JP3529520B2 - Welding method to prevent ductility degradation crack of ferritic stainless steel - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a welding method for preventing ductility-reducing cracks in a heat affected zone of a ferritic stainless steel. [0002] Ferritic stainless steel has a smaller coefficient of thermal expansion than austenitic stainless steel, is advantageous for applications where heating and cooling are repeated, does not generate stress corrosion cracking, and is relatively inexpensive. Therefore, it is used in various fields such as automobile exhaust gas path members, various plant materials, and building materials. For these applications, welding work is frequently performed. [0003] Problems in welding ferritic stainless steel include a decrease in toughness due to coarsening of the grains in the weld zone and the heat-affected zone, a reduction in corrosion resistance due to a sharpening of the heat-affected zone,
Hot cracking during welding work, and the like. To address these problems, several countermeasures have been taken in the past, such as adding an alloying element to the base metal and the welding core wire to suppress the coarsening of crystal grains. In addition, the use of stabilized steel containing Nb, Ti, etc., has been attempted to reduce alloying elements such as P, S, etc., which increase hot cracking susceptibility. However, cracks not only occur in the weld metal but also in the heat-affected zone depending on the welding method. The cracks generated in the heat affected zone can be classified into liquefaction cracks and ductility-reduced cracks. In the former, the low-melting compound segregated at the grain boundaries is liquefied by heating to cause cracking, and in the latter, cracking occurs in a temperature range where the ductility of the material itself is reduced. Liquefaction cracking can be prevented to some extent by the above measures, but effective ductile cracking has not been clarified yet. . SUMMARY OF THE INVENTION An object of the present invention is to provide a welding method for preventing ductility-reducing cracks which may occur during welding of ferritic stainless steel with good reproducibility. In order to achieve the above object, the present inventors have clarified a temperature range in which ductility-reducing cracks are generated by a tensile test using a welding heat cycle reproducing apparatus. The conditions under which ductility-reducing cracks occur during actual welding were studied in detail. As a result, ductility reduction cracking occurs in a very narrow temperature range just below the melting point of ferritic stainless steel.To stably prevent this, the maximum temperature of the back surface of the weld during welding and the penetration rate of the weld are determined. The present inventors have found that it is necessary to regulate, and have led to the present invention. That is, the present invention relates to
C: 0.03% or less, Si: 2.0% or less, Mn: 2.0
%, Cr: 5.0-30.0%, N: 0.03% or less, and Nb: 0.05-1.0%, Ti: 0.03%.
0.05 to 1.0%, Mo: 0.05 to 3.0%, Cu: 0.0
2 to 1.0% of one or more, the balance being F
In welding of ferrite stainless steel consisting of e and unavoidable impurities in production, the maximum temperature of the back surface of the welded portion during welding is set to a temperature lower than the melting point of the steel by 100 ° C. or more, and the following formula (1) is used. Provided is a welding method for preventing ductility-reducing cracks in a heat-affected zone of a ferritic stainless steel with a defined penetration ratio (%) in a range of 20 to 80%. Penetration rate (%) = Penetration depth / Thickness of the base metal to be welded × 100 (1) Here, the penetration depth is welded to the highest point (deepest part) of the melted portion of the base metal to be welded. Defined as the distance between the surface and the surface. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The reasons for specifying the present invention will be described below. C and N are generally important elements for increasing the strength at high temperatures. However, when the content increases, the workability and the low-temperature toughness decrease. [0010] Si is an element that improves high-temperature oxidation resistance. However, if added excessively, it becomes hard, resulting in deterioration of workability and toughness. [0011] Mn is sometimes added aggressively for heat-resistant applications in order to significantly improve the adhesion of the surface oxide.
However, if added in excess, it will deteriorate the corrosion resistance and become hard, lowering the low-temperature toughness and workability,
2.0% or less. [0012] Cr is an element effective for imparting corrosion resistance and high-temperature oxidation resistance, and requires addition of 5.0% or more. On the other hand, if it is added excessively, the steel will be embrittled, and it will become hard and deteriorate workability, and the raw material price will increase. Therefore, the range of Cr is 5.0-30.0%.
And Nb effectively acts on the increase in high-temperature strength. At least 0.05 to increase high temperature strength
% Must be added. On the other hand, if Nb is added excessively, low-temperature toughness and workability are reduced. The range of Nb is set to 0.05 to 1.0% so that high-temperature strength is maintained and low-temperature toughness and workability are not significantly affected. [0014] Ti is an element effective for improving the r-value of the steel sheet and improving the deep drawability. In order to exert its effect, 0.05% or more must be added. However, if added excessively, TiN is likely to be generated, leading to a decrease in yield due to generation of barbed flaws in the steel sheet and a cause of deterioration in weldability. Therefore, the range of Ti is 0.0
5 to 1.0%. Mo is an element effective for improving corrosion resistance, oxidation resistance and high-temperature strength. In order to exhibit these effects, 0.05% or more must be added. But,
Excessive addition causes brittleness of the steel. Therefore, the range of Mo is set to 0.05 to 3.0%. Cu is an element effective in improving both low-temperature toughness and workability. The effect becomes remarkable by adding 0.02%. However, when added in a large amount, workability is impaired. Therefore, the range of Cu is set to 0.02 to 1.0%. The highest temperature reached on the back surface of the weld is the most important factor in preventing ductility-reducing cracks. The present inventors have repeatedly conducted the following test and research, and have determined that the highest ultimate temperature of the back surface of the welded portion is a temperature lower than the melting point by 100 ° C. or more. Investigation of some examples of the fracture surface of ductility-reduced cracks actually occurring in the heat-affected zone reveals that all fracture surfaces are fracture surfaces of grain boundary fracture, and that the grain boundaries are liquefied. It turned out not to be. Therefore, first, an attempt was made to reproduce a fracture surface with reduced ductility using various ferritic stainless steels. FIG. 1 shows the relationship between the reduction ratio of the sheet width and the tensile test temperature after performing a high-temperature tensile test with a welding heat cycle reproduction device using SUS429 equivalent steel, SUS444 equivalent steel, and SUS430J1L equivalent steel. 1000 ° C to 14
It is understood that all steels show good ductility up to 00 ° C., and in this temperature range, even if a tensile stress is applied to the heat-affected zone during welding, it is considered that the steel is ductilely deformed. On the other hand, if it exceeds 1400 ° C., the ductility sharply decreases. At 1440 ° C. or higher, grain boundary destruction occurs. It was confirmed that the fracture surface of the grain boundary fracture was in good agreement with the fracture surface of the ductility-reduced crack observed in the case study described above. From this, it became clear that the ductility reduction of the heat affected zone and the cracks caused by it occur in a very narrow temperature range from over 1400 ° C. to the melting point (about 1500 ° C.). That is, it was found that good ductility was exhibited at a temperature lower than the melting point by 100 ° C. or more. Next, based on this finding, various investigations were made on the relationship between the temperature of the back surface of the weld and the ductility-reduced crack in actual welding. as a result,
It has been found that if the temperature of the back surface of the welded portion of the base material to be welded is lower than the melting point by 100 ° C. or more, ductility-reducing cracks can be prevented with good reproducibility. This is because if there is a region with good ductility even in a part of the unmelted part (= part expressed by thickness-penetration depth) in the thickness of the welded portion of the base metal to be welded, It is considered that the occurrence of cracks with reduced ductility as described above can be suppressed. From the above results, as a means for suppressing the ductility-reducing cracks during welding of ferritic stainless steel with good reproducibility, the highest temperature during welding on the back surface of the welded portion as shown in FIG. It specified that the temperature should be low. The melting point of each of the steels shown in FIG. 1 is about 1500 ° C., but the melting point varies depending on the composition of the steel. Therefore, it is necessary to define the maximum temperature at the back surface of the welded portion according to the steel used. For example, SUS4
For 29 series, SUS444 series, and SUS430 series steels, the maximum temperature at the back surface of the welded portion may be set to 1400 ° C. or less. If the penetration rate is less than 20%, poor joining or insufficient joining strength may occur depending on the variation of welding conditions, and therefore it is necessary to set the penetration rate to 20% or more. However, if the penetration rate exceeds 80%, the maximum temperature of the back surface of the welded portion during welding exceeds the temperature of the melting point of −100 ° C., which may cause ductility-reducing cracks. Here, specific means for setting the maximum attained temperature and penetration rate of the back surface of the welded portion will be described. With the joint shape as shown in FIG. 2, it is practically impossible to measure the temperature and the penetration rate during actual welding. Therefore, for example, a model of the joint suitable for the actual work is manufactured in advance, and this is welded under various welding heat input conditions. Can be selected such that the welding heat input condition range is in the range of 20 to 80%, and this welding heat input condition range is applied at the time of actual welding. As the temperature measurement of the back surface of the welded portion, for example, a method of arranging a plurality of thermocouples and pasting them, and measuring the temperature, or a measuring method by thermography can be adopted. The penetration rate can be determined, for example, by collecting a plurality of cross sections per test piece from a test piece welded under various welding heat input conditions, measuring the cross section after polishing, and then observing the cross section with an optical microscope. Incidentally, the stress generated in the heat-affected zone at the time of welding is also an important factor relating to the occurrence of cracking with reduced ductility. FIG. 3 shows the results of measuring the temperature at which a test piece breaks when a test piece is heated under a constant stress while using the same test material as in FIG. As the stress decreases, the rupture temperature increases, and at the temperature of 1440 ° C. or higher, grain boundary fracture occurs as in the result of FIG. The stress at this time is 2.0 to 2.5 N / mm ² . On the other hand, when a stress of 1.5 N / mm ² or less is applied, the test piece does not break until it begins to melt. From this, it was found that ductility-reducing cracks may occur when the stress during welding exceeds 1.5 N / mm ² . Therefore, when applying the welding heat input conditions selected from the maximum temperature at the back of the weld and the penetration rate to actual welding work, elasto-plastic analysis of welding thermal stress (for example, welding mechanics and its applications; published by Asakura Shoten) ), The value is 1.5 N / m
it is desirable to apply to confirm that this is m ² or less. The method of the present invention provides a method for welding joints comprising:
The present invention can be applied to a welded joint having a construction method in which the penetration of one or both base materials of the material to be welded constituting the welded joint does not penetrate from the front surface to the back surface, such as a lap joint, a T-joint, and a barbed joint referred to in ISZ3001. The welding method is not particularly limited. Further, the shielding gas and the core wire used for welding are not limited. Furthermore, in the dissimilar joint welding of ferritic stainless steel and another alloy, the application of the method of the present invention can prevent the ductility-reducing cracks in the heat-affected zone of ferritic stainless steel. EXAMPLES Three types of steel shown in Table 1 were melted, hot-rolled,
Annealed and cold rolled to a 2.0mm thick plate,
After annealing in the range of 0 to 1050 ° C., the specimen was pickled to obtain a test material for a welding test. A welding test was performed using these test materials to evaluate ductility-reducing cracks. Table 2 shows the welding conditions and results. [Table 1] [Table 2] The ductility-reduced cracks were evaluated by the presence or absence of cracks after lap joint welding was carried out by fixing two plates and fixing them with a jig. The determination of the presence or absence of cracks was performed by cutting and collecting five sections of the welded portion, inspecting the presence or absence of cracks with an optical microscope after buffing. Thereafter, etching was performed and the penetration ratio was measured. In addition, the thermocouple was pasted on the back surface of the welded part in advance on the cut specimen of the welded part, and the maximum temperature during welding was measured. The melting point of each test material was about 15
00 ° C. Nos. 1 to 11 are based on the method of the present invention. Regardless of the method of TIG welding, MIG welding or MAG welding, the maximum temperature of the back surface of the welded portion is set to 1400 ° C. (= melting point−100 ° C.) or less, and the penetration rate is in the range of 20 to 80%. It was confirmed that if the heat input conditions were selected and welding was performed, no ductility-reducing cracks occurred in the heat-affected zone. Nos. 12 to 16 show comparison methods. In No. 12 and No. 16, since the maximum attained temperature and the penetration ratio deviated from the method of the present invention, ductility-reduced cracking occurred. Further, in No. 13, although the penetration rate was included in the range of the present invention, the highest ultimate temperature of the back surface of the welded portion was out of the range of the method of the present invention, so that ductility-reduced cracking occurred. In addition, No. 14 and No. 15 could not be used as a product with a hole in the welded portion because they had melted down. The present invention provides a welding method capable of preventing ductility-reduced cracking with good reproducibility in the welding of ferritic stainless steel, and in particular, has a versatility applicable to welded joints having various structures. Is high. Therefore,
At the time of actual welding, by setting appropriate welding conditions according to each situation so as to satisfy the items stipulated in the present invention, the allowable range of welding conditions, which was conventionally unclear, is appropriately set. It is possible to optimize welding conditions at various welding sites.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a graph showing a relationship between a test temperature of a high-temperature tensile test and a reduction ratio of a sheet width. FIG. 2 is a cross-sectional view showing the positions of the back surfaces of welded portions of a lap joint and a T joint. FIG. 3 is a graph showing a relationship between a stress value and a breaking temperature when heating is performed in a state where a stress is applied.

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification code FI // B23K 103: 04 B23K 103: 04 (72) Inventor Shoji Inoue 1 Tsurumachi, Amagasaki-shi, Hyogo Nisshin Steel Co., Ltd. (72) inventor Katsuhiko Fukumura Amagasaki, Hyogo Prefecture Tsurumachi address 1 Nisshin Steel Co., Ltd. intra-technology research Institute (56) reference Patent Sho 60-83792 (JP, a) (58 ) investigated the field (Int.Cl. ⁷ , DB name) B23K 9/095 B23K 9/23 B23K 103: 04

Claims (1)

(57) [Claims 1] In mass%, C: 0.03% or less, Si: 2.0% or less, Mn: 2.0% or less, Cr:
5.0-30.0%, N: 0.03% or less, Nb: 0.05-1.0%, Ti: 0.05-1.0%,
Mo: 0.05 to 3.0%, Cu: 0.02 to 1.0%
In welding of ferritic stainless steel containing one or more species and the balance being Fe and inevitable impurities in production, the highest ultimate temperature of the back surface of the weld during welding is 100 ° C. or more lower than the melting point of the steel. Temperature and the penetration rate (%) defined by the following equation (1) is 20 to 80%
A welding method for preventing ductility-reducing cracks in the heat-affected zone of a ferritic stainless steel within the range described above. Penetration rate (%) = Penetration depth / Thickness of the base metal to be welded × 100 (1)