JP2002275590A - Ferritic stainless steel for welding having excellent workability in weld zone - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide ferritic stainless steel for welding in which a fine metallic structure can be obtained even in the weld zone, the toughness and ductility in the weld zone are not reduced, and the workability of the weld zone is improved. SOLUTION: The ferritic stainless steel has a composition containing, by mass, <=0.03% C, <=2.0% Si, <=2.0% Mn, 10.0 to 20.0% Cr and <=0.15% Al, and further containing Ti, N and O in the relation satisfying (Ti-5×O)×N/ Cr<=0.00012 and <=0.010% O, and the balance iron with inevitable impurities. If required, one or more kinds selected from <=0.8% Nb, <=0.5% Zr, <=0.5% V, <=0.1% rare earth metals, <=0.05% B, <=3.0% Mo and <=2.0% Cu can be incorporated therein.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a ferrite for welding in which the amount of TiN generated in a weld is controlled by regulating the contents of Ti, N, and O to improve the workability of the weld. Related to stainless steel.

[0002]

2. Description of the Related Art Ferritic stainless steel has a smaller coefficient of thermal expansion than austenitic stainless steel, and is advantageous for applications in which overheating and cooling are repeated, and is relatively inexpensive. It is used for heat-resistant applications such as plant components. Particularly, since stainless steel pipes used as exhaust gas members are subjected to severe processing, workability is required in addition to heat resistance. Generally, in welding a stainless steel pipe, pipes are formed by welding both ends in the width direction of a steel plate or a steel strip. Examples of the type of welding include TIG welding, MIG welding, high-frequency welding, and laser welding. By the way, as a problem related to welding, ductility of the weld due to coarsening of the crystal grains of the weld,
There is a problem of a decrease in toughness.

[0003]

In order to improve the ductility and toughness of such a ferritic stainless steel weld,
JP-B-55-47102 and JP-A-2-10774
As disclosed in Japanese Patent Publication No. 4 (1994) -4, the regulation of the amounts of C and N, the addition of stabilizing elements such as Ti and Nb and the like are disclosed in JP-B-55-47102 and JP-A-50-109809. As described above, techniques such as addition of Al have been proposed. In these inventions, since C and N are harmful to toughness and ductility in ferrite, C and N are kept low from the beginning, or C and N are replaced with Ti, Nb,
It is intended to be fixed by Al. On the other hand, even when these techniques are used, when a material including a weld is processed, insufficient ductility of the weld may be a problem. That is, a fundamental problem is that the metal structure of the welded portion becomes coarser than that of the base metal.

[0004] Further, there is also a means for positively adding Ti and utilizing TiN to make the structure finer. Since TiN is generated during solidification and acts as a ferrite nucleation site,
This is an effective method for refining the solidification structure during casting. However, in the case of welding, the conditions are completely different from those at the time of casting. It is not possible to miniaturize the structure of the part. Further, since TiN causes surface defects in a product, it is not always preferable to add a large amount of TiN.

The present invention has been devised in order to solve such a problem. A fine metal structure can be obtained in a welded portion, and the toughness and ductility of the welded portion are not reduced. An object of the present invention is to provide a ferritic stainless steel for welding with improved weldability.

[0006]

In order to achieve the object, the ferritic stainless steel of the present invention having excellent workability of a welded portion has a C content of 0.03% by mass or less and a Si content of 2.0% by mass or less. , Mn: 2.0% by mass or less, Cr: 10.0 to 2
0.0% by mass or less, Al: 0.15% by mass or less, and further, Ti, N and O are expressed by (Ti-5 × O) × N
The composition is adjusted so that /Cr≦0.00012 and O: 0.010% by mass or less, with the balance being iron and unavoidable impurities. Further, if necessary, Nb: 0.8% by mass or less, Zr: 0.5% by mass or less, V: 0.5% by mass or less, REM: 0.1% by mass
Hereinafter, one or more of B: 0.05% by mass or less, Mo: 3.0% by mass or less, and Cu: 2.0% by mass or less may be contained.

[0007]

The present inventors have conducted various studies to solve the above-mentioned problems, and as a result, have obtained the following findings and reached the present invention. Ti
As described above, N can be used as a solidification nucleus of ferrite and is a very effective means for refining the solidification structure. However, the microstructure of the weld cannot be reduced by simply adding Ti and N. Therefore, TIG welding was performed on ferrite stainless steel samples of various components, and the relationship between the components and the metal structure of the welded portion was investigated. As a result, it was found that TiN existing before welding was also dissolved before the weld was melted. Furthermore, it has been found that TiN cannot be sufficiently generated because the solidification rate during welding is much higher than the solidification rate during casting.

[0008] Even when Ti and N are simply contained in a predetermined amount or more, the refinement of the welded structure may not be achieved. As a result of further study, when the oxygen content of the base material is high, In addition, it was found that Ti is oxidized during welding and the amount of TiN produced is reduced, so that the refinement of the weld structure is insufficient. Therefore, when an investigation was conducted with attention paid to the cleanliness of the welded portion, it was found that a predetermined amount or more of TiN had to be generated in the welded portion after welding in order to reduce the size of the welded structure. .

As described above, TiN is an inclusion that causes surface defects of a product, and when a large amount of Ti and N is contained, a large amount of TiN is generated, which may cause a problem.
Examination of the component range in which surface flaws do not occur revealed that (Ti
−5 × O) × N / Cr is not more than 0.00012 and O:
When the content was 0.010% by mass or less, it was found that the surface flaws of the product were at a level without any problem. The ease of generation of TiN can be approximated by the value of Ti × N. The larger the value of Ti × N, the more easily TiN is generated. O in addition to Ti and N
Since Cr and Cr also affect the formation of TiN, considering this point, O is easily associated with Ti, and Cr lowers the activity of N. Reached.

When a cold-rolled steel sheet having the contents of Ti, N, and O defined as described above is welded by a normal welding method, TiN having a cleanliness of 0.01% or more is formed in the welded portion, and the microstructure is formed. It is possible to obtain a welded product such as a steel pipe which has been refined, has sufficient ductility and toughness thereafter, and has excellent workability. In addition, it was found that there was no flaw caused by TiN on the product surface. The cleanliness of TiN is measured according to JIS G055.
It shall be performed according to the inclusion cleanliness measurement method of No. 5. Ti
N includes a type in which TiN exists alone and a type in which TiN is present in combination with an oxide-based inclusion. In the present invention, TiN means both types of TiN.

The term "welding" as used in the present invention means welding such as TIG welding, MIG welding, covered arc welding, high frequency welding, laser welding, plasma welding, electron beam welding, full penetration welding and the like. Any of the following welding methods may be used: a welding method that does not add a metal, and a welding method that adds an additive. However, what is important for refining the structure of the weld is to generate a certain amount or more of TiN in the weld. is there. The welding conditions are selected according to various conditions such as the composition of the material, the thickness of the material, and the purpose. Further, the term “welded portion” as used in the present invention means both a portion called a so-called weld metal which is once melted and solidified and a bond portion which is solidified in a semi-molten state in a portion welded by the above welding method. Does not include so-called HAZ which is affected by heat at the time of heating.

The reasons for limiting the components and compositions in the ferritic stainless steel of the present invention will be described below. C: 0.03% by mass or less C is an effective element for improving the high-temperature strength.
Since it is an element that inhibits oxidation resistance, workability, and toughness, the upper limit is specified as 0.03% by mass. Si: 2.0% by mass or less Si is an element having an effect of improving high-temperature oxidation resistance. However, if added excessively, workability and toughness are reduced. Therefore, the upper limit is specified as 2.0% by mass. Mn: 2.0% by mass or less Like Mn, Mn is an element having an effect of improving high-temperature oxidation resistance. However, excessive addition of Mn causes a decrease in workability and low sound toughness. % By mass.

Cr: 10.0 to 20.0% by mass Cr is an important alloying element for maintaining the corrosion resistance of ferritic stainless steel, and needs to be 10.0% by mass or more. However, a large Cr content exceeding 20.0% by mass deteriorates the manufacturability. For this reason, the Cr content is specified to be 10.0 to 20.0% by mass. Al: 0.15% by mass or less Al is an element used as a deoxidizing agent, but the workability is improved by adding a small amount. However, addition exceeding 0.15% by mass causes deterioration of hot workability and weldability, so the upper limit of the content is specified as 0.15% by mass.

The range of Ti, N and O, (Ti-5 × O) × N / Cr ≦ 0.00012 and O:
0.010% by mass or less of Ti has an effect of fixing C and N in steel and improving workability. To fix C and N, it is necessary to contain 0.01% by mass or more of Ti. However, as described below, in order to generate a predetermined amount of TiN in the welded portion, 0.05% by mass or more is required. It is preferable to contain them. Ti and N are elements necessary for generating TiN which is a nucleation site of ferrite and for achieving a finer structure of a welded portion. O is present as free oxygen and inclusions, and the higher the content, the easier it is to generate oxides of Al and Ti during welding, hindering the generation of TiN and hindering the refinement of the structure of the weld. The lower the O content, the greater the amount of TiN generated, which is effective for miniaturization. Therefore, the upper limit of the O content is 0.010% by mass.

As described above, it is necessary to generate a certain amount of TiN in the welded portion in order to reduce the structure of the welded portion. It is preferable that the amount is 0.01% or more in terms of cleanliness. When the contents of Ti and N are high and a large amount of TiN is generated, it causes a surface flaw of the product. Examination of the component range free from surface flaws revealed that (Ti-5 ×
When the relationship of O) × N / Cr ≦ 0.00012 is satisfied, the surface flaw of the product is at a level that does not cause any problem, so the upper limit is set to 0.00012. T defined as above
i, the cold-rolled steel sheet containing N and O, for example, when welding in a conventional manner as illustrated in the following examples, welds reached a maximum 1800 to 2000 ° C., the cooling rate during solidification is 10 ² ° C. / S, which indicates that 0.01% or more of TiN is generated in the welded portion under such conditions.

Next, other components contained as necessary will be described. Nb: 0.8% by mass or less Nb is an element that fixes C and N in steel and has an effect of improving high-temperature strength. However, excessive addition of Nb causes deterioration in workability and low-temperature toughness. It was defined as 0.8% by mass. Zr: 0.5% by mass or less Zr, like Nb, is an element that fixes C and N in steel and has an effect of improving high-temperature strength. However, excessive addition of Zr causes deterioration of workability and low-temperature toughness. Invite, the upper limit is 0.5
It was defined as% by mass.

V: 0.5% by mass or less V, like Nb and Zr, is an element having the effect of fixing C and N in steel and improving high-temperature strength. Since the low-temperature toughness is deteriorated, the upper limit is set to 0.5% by mass. B: 0.05% by mass or less B has an effect of improving hot workability. However, since excessive addition adversely affects hot workability, the upper limit is set to 0.05% by mass. . REM: 0.1% by mass or less REM has the effect of improving hot workability similarly to B, but excessive addition adversely affects hot workability. It was defined as% by mass.

Mo: 3.0% by mass or less Mo is an element having an effect of improving corrosion resistance and strength. However, an excessive addition causes a reduction in productivity and workability, so the upper limit is 3.0. It was defined as% by mass. Cu: 2.0% by mass or less Cu is an element having an effect of improving low-temperature toughness and workability, but excessive addition impairs hot workability.
The upper limit was set to 2.0% by mass. Further, other optional components such as Ni, Co, Sn, Pb, Ca, Mg, W, etc.
Species or two or more species may be included. It is preferable that S and P contained as impurities be specified at 0.02% by mass or less and 0.05% by mass or less, respectively.

[0019]

The present invention will be described below with reference to examples. 30 kg of ferritic stainless steel having the chemical components shown in Table 1
It was melted using a vacuum melting furnace, and the obtained steel ingot was forged and hot-rolled to produce a hot-rolled sheet. Thereafter, hot rolled sheet annealing was performed, and pickling, cold rolling, finish annealing, and pickling were performed to produce a cold rolled sheet having a thickness of 2 mm. TIG welding (welding current: 100 to 150 A, welding voltage: 10 V,
(Welding speed: 30 to 60 cm / min).

The metal structure of the cross section of the weld was evaluated. The evaluation of the weld was evaluated by measuring the amount of TiN generated and by a bending test of the weld. In the bending test, a 180 ° bending process was performed so that the welded portion was located at the center, and when no crack was generated, the workability was evaluated as good. The table also shows the occurrence of surface flaws on product materials. Table 1 shows the test materials Ti, O,
The calculation formula value of the relationship between N and Cr, the generation amount of TiN, and the evaluation result of the welded portion are also shown.

[0021]

Test No. 1 within the range defined by the present invention
In Nos. 5 to 5, TiN is produced in a preferable amount of 0.01% or more, and no crack occurs even in bending of the welded portion.
The workability of the weld can also be evaluated as good. No flaw was observed on the surface of the cold-rolled sheet. In Comparative Examples of Test Nos. 6 and 7 out of the range specified in the present invention, the generation amount of TiN was less than 0.01%, and it was recognized that cracks were generated by bending of the welded portion. Test number 8,
In No. 9, the generation amount of TiN was 0.01% or more, and no crack was generated by bending of the welded portion.
It was confirmed that flaws occurred on the surface of the cold-rolled sheet because it contained a large amount of Ti and N.

[0023]

As described above, according to the present invention, by controlling the contents of Ti, N, and O to generate a predetermined amount of TiN in the welded portion, the workability of the welded portion is improved. Ferritic stainless steel can be obtained.

Continued on the front page (72) Inventor Shuhisa Hiruhama 4976 Nomura Minami-cho, Shinnanyo-shi, Yamaguchi Nisshin Steel Co., Ltd.Stainless Steel Business Division (72) Inventor Manabu Oku 4976 Nomura-minami-cho, Shinnanyo-shi, Yamaguchi Steel Co., Ltd.

Claims (2)

[Claims] C: 0.03% by mass or less, Si: 2.0
Mass% or less, Mn: 2.0 mass% or less, Cr: 10.0
２20.0% by mass or less, Al: 0.15% by mass or less, and furthermore, Ti, N and O are (Ti-5 × O) ×
The composition is adjusted so that N / Cr ≦ 0.00012 and O: 0.010% by mass or less, and the balance consists of iron and inevitable impurities. Ferritic stainless steel. 2. If necessary, Nb: 0.8% by mass or less, Zr: 0.5% by mass or less, V: 0.5% by mass or less, REM: 0.1% by mass or less, B: 0 The ferrite for welding according to claim 1, which contains one or more of 0.05% by mass or less, Mo: 3.0% by mass or less, and Cu: 2.0% by mass or less. Series stainless steel.