasing] → [heat treatment], and degreasing by mechanical grinding. This process is not included. Also in the aforementioned Patent Document 1, (i) a cold-worked stainless steel material is prepared, (ii) immersed in an aqueous solution containing a water-soluble inorganic silicate (also serves as degreasing), and (iii) annealed in an oxidizing atmosphere. The process of [cold working] → [degreasing] → [heat treatment] is described, but nothing is described about degreasing by mechanical grinding (or electropolishing).

  Below, the process of (i)-(d) prescribed | regulated with the manufacturing method of the stainless steel of this invention is demonstrated in detail.

  The step (a) is a step of “cold working stainless steel”.

  The surface of the stainless steel obtained by the hot process has a rough surface due to the generation of scale, and the variation in roughness is large. On the surface, there is a Cr removal layer that accompanies scale formation, and this Cr removal layer reaches deeper, especially at the grain boundary portion. The surface of the stainless steel after the scale has been removed is in a state where the crystal grain boundaries are dug into a groove shape due to corrosion during descaling by pickling. This groove exists on the entire surface in a mesh shape, and usually depends on the time, degree of processing, and atmosphere exposed to high temperature during hot working, but usually reaches about several μm to several tens of μm. And the shape that is easy to collect contaminants.

  When this stainless steel is cold worked, the surface is crushed by contact with the tool under high surface pressure, and when it is stretched, it undergoes shear deformation due to frictional force, and the groove-like portion is crushed. The part which deform | transformed into this and opened in groove shape is contact | adhered. That is, the lubricant and dirt accumulated in the groove are squeezed out, and the surface roughness is also improved. This is more conspicuous as the degree of processing is higher. Thus, the stainless steel for applications in which corrosion resistance is particularly required must be a cold-worked product with improved surface smoothness and cleanliness.

  The rough surface after pickling and removing the de-Cr layer can be smoothed by cutting and grinding. However, the efficiency is lower than that of cold working, and chips are discharged, resulting in poor yield and increased cost. Therefore, cold working is an essential process in the method for producing stainless steel of the present invention.

  Cold work is performed at least once to create a new surface and improve roughness. The cross-sectional reduction rate during cold working is desirably 20% or more. More desirably, it is 40% or more. As a result, a new surface is created, the removed Cr layer is crushed and averaged to normalize the Cr concentration in the surface layer portion, and the lubricant and dirt are removed by improving the surface roughness.

  The processing method is not particularly limited. If a stainless steel pipe is produced, either drawing or rolling may be used, and drawing and rolling may be combined. The drawing is a core drawing with a tool on the inner surface. In the case of emptying, generation of a new surface on the inner surface is insufficient, which is not desirable.

  In order to obtain a good new surface, it is desirable to carry out rolling at least once so as to obtain a large cross-sectional reduction rate.

  In addition, although a lubrication process is performed before cold working, it is performed by a chemical conversion film lubrication process or an oil lubrication process. Chemical film lubrication treatment is a lubrication method that imparts lubricity by forming a chemical film by chemical reaction. Surface roughness is degraded by etching (corrosion), and it is difficult to remove the lubricating film (chemical film). There is a problem that it is necessary to remove the film by “pickling” after cold working. On the other hand, in the case of oil lubrication, since the lubricant (oil) is only physically attached to the pipe surface, the lubricant is almost removed by immersion degreasing performed after cold working. Therefore, it is desirable to perform oil lubrication before cold working, and when performing cold working a plurality of times, it is desirable to perform oil lubrication at least in the final pass.

  The step (b) is a step of “performing first degreasing by immersion”.

  Lubricant adheres to the stainless steel after cold working, and if this is heat-treated as it is, carburization occurs, resulting in quality problems. In order to remove the lubricant, degreasing is first performed by immersion. “Degreasing by dipping” is degreasing performed by dipping in a degreasing solution, and may be performed by a commonly used method. For the immersion degreasing, a solution such as an organic solvent can be used in addition to an alkaline solution. When performing immersion degreasing, depending on the solution used, it may be heated.

  When the chemical conversion film lubrication treatment is performed during the cold working of (a), pickling is further required after immersion degreasing. However, when the pickling treatment is performed, the grain boundaries are particularly eroded and the surface roughness is deteriorated. When oil lubrication is performed, the lubricant is only physically attached and is almost removed by immersion degreasing, so pickling is not required.

  The step (c) is a step of “performing second degreasing by mechanical grinding or electrolytic polishing”.

  It is judged that the degreasing agent (lubricating oil used in the oil lubrication treatment) is apparently removed on the surface of the stainless steel after the cold working and immersion degreasing to perform such treatment. This is because a slight amount remains (deposits), and when heated, hydrocarbons and other corrosive gases are generated from the remaining deposits. In particular, in the case of a stainless steel pipe, as described below, it is difficult to remove deposits on the inner surface of the pipe, and when used as a pipe, corrosion of the inner surface of the pipe and contamination with substances passing through the pipe are likely to occur.

  Lubricating oils used during metal processing generally contain oils and fats as main components and contain compounds such as F, Cl, S, and P as extreme pressure additives. The deposits are present on the inner and outer surfaces of the tube, but during heat treatment, the gas generated from the deposits on the outer surface scatters and dilutes into the furnace and is further diluted with the ambient gas supplied continuously. Few. However, the gas generated from the deposits on the inner surface stays in the pipe and becomes a non-negligible amount with respect to the internal volume in the pipe.

Without performing the second degreasing step of the present invention, that is, the step (c), the stainless steel pipe after the oil lubrication treatment and the cold working was degreased by dipping in the step (b). Immediately thereafter, the gas in the tube when heat-treated (bright heat treatment) in a conventional hydrogen furnace using hydrogen as the atmospheric gas was collected and analyzed. As a result, even after the inside of the tube is sufficiently replaced with hydrogen, the gas mainly composed of hydrocarbon, CO, CO ₂ , N ₂ , and O ₂ is combined even if the material to be treated is charged into the furnace and heat treated. It was found that 6 to 10% by volume (the balance was H _{2 of} the atmospheric gas) was contained. Hydrocarbon, CO, and CO ₂ are gases generated from residual deposits after degreasing, and N ₂ and O ₂ are considered to be derived from the air that was present in the pipe before charging. It was also confirmed that a gas containing F, Cl, S, etc. contained in the extreme pressure additive in the lubricating oil was generated from the residual deposits and stayed in the pipe although it was a small amount. In addition, these components are components which are problematic even in a minute amount from the viewpoint of “contamination” with respect to substances passing through the tube when used as piping or heat transfer tubes.

  Furthermore, it was found that the generation of gas from such residual deposits was found to exceed 200 ° C., and the generation amount gradually decreased when the temperature exceeded 300 ° C., and almost no generation occurred at 500 ° C. or higher.

  If the heat treatment is continued while these gases (the above-mentioned hydrocarbons and CO) remain in the pipe, the C (carbon) activity of the gas increases. For example, if the C activity of stainless steel is higher, carburization may occur.

  In addition, halogen elements such as fluoride and chloride exist as halogen compounds of iron and Ni that react with the surface of the stainless steel tube during cold working to gasify and condense in the range from room temperature to about 300 ° C. There are many cases. The iron and Ni halogen compounds are gasified during heat treatment, and those on the outer surface of the tube are scattered and diluted as described above, but those gasified on the inner surface of the tube stay in the tube and reappear on the inner surface of the tube during cooling. Adhere to. The reattached material is concentrated and has adverse effects such as generating rust even in the atmosphere.

  Therefore, in the present invention, deposits remaining on the inner surface of the tube, particularly after cold working (step (b)) and before heat treatment (step (d)) after immersion degreasing (step (b)). The second degreasing (step (c)) is performed by mechanical grinding or electropolishing to remove mechanically.

  As the method, grinding with abrasive grains, felt or sponge cleaning can be applied. At that time, it is desirable that the polishing agent used for polishing, such as a polishing belt and a grindstone, and the coolant such as the coolant do not include those which are the sources of gas as described above.

  For abrasives, binders are used to solidify and hold abrasive grains like abrasive belts and grindstones. When these abrasives are used for polishing, the surface is ground and the binder is rubbed into the surface at the same time. Occurs. In general, a resin is often used for the binder and can be a C source. Therefore, a method using only abrasive grains and compressed air (air), such as a blast method, is desirable.

  As abrasive grains, it is usually preferable to use sand, alumina, zirconia or the like. It is desirable that these do not contain a low melting point metal such as a halogen element, a C source, Zn, Sn, or Cu. For example, when sand is used, if the object of treatment is stainless steel, river sand containing as little chlorine as possible is used. Sea sand is not desirable because it contains a large amount of chlorine.

  Since there is a possibility that the lubricant used in the compressor may be mixed into the air, it is desirable to filter it and keep it free from oil.

  Felt or sponge cleaning is an effective way to wipe off dirt. However, if it is pressed strongly against the surface and used in a manner that generates a large amount of heat, there is a risk that carbonized by heat will adhere to the steel surface. Felt and sponge are also used to confirm the removal of dirt.

  When an organic solvent is used for coolant (cooling material) or dirt removal, it becomes a gas generation source. Therefore, it is better not to use an organic solvent as much as possible. Pure water may be used as a solvent for cooling and cleaning to remove dirt, but tap water contains chlorine and is not desirable.

  Even when processing a large amount of a small diameter and long stainless steel pipe, a blasting method using sand or the like as an abrasive is suitable. Since the first degreasing by dipping has already been carried out in the process (b), the surface remains slightly ground so that the abrasive grains are distributed over the entire inner surface of the tube, and the deposits remaining on the surface are removed. it can. For this reason, a sufficient removal effect can be obtained by blasting for several tens of seconds. Blasting for about 2 minutes including the time required for stabilizing the flow rate in the pipe may be used. Therefore, the inner diameter change (thickness reduction) before and after blasting in this case is extremely small and within the measurement error range.

  In addition, when manufacturing a stainless steel pipe by the method of this invention, especially a long steel pipe with a small diameter, the surface roughness of a pipe inner surface has big influence. That is, if the surface roughness is large, dirt such as lubricant is likely to accumulate in the concave portion, and it is difficult to wash and remove this by immersion degreasing, and abrasive grains are also present in the concave portion during degreasing by mechanical grinding. It is necessary to grind a large amount of the periphery before it reaches. For this reason, the time and labor required to obtain a certain degree of cleanliness become extremely large. Therefore, in particular, when manufacturing a long stainless steel pipe with a small diameter, it is important to increase the smoothness of the inner surface by performing cold working that can increase the cross-section reduction rate in the step (a). It is.

  In the second degreasing performed after the immersion degreasing, degreasing by electropolishing may be performed instead of degreasing by mechanical grinding. Electropolishing is a method of obtaining a polishing effect by using a stainless steel as an anode and passing a direct current through an electrolytic polishing solution to dissolve the steel surface. Since the steel surface is also ground by electrolytic polishing, the same effect as when mechanical grinding is performed can be obtained.

As the electrolytic polishing liquid, sodium nitrate (NaNO ₃ ) or the like that is usually used may be used. The current density and electrolysis time during electropolishing are usually 0.1 to 0.5 A / cm ² for 30 to 500 seconds, although depending on the amount of deposits remaining on the stainless steel surface and the difficulty of removal. It is good to energize.

  The step (d) is a step of “performing heat treatment”.

  Although the atmosphere during the heat treatment is not particularly limited, it is usually preferable to perform an air furnace heat treatment using air and combustion gas. That is, the heat treatment cost can be reduced by using an atmospheric furnace as a heat treatment furnace and passing the combustion exhaust gas of LNG as a fuel through the heat treatment furnace, for example. However, in this case, in order to prevent carburization, it is necessary to lower the C activity of the atmospheric gas than the C activity of stainless steel. For this purpose, it is preferable to reduce the carbon monoxide concentration in the atmospheric gas by completely burning the fuel gas. Normally, there is no problem if the supply amount of combustion air is set to a theoretical air amount or more.

  Heat treatment (bright heat treatment) in a hydrogen furnace containing hydrogen as a main component may be used. Such an atmospheric gas is used in order to suppress the surface oxidation of the stainless steel, and only hydrogen may be used, or an inert gas such as He or Ar may be mixed with hydrogen. However, since inert gas is expensive, it is usually not added aggressively.

  Nitrogen is an inexpensive gas and can be used by mixing with hydrogen. However, depending on the nitrogen content level of the stainless steel pipe, penetration (nitriding) or denitrification of the base material occurs. Therefore, the mixing ratio is preferably adjusted in the range of 0 to 10% by volume according to the nitrogen level of the base material. Note that when an atmospheric gas containing hydrogen as a main component is used, an inevitably mixed impurity gas is allowed.

  The atmospheric gas may be appropriately selected from the above-mentioned various gases according to restrictions on the production of stainless steel or stainless steel pipe, the use of the product, and the like.

  As described above, the stainless steel manufacturing method or stainless steel pipe manufacturing method of the present invention includes the steps of [cold working] → [immersion degreasing] → [degreasing by mechanical grinding or electrolytic polishing] → [heat treatment]. It is a manufacturing method of a stainless steel pipe. On the other hand, the process example of the conventional method is [cold working] → [degreasing] → [heat treatment] and does not include the [degreasing by mechanical grinding or electrolytic polishing] process.

  On the other hand, conventionally, descaling may be performed after heat treatment. This is shown as a comparative example in FIG. That is, after [cold working] → [degreasing] → [heat treatment], a mechanical grinding means is used in a method having a process of [mechanical descale], and formally, the method for producing stainless steel of the present invention Or the manufacturing method of a stainless steel pipe can be seen only by exchanging [heat treatment] and [mechanical grinding (mechanical descale)] in the comparative example.

  However, the method of the comparative example and the method of the present invention are fundamentally different in the following points.

  First, according to the method of the comparative example, the deposits remaining on the surface of the stainless steel or the stainless steel pipe after immersion degreasing are gasified at a temperature of less than 500 ° C. during the heat treatment, and the gasified component is stainless steel during the soaking. It diffuses and penetrates into the base material. Therefore, the polishing time required to remove the portion where the gasification component has diffused and penetrated is enormous, and the diffusion depth cannot be grasped, and it is extremely difficult to confirm whether or not the portion has been removed. On the other hand, in the method of the present invention, only a small amount of deposits that could not be removed by immersion degreasing are removed, so that the object can be achieved in a short time as described above.

  Secondly, in the method of the comparative example, the grinding allowance must be increased in order to remove the invasion portion of the gasification component, so that the inner diameter of the tube is increased, the wall thickness is reduced, and the tolerance is not entered. Cases can also occur. In the method of the present invention, since the processing is performed for a short time, the dimensional change is small.

  Third, in the method of the comparative example, after [heat treatment] and when [mechanical descaling] is performed, if there is time in standby or the like, the gas component diffused and entered into the base material is, for example, a halogen compound. Since corrosion proceeds promptly, it is difficult to guarantee the depth of corrosion, and it must be rejected. In the method of the present invention, such a problem does not occur because the deposit is removed before being decomposed and gasified by the heat treatment.

  Thus, the stainless steel or stainless steel pipe obtained by the method of the present invention is excellent in surface cleanliness and therefore excellent in corrosion resistance. Since this stainless steel or stainless steel pipe is degreased by the minimum necessary mechanical grinding or electrolytic polishing after immersion degreasing, for example, even when abrasive grains are blasted as mechanical grinding, the surface processing strain layer is minimized, and It is also possible to minimize the amount of adhesion of chloride, sulfide and the like.

  It is well known that when there is a work strain layer on the surface, sensitization is accelerated, dislocation density is high and it is easy to crack, and chloride and sulfide accelerate stress corrosion cracking (SCC) of stainless steel or stainless steel pipe. It is well known that there is an action, but the stainless steel or stainless steel pipe obtained by the method of the present invention has a very excellent property with respect to stress corrosion cracking resistance by suppressing these adverse effects slightly. Is also a big feature.

  FIG. 1 (c) shows a stainless steel base tube obtained by a seamless pipe manufacturing method using SUS304L (austenitic stainless steel) obtained through melting, refining process, ingot making, and raw material manufacturing process. A stainless steel pipe is manufactured in a process according to the pipe making process after hot pipe exemplification as shown in, and the thickness reduction, surface roughness, and surface processing by mechanical grinding or electrolytic polishing (hereinafter referred to as “mechanical grinding, etc.”) The depth of strain layer, carbide precipitation (corrosion test with 10% oxalic acid) and the amount of deposits (chloride, sulfide) were investigated. In addition, the dimensions of the manufactured tube are an outer diameter of 16 mm, a thickness of 1.2 mm, and a length of 14 to 20 m.

  The evaluation method is as follows.

[Thickness reduction due to mechanical grinding, etc.]
When the thickness reduction amount is measured at three locations (both ends and the center in the length direction) of the manufactured tube, and the average value is 2/100 mm or less, “○ (good)”, if it exceeds 2/100 mm "X mark (defect)".

〔Surface roughness〕
The surface roughness of the three inner surfaces of the tube is indicated by the centerline average roughness Ra (μm). If the average value is 0.5 μm or less, “◎ (very good)”, 0.5 μm to 1 μm or less If there was “◯ mark (good)”, if it exceeded 1 μm, “△ mark (good)”.

[Surface depth of surface processed strain]
Surface processed strained layer depth is the thickness of the processed strained layer on the surface of the material by mechanical grinding, etc. performed after immersion degreasing, and the crystal in the deepest part of the crystal grains where a sliver line is observed by micro observation of the cross section of the stainless steel pipe The depth of the grain. If there was no slip line or the depth of the crystal grain in the deepest part was 3/100 mm or less, it was rated as “◯ (good)”, and if it exceeded 3/100 mm, it was marked “x (defect)”.

[Carbide precipitation (corrosion test with 10% oxalic acid)]
For each of the stainless steel pipes that had been heat-treated after cold working, specimens were taken at 5 m regular intervals in the length direction and subjected to sensitization treatment at 625 ° C. for 2 hours to precipitate Cr carbide at the grain boundaries. After forming a Cr-deficient layer (region) in the grain boundary adjacent portion, a corrosion test with 10% oxalic acid was performed. The test was performed according to the method specified in JIS G 0571 (10% oxalic acid etch test method for stainless steel).

  In the above test, if no groove-like structure is observed at the grain boundary in any crystal grain, “◎ (very good)”, even in the case where a groove-like structure is observed, the groove is found in any crystal grain. "○ mark (good)" when the grain structure is recognized only at 5% or less of the grain boundary, "X mark (when the crystal grain structure is found at a portion where the groove structure exceeds 5% of the grain boundary" Bad) ”.

[Amount of deposit (amount of chloride and sulfide)]
After the pure water is sealed in the tube and the deposits on the inner surface are eluted, the concentration of Cl ion and SO ₄ ion in the sealed water is obtained by ion chromatography, and the amount of chloride per unit surface area is determined from the amount of sealed water and the surface area in the tube ( mg / m ² ) and the amount of sulfide (mg / m ² ) were calculated. When the total amount of chloride and sulfide was 1 mg / m ² or less, “◯” (good), and when it exceeded 1 mg / m ² , “×” (bad).

  The pipe making conditions are shown in Table 1, and the survey results are shown in Table 2.

  In Table 1, “alkali” in the column of “degreasing method” means degreasing by immersing in an alkaline solution and then washing with warm water, and “alkali → pickling” is degreasing by immersing in an alkaline solution. Then, after washing with warm water, pickling and then washing with water.

  In mechanical grinding, 0.4 MPa (gauge pressure) clean air was used in all cases. In the “type” column of “mechanical grinding or electropolishing”, for example, “blast / minute” means that the blast injection time is one minute, “reciprocating grinding / 10 times” means paper, etc. “Cleaning / 5 times” means that the polishing by reciprocating was performed 10 times, and “cleaning / 5 times” represents that cleaning by felt or the like was performed 5 times.

The “electropolishing” was performed by using NaNO ₃ (temperature 20 ° C.) as the electrolytic polishing liquid and applying a direct current with a current density of 0.3 A / cm ² for 120 seconds.

“Hydrogen” in the “Atmosphere gas” column of “Heat treatment” means that H ₂ is substantially 100% by volume.

  As is clear from Tables 1 and 2, Examples 1 to 6 and Examples 8 to 11 of the present invention in which machine grinding or the like was performed after immersion degreasing and before heat treatment, included reduction in thickness due to mechanical grinding or the like. All items surveyed were good. In Example 7 of the present invention, the surface roughness Ra was slightly large, but this was because the chemical conversion film was lubricated before the cold working. Good results were obtained in the depth 10% oxalic acid corrosion test and the amount of deposits (chlorides, sulfides).

  On the other hand, among the conventional examples 12 to 14 in which mechanical grinding or the like was not performed before the heat treatment after the immersion degreasing, the conventional example 12 in which the chemical conversion film lubrication treatment was performed before the cold working had a poor surface roughness Ra. Even in a corrosion test with 10% oxalic acid, good results were not obtained. Moreover, in the prior art examples 13 and 14 which performed the oil lubrication process, since the removal of the deposit | attachment in a pipe | tube was not enough, the deposit | attachment was recognized in the pipe | tube.

On the other hand, although mechanical grinding was performed after immersion degreasing, in Comparative Examples 15, 17, and 18 performed not after “before heat treatment” as defined in the present invention but after “heat treatment → after pickling” or “after heat treatment”, In Comparative Example 15 in which the surface processed strained layer depth was poor and the chemical conversion film lubrication treatment was performed before cold working, generally good results were not obtained except for the amount of deposits (chlorides and sulfides). . Since Comparative Example 16 was felt cleaning, there was no cutting force and the carburized portion was not removed, and the 10% oxalic acid corrosion test was poor. Deposits in Comparative Example 18 (chlorides, sulfides) of the amount was poor, due to the blast treatment with sea sand (mechanical grinding) was performed after the heat treatment, chlorine contained in the sand This is because it adhered and remained as chloride .

  In addition, although this Example is the investigation result about the stainless steel pipe which uses SUS304L as a raw material, it is the same also about stainless steel containing chromium, such as other austenitic stainless steel and ferritic stainless steel, and also Ni base alloy Results were obtained.

  The method for producing stainless steel or the method for producing stainless steel pipe of the present invention can be widely applied as a component of equipment and equipment in industrial fields such as semiconductor production, chemical industry, food industry, thermal power and nuclear power generation equipment. Suitable for manufacturing stainless steel pipes with excellent cleanliness and corrosion resistance, especially stainless steel pipes used as small and long pipes, such as heat transfer pipes for water heaters installed in nuclear power generation facilities, etc. Can do.

It is a figure which shows the example of a pipe making process after the hot pipe making of the general stainless steel pipe which applied the manufacturing method of the stainless steel pipe of this invention, (a) is a process example of a conventional system, (b) is for a comparison. A process example, (c) is a process example to which the production method of the present invention is applied.

Claims (2)

A stainless steel pipe is cold-worked, degreased, and then subjected to heat treatment, which is a method for producing a stainless steel pipe, wherein the degreasing step includes:
(1) A first degreasing step by dipping and (2) a second degreasing step of grinding the surface of the tube by blasting using abrasive grains . The method for producing a stainless steel pipe according to claim 1 , wherein cold working is performed by oil lubrication.

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