JP5467673B2 - Ferritic stainless steel for corrugated tubes - Google Patents

Description

  The present invention relates to a ferritic stainless steel used for a corrugated tube.

  Conventionally, stainless steel pipes and copper pipes are often used for hot and cold water pipes, gas pipes, refrigerant pipes, heat exchangers, and the like. In particular, stainless steel pipes have higher corrosion resistance than copper pipes, and are therefore suitable for use in corrosive liquid piping and general piping in salt damage areas where outer surface corrosion resistance is a problem. On the other hand, stainless steel pipes have a higher strength and lower bendability than copper pipes, so there are cases where they are avoided as applications when there are many manual operations.

  Thus, a flexible pipe having a groove formed on the peripheral surface by corrugating or bulging to improve bendability may be used.

  Here, the corrugating process is a processing method in which a die is pressed from the outer surface side to form a groove on the peripheral surface of the pipe, and the stainless steel pipe manufactured by the corrugating process is called a so-called corrugated pipe. Bulge processing is to perform overhang processing on a part of a pipe using hydraulic pressure or the like.

  When corrugating and bulging are performed on a stainless steel pipe, austenitic stainless steel that is large in elongation and easy to process is often used. On the other hand, ferritic stainless steel has been considered unsuitable for these processes because of its small uniform elongation.

  Therefore, conventionally, as a ferritic stainless steel for such steel pipes, ferritic stainless steel that has improved workability by reducing ductility or toughness by reducing C and N to enable corrugating and bulging. Stainless steel plates and corrugated pipes are known (see, for example, Patent Document 1 and Patent Document 2).

JP-A-8-121991 (pages 2 and 3) JP-A-9-143630 (pages 2 and 3)

  However, in the above-described corrugated pipe of Patent Document 1 and stainless steel of Patent Document 2, in the case of forming a steel pipe having a small plate thickness and a small diameter, compared to a steel pipe having a large plate thickness or a steel pipe having a large diameter, welding is performed. The shape of the part is likely to be unstable, and cracks are likely to occur due to the difference in strength between the welded part and the material part.

  This invention is made | formed in view of such a point, and it aims at providing the ferritic stainless steel excellent in the manufacturability of a corrugated pipe.

The ferritic stainless steel for corrugated pipes according to claim 1 is, in mass%, 0.03% or less C, more than 0.1 to 2% Si, 2% or less Mn, 0.05% or less P. 0.005% or less of S, from 10.5 to 26% of Cr, 0.70% or less of Cu, 0. 4% or less of Al, 0.03% or less of N , 0.8% or less of Nb, and 0.8% or less of Ti and at least one of Ti, with the balance being Fe and inevitable impurities, (Ti % + Nb%) ≧ (7 × (C% + N%) + 0.05), and contains 0.004 to 0.20% of inclusions excluding MnS with the cleanliness specified by JISG0555 The surface hardness is 130 to 200 HV in terms of Vickers hardness.

  The ferritic stainless steel for corrugated pipes described in claim 2 is the ferritic stainless steel for corrugated pipes described in claim 1, and is Mo, V, and so that the total content is 4% or less in mass%. It contains at least one of W.

  The ferritic stainless steel for corrugated pipes according to claim 3 is the ferritic stainless steel for corrugated pipes according to claim 1 or 2, and is Ni and Co so that the total content is 5% or less by mass%. It contains at least one of these.

  According to the first aspect of the present invention, C, Si, Mn, P, S, Cr, Cu, Nb, Ti, Al, and N are contained within a specified range, and (Ti% + Nb%) ≧ (7 × (C% + N%) + 0.05), the composition contains 0.004 to 0.20% of inclusions excluding MnS with the cleanliness specified by JISG0555, and the surface hardness is Vickers Since it is 130-200HV in hardness, generation | occurrence | production of the crack by corrugation processing can be suppressed, Therefore The productivity at the time of manufacturing a corrugated pipe | tube can be improved.

  According to the invention described in claim 2, by including at least one of Mo, V, and W so that the total content is 4% or less, excellent productivity is more reliably ensured. It can be secured.

  According to the invention described in claim 3, by including at least one of Ni and Co so that the total content becomes 5% or less, excellent productivity can be ensured more reliably.

It is a graph which shows the relationship between the cleanliness of this invention, and the fluctuation | variation range of a shear surface rate. It is a top view which shows the state which gave the corrugation process to the pipe formed in this invention. It is a graph which shows the relationship between the presence or absence of the crack in the corrugating process of this invention, the cleanliness before welding, and the surface hardness of the stainless steel before welding.

  Hereinafter, an embodiment of the ferritic stainless steel for corrugated pipe of the present invention will be described in detail. The content of each element is mass% unless otherwise specified.

  Each element constituting the ferritic stainless steel in the present embodiment will be described.

  C (carbon) and N (nitrogen) are elements inevitably contained in stainless steel. If the C and N contents are reduced, the formation of carbides and nitrides is reduced, and the weldability and corrosion resistance of the welded portion can be improved. Therefore, it is preferable that the C and N contents are low. However, in order to reduce the contents of C and N, it is necessary to lengthen the refining time. If the refining time is long, the cost of producing stainless steel increases. Therefore, considering the balance between corrosion resistance and cost, the upper limit of the C content is set to 0.03%, and the upper limit of the N content is set to 0.03%.

Si (silicon) is an effective element for forming SiO ₂ inclusions and stabilizing the slit end face properties. However, the excessive Si content exceeding 2% hardens the ferritic stainless steel phase and causes deterioration of workability. Further, when the Si content is 0.1% or less, the shape of the slit end face cannot be stabilized. Therefore, the Si content is in the range of more than 0.1% and 2% or less.

  Mn (manganese) is an effective element for forming MnO-based inclusions and stabilizing the slit end face properties. However, if Mn is excessively contained in excess of 2%, MnS is present in the steel and MnS acts as a starting point of cracking in the shearing process. The ratio with the cross section changes, and the slit end face properties become unstable. If the slit end face properties become unstable, the corrugation processability deteriorates. Therefore, the Mn content is 2% or less.

  Since P (phosphorus) is an element that lowers the toughness of the base material and the brazed part, the content is preferably low. However, since dephosphorization by refining is difficult in the production of Cr-containing steel, it is necessary to carefully select raw materials to reduce the P content extremely, and the cost increases. Therefore, if the P content is 0.05% or less, sufficient toughness of the base material and the brazed part can be secured. Therefore, considering the toughness and cost of the base material and the brazed part, the P content Is 0.05% or less.

  S (sulfur) is an element that forms MnS together with Mn. Since MnS acts as a starting point of cracks in shearing processing, the ratio of the shear surface to the fracture surface at the slit end surface varies depending on the position of MnS. Can do. And when S is contained excessively exceeding 0.005%, the amount of MnS increases and the starting point of the crack increases, so the slit end face properties become unstable. If the slit end face property is unstable, the shape of the welded portion becomes unstable, and the corrugation workability deteriorates. Therefore, the S content is 0.005% or less.

  Cr (chromium) is a main constituent element of the passive film, and improves local corrosion properties such as pitting corrosion resistance and crevice corrosion resistance. Moreover, in order to fully exhibit the effect, it is necessary to contain 10.5% or more of Cr. However, if the Cr content exceeds 26%, it becomes difficult to reduce C and N, and mechanical properties and toughness are lowered, and the cost is increased. Therefore, the Cr content is in the range of 10.5% to 26%.

  If Cu (copper) is contained in a large amount, the ferrite single-phase structure cannot be maintained, and the productivity deteriorates. Moreover, when Cu is contained excessively exceeding 0.70%, pitting corrosion resistance and crevice corrosion resistance will fall. Therefore, the Cu content is 0.70% or less.

Nb (niobium) is an element effective for fixing C and N and preventing intergranular corrosion which is a problem in stainless steel. However, when Nb is contained excessively exceeding 0.8%, the toughness of the stainless steel is lowered and the productivity is deteriorated. Therefore, the Nb content is 0.8% or less (including no addition) .

Ti (titanium) is an element effective for fixing C and N similarly to Nb and preventing intergranular corrosion which is a problem in stainless steel. TiN also has the effect of stabilizing the slit end face properties. However, when Ti is excessively contained exceeding 0.8%, a decrease in corrugation workability may be observed. Therefore, the Ti content is 0.8% or less (including no addition) .

Al (aluminum) is an effective element for forming Al ₂ O ₃ -based inclusions and stabilizing the slit end face properties. However, when Al is contained excessively exceeding 0.4%, hot workability is deteriorated. Therefore, the Al content is 0.4% or less.

  Mo (molybdenum), V (vanadium) and W (tungsten) are effective elements for improving the corrosion resistance together with Cr. However, when it contains excessively, it will have a bad influence on hot workability. Therefore, when these Mo, V, and W are contained, the total content needs to be 4% or less. Mo, V, and W may be contained alone or in combination as long as the total content is 4% or less.

  Ni (nickel) and Co (cobalt) are effective elements for improving the corrosion resistance. However, since it is an austenite generating element, if it is contained excessively, an austenite phase is generated at a high temperature, and a ferrite single phase cannot be maintained. Therefore, when these Ni and Co are contained, the total content needs to be 5% or less. Ni and Co may be contained alone or in combination as long as the total content is 5% or less.

  The balance other than the elements described above is Fe (iron) and impurities inevitably contained, and these unavoidable impurities are not intentionally contained.

  Here, since Ti and Nb are contained for the purpose of fixing C and N as described above, it is necessary to set the content according to the contents of C and N. Therefore, the composition is adjusted so that the relationship between the contents of Ti and Nb and C and N satisfies (Ti% + Nb%) ≧ (7 × (C% + N%) + 0.05). In addition, when content of Ti and Nb is insufficient, generation | occurrence | production of a grain boundary corrosion is anxious.

In stainless steel, TiN, MnS, Al ₂ O ₃ produced by a deoxidizer, MnO, SiO ₂ and composite inclusions thereof exist as inclusions. Among these inclusions, inclusions other than MnS are hard, and when the separation tip is associated with inclusions during the progress of shear deformation, separation proceeds in a manner that avoids inclusions, leading to local stress concentration. There is no. Moreover, since the form is a block shape, the shear deformation proceeds without branching the separation. Therefore, the ratio between the shear surface of the slit fracture surface and the fracture surface is stabilized. The effect of stabilizing the ratio of the sheared surface to the fractured surface of the slit fracture surface can be sufficiently exerted by setting the cleanliness of inclusions excluding MnS defined in JISG0555 to 0.004% or more. However, if the cleanliness of inclusions excluding MnS specified in JISG0555 exceeds 0.20%, cracks are likely to occur due to the inclusions, and the cracking susceptibility in corrugating becomes high. Therefore, the cleanliness of inclusions excluding MnS specified by JISG0555 is set in the range of 0.004% to 0.20%.

  The welded part may have higher strength than the material part due to the effect of thick plate thickness or welding distortion. On the other hand, in the case of a material that is work-hardened by rolling or the like, strain is removed by heat from welding in the welded portion, and the strength may be lower than that in the material portion. Thus, when corrugating is performed when there is a difference between the strength of the welded portion and the strength of the material portion, deformation in the corrugated processing may concentrate on the soft portion and cause cracks. In particular, when the stainless steel plate is thin, it is easily affected by the strength due to the difference in plate thickness. And since the ductility and toughness are inferior to the material part in the welded part, in order to perform corrugating stably, the material part is made slightly soft, the strength difference between the welded part and the material part is reduced, and due to the difference in strength It is necessary to suppress the occurrence of cracks. Therefore, the Vickers hardness of stainless steel is adjusted and managed in the range of 130 HV or more and 200 HV or less. In addition, adjustment of hardness may be any known method as long as it is a method capable of adjusting hardness adjustment, such as components, annealing temperature, and temper rolling.

  According to the ferritic stainless steel described above, C, Si, Mn, P, S, Cr, Cu, Nb, Ti, Al, and N are contained within a specified range, and (Ti% + Nb%) ≧ (7 × (C% + N%) + 0.05), and by containing 0.004 to 0.20% of inclusions excluding MnS with the cleanliness specified by JISG0555, It is possible to stabilize the end face properties and prevent the inclusions from becoming more susceptible to cracking in corrugating, and to suppress the occurrence of cracking due to inclusions, for example, when the plate thickness is set thin, etc. Can be welded stably. Moreover, since the difference in strength between the welded portion and the material portion can be reduced by setting the surface hardness to 130 to 200 HV in terms of Vickers hardness, it is possible to suppress the occurrence of cracks due to concentration of strain due to the strength difference in corrugating. . Therefore, productivity can be improved when manufacturing a corrugated pipe.

  In addition, by manufacturing corrugated pipes with ferritic stainless steel for corrugated pipes with excellent manufacturability in corrugating as described above, ferritic stainless steel for corrugated pipes has conventionally been used for pipes that used austenitic stainless steel flexible pipes. Therefore, the cost can be reduced. Further, since ferritic stainless steel is softer than austenitic stainless steel, bending work can be performed more easily.

  By containing at least one of Mo, V and W so that the total content is 4% or less, the corrosion resistance can be improved and the hot workability can be prevented from being lowered. Excellent manufacturability can be secured.

  By containing at least one of Ni and Co so that the total content is 5% or less, the corrosion resistance can be improved and the formation of the austenite phase at high temperatures can be prevented. Manufacturability can be secured.

  Next, examples of the present invention will be described.

  Table 1 shows the composition of each ferritic stainless steel for which the presence or absence of cracks after production of the corrugated pipe was confirmed, the value by Ti% + Nb%, the value by 7 × (C% + N%) + 0.05, and specified by JISG0555. Vickers hardness indicating the cleanliness of inclusions excluding MnS and the surface hardness of each stainless steel is shown.

  The degree of cleaning was measured by the following method. The cross section of the steel sheet is enlarged with a microscope, and a mesh (for example, 20 × 20) having intersections of 400 at intervals of 6.25 μm is used, and the intersections of meshes that overlap with the inclusions are counted. This operation was performed for 10 or more fields of view, and the area ratio of inclusions was determined by dividing the number of intersections overlapping with inclusions by the total number of intersections, and this was defined as the degree of cleaning.

Each of the stainless steels of Steel Type No. 1, Steel Type No. 2, Steel Type No. 4 to Steel Type No. 9 has different compositions, but contains each element within the specified range, and (Ti% + Nb%) ≧ (7 × ( C% + N%) + 0.05), the composition is 0.004 to 0.20% of inclusions excluding MnS with the cleanliness specified by JISG0555, and the surface hardness is Vickers hardness This example was prepared to be 130 to 200 HV.

On the other hand, each stainless steel of steel type number 10 thru | or steel type number 15 is a comparative example which does not satisfy | fill the conditions mentioned above.

  First, each stainless steel having the chemical composition shown in Table 1 was melted and subjected to hot rolling, annealing and cold rolling to produce a plate material having a thickness of 0.3 mm. In addition, finish annealing was performed at 1000-1070 degreeC, and the pickling was performed after that. The surface hardness of the plate material thus obtained was measured by Vickers hardness. Further, the cleanliness of inclusions except MnS defined by JISG0555 was determined by SEM observation, and the composition of the inclusions was analyzed by EDX to determine whether it was MnS.

  Each of these plate materials was slit at a width of 31.4 mm, the shear fracture surface was observed every 1 m within a length of 100 m, the ratio between the shear surface and the fracture surface was determined, and the stability of the slit end face property was confirmed. . FIG. 1 shows the relationship between the cleanliness of inclusions excluding MnS in each stainless steel plate and the fluctuation range of the shear fracture surface ratio. In addition, the broken line of FIG. 1 shows 0.004% of the cleanliness of inclusions excluding MnS. Moreover, when the slit end face of stainless steel cracked by corrugating was observed, the fluctuation of the shear fracture surface ratio exceeded 10%, so the standard value of the stability of the slit end face shape due to the shear fracture surface ratio was 10%. It was.

  As shown in FIG. 1, the cleanliness is 0.004% or more, that is, in the stainless steels of steel type number 1 to steel type number 12 and steel type number 15, the fluctuation range of the shear fracture surface ratio is 10% or less. In stainless steel, it can be said that the slit end face shape is stable.

  Furthermore, each slit coil was welded at a speed of 3 m / min by TIG welding to produce a pipe having a diameter of 10 mm. Thereafter, as shown in FIG. 2, corrugation was performed by setting the pitch a to 3 mm and the groove depth b to 1 mm, thereby producing a corrugated tube.

  And in each corrugated pipe | tube produced in this way, the presence or absence of the crack by corrugation processing was confirmed. FIG. 3 shows the relationship between the presence or absence of cracks in corrugating, the cleanliness before welding, and the surface hardness of stainless steel before welding.

Cleanliness is 0.004 to 0.20%, this example the surface hardness Ru 130~200HV der could produce corrugated tube without any cracking occurs.

  On the other hand, all the comparative examples of steel type number 10 to steel type number 15 were cracked.

  The stainless steel as a comparative example of steel type No. 13 and steel type No. 14 having a cleanliness lower than 0.004% was cracked by corrugating. In these stainless steels, the slit end face properties are unstable, and a portion where the weld bead is large and a portion where the weld bead is small are mixed. And especially the crack was recognized in the location where a weld bead is large.

  Stainless steel as a comparative example of steel type No. 11 and steel type No. 12 with a cleanliness degree exceeding 0.20% had a stable slit end face property and a stable shape of the welded portion, but a crack occurrence portion was observed. However, inclusions were observed. Therefore, it is considered that cracking occurred due to inclusions.

  In the comparative example of steel type number 10 whose surface hardness is lower than 130HV and the comparative example of steel type number 15 whose surface hardness is higher than 200HV, the degree of cleanliness is in the range of 0.004 to 0.20%. It is considered that the difference between the strength and the strength of the material portion became large, and the strain concentrated on the material portion or the welded portion, leading to cracking.

  Since the ferritic stainless steel for corrugated pipes of the present invention is excellent in manufacturability when producing corrugated pipes, for example, hot and cold water pipes, gas pipes, refrigerant pipes and heat exchanger pipes that require bending work. Can be used.

Claims (3)

In mass%, C: 0.03% or less, Si: more than 0.1 to 2%, Mn: 2% or less, P: 0.05% or less, S: 0.005% or less, Cr: 10.5 to 26%, Cu: 0.70% or less , Al: 0.4% or less, N: 0.03% or less , and Nb: 0.8% or less and Ti: 0.8% or less And the balance consists of Fe and inevitable impurities,
The composition is prepared so that (Ti% + Nb%) ≧ (7 × (C% + N%) + 0.05)
Contains 0.004 to 0.20% of inclusions excluding MnS with the cleanliness specified by JISG0555,
A ferritic stainless steel for corrugated pipes having a surface hardness of 130 to 200 HV in terms of Vickers hardness. The ferritic stainless steel for corrugated pipes according to claim 1, characterized by containing at least one of Mo, V, and W so that the total content is 4% or less by mass. The ferritic stainless steel for corrugated pipes according to claim 1 or 2, characterized by containing at least one of Ni and Co so that the total content is 5% or less by mass%.

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