JP2940625B2 - Ferritic stainless steel ERW pipes with excellent corrosion resistance, flat workability, and spreadability - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Application Field> The present invention relates to an electric resistance welded pipe made of ferritic stainless steel having excellent corrosion resistance and workability, and particularly relates to an oil-fired equipment and an automobile exhaust pipe. Thus, it is suitable for a field in which workability and corrosion resistance as a pipe are required.

<Conventional technology> As is well known, ferritic stainless steel has poor workability and weldability compared to austenitic stainless steel, but in an environment subjected to repeated heating, it is more resistant to oxidation than austenitic stainless steel. Ferritic stainless steel such as SUH409 is used for petroleum combustion equipment and automobile exhaust pipes because of its excellent properties and low cost.

However, in recent years, the pipe shape has become complicated, and the SUH409 electric resistance welded pipe has a problem that brittle cracks are apt to occur in the welded portion at the time of flattening or the like, particularly when the temperature is lowered as in winter.

In this regard, C and N are both 50pp using so-called VOD technology.
It is known that the toughness of the weld is improved by making the carbon extremely ultra-low carbon, such as m or less, but this method is extremely costly and departs from the feature of the present material that it is inexpensive. Furthermore, pipes for automobile exhaust systems are required to have sufficient corrosion resistance because of the environment in which exhaust gas condenses. Therefore, there is a strong demand for the development of an inexpensive material that does not crack even in flattening at low temperature without using the VOD technology and has excellent corrosion resistance.

On the other hand, Japanese Patent Publication No. 55-47103 discloses that in a ferritic stainless steel, Ti which is at least 5 times the C + N amount is added up to 0.7% and Ni is added in an amount of 0.5 to 2.0%. However, in a flat test at a low temperature such as 0 ° C. or less, it cannot be said that there is sufficient workability.

<Problems to be Solved by the Invention> In view of such circumstances, the present invention is made of a corrosion-resistant ferritic single-phase steel which is less expensive than the conventional steel in terms of cost, has excellent corrosion resistance, and can prevent work embrittlement cracking of a weld. It is an object of the present invention to provide an electric resistance welded pipe excellent in flat workability and push-out workability.

<Means for Solving the Problems> That is, in the present invention, C: 0.005 to 0.012% by weight,
N: 0.005 to 0.010%, C + N: 0.019% or less, Si: 0.45 to
0.7%, Mn: 0.35% or less, P: 0.025% or less, S: 0.005% or less, Cr: 10 to 12%, Ni: less than 0.5%, Al: 0.03% or less, Ti:
An electric resistance welded tube excellent in corrosion resistance, flat workability and push-out workability, characterized in that it is composed of ferritic stainless steel containing 0.1 to 0.3% and the balance being Fe and unavoidable impurities.

<Action> The present inventors have repeatedly conducted various experimental studies to determine the cause of the embrittlement cracking of the ERW pipe, and as a result, have discovered the following new findings. Provided is an electric resistance welded pipe made of inexpensive ferritic stainless steel that improves the toughness of a welded portion, prevents embrittlement, is excellent in flat workability and push-out workability, and has excellent corrosion resistance in an electric resistance welded portion. It has been reached.

That is, Ti is usually added to fix C and N and to prevent precipitation of Cr carbonitride. For that purpose, Ti is added in an amount not less than that related to C and N. For example, in the example of Japanese Patent Publication No. 55-47103, an addition of 10 times or more the C + N amount is performed. However, the present inventors have found that cracking of the ERW tube occurs in the HAZ, and that only the C and N control the toughness of the thermal history of the HAZ. That is, C and N fixed by Ti before welding receive the thermal history of HAZ, and C and N form a solid solution. During the subsequent cooling, it was discovered that Cr, which is higher than the content of Ti, combines with C and N earlier than other elements, and becomes Cr carbonitride, lowering the corrosion resistance and toughness.

Therefore, the HAZ toughness of the ERW pipe has almost no effect when Ti is added.

In order to prevent this precipitation of Cr carbonitride, the present inventors, C,
By controlling the total amount of N particularly strictly, the electric resistance welded tube made of ferritic stainless steel which is inexpensive and has excellent corrosion resistance and workability can be manufactured.

 Next, the reason for limiting the content range of each element will be described.

C and N are preferably as low as possible to improve the toughness of HAZ. Generally, it is considered that C and N are fixed by the addition of Ti, but the present inventors studied in detail the HAZ of the electric resistance welded tube, and C and N are hardly fixed by Ti. I found something. From this finding, the toughness of HAZ, that is, HAZ cracking in pipe flattening, is strictly independent of Ti.
The fact that C and N must be controlled was experimentally found. As a result, the upper limit of C is 0.012 wt%, and the upper limit of N is
As shown in FIG. 1 and 0.01% by weight, if N ≦ 0.01 and C + N is not less than 0.019% by weight, brittle cracking occurs in a pipe flatness test at 0 ° C. On the other hand, both C and N
In order to reduce the content to 0.005 wt% or less, a so-called VOD technique is required, and the cost is remarkably increased. In the present invention, in particular, both C and N are set to 0.005 wt% or more from the viewpoint of cost.

Si is an element effective for improving oxidation resistance, and is 0.45
% Or more, but if it exceeds 0.70%, the toughness of the HAZ is reduced, and cracks are likely to occur during flattening of the ERW pipe.
Therefore, the content of Si is set to 0.45 to 0.70%.

Mn is preferably as low as possible for the corrosion resistance of the electric resistance welded portion. However, since it remains to some extent in steelmaking, the upper limit is set to 0.35%.

Further, P and S are preferably as low as possible from the viewpoint of toughness and workability. However, in this application, up to 0.025% by weight and 0.005% by weight, respectively, are acceptable.

Cr is an essential element for heat-resistant stainless steel,
If it is less than 10 wt%, sufficient corrosion resistance cannot be obtained at the welded portion of the ERW pipe. If it exceeds 12 wt%, the toughness of the ERW pipe will decrease,
Cracks are likely to occur during flattening. Therefore, the Cr content is set to 10 wt% or more and 12 wt% or less.

Ni is 0.5 wt.% In the invention of Japanese Patent Publication No. 55-47103.
% Is essential, but in the present invention, C and N are strictly controlled, so that it is not particularly necessary to add C and N. However, it is an element mixed from the steelmaking raw material. If the content is 0.5 wt% or more, the oxidation resistance of the welded ERW pipe is deteriorated, so the content is limited to less than 0.5 wt%.

Al is added for the purpose of deoxidation, and 0.03 wt% is sufficient for this purpose. Therefore, the upper limit is limited to 0.03 wt%.

Ti has no effect on the toughness of HAZ in ERW. The addition of Ti is for improving the expansion ratio of the ERW pipe. If it is less than 0.1 wt% or more than 0.3 wt%, the expansion ratio will decrease.
Limited to not less than wt% and not more than 0.3 wt%.

Hereinafter, the present invention will be described in further detail with reference to Examples.

<Examples> The steels shown in Table 1 were subjected to ordinary hot rolling and cold rolling, and then annealed.
An electric resistance welded tube of mt × 38.1 mmφ was used. The electric resistance welded portions of these electric resistance welded tubes were set at positions as shown in FIG. 2, and flattening tests were performed at -10, 0, 10, and 20 ° C. FIG. 3 shows an outline of the push-spread test. As shown in FIG. 4, the corrosion resistance test used a test piece cut out from the pipe with the electric resistance welded at the center.

 Table 2 shows the results of each test.

No.1 SUH409 cannot be flattened even at 20 ° C because both C and N are excessive. No.2 SUS410 is inferior in corrosion resistance in particular because it has two phases of an electric resistance welded part or ferrite and martensite. One of the features of the present invention is that, when N> 0.010 as shown in FIG. 1, as shown in No. 4, No. 14 and No. 15, the ERW pipe is independent of C + N. Flat cracks occur below 0 ° C. If N ≦ 0.01, as shown in No. 11, No. 12, and No. 13, flat cracking does not occur even at 0 ° C. with C + N ≦ 0.019. No. 7 has flat cracks even at 10 ° C due to excess Si.
No. 8 has inferior corrosion resistance of the ERW part due to excess Mn.
At 10 ° C, brittle cracks occur. No.9 has too little Ti,
Also, No. 10 has poor spreadability due to excessive Ti. No.
No. 16 has poor corrosion resistance due to insufficient Cr, and No. 17 has flat cracks at 0 ° C. due to excessive Cr.

ERW pipes made of ferritic stainless steel and having excellent flat workability and push-out workability and excellent corrosion resistance are limited to the scope of the present invention as shown in Nos. 11, 12, and 13.

<Effects of the Invention> As is clear from the above description, according to the present invention, an inexpensive ferritic stainless steel electric resistance welded pipe excellent in flat workability, push-out workability, and corrosion resistance as an electric resistance welded pipe is provided. Since it can be manufactured by a normal method, various combustion appliances and automobile exhaust treatment parts can be provided industrially advantageously.

[Brief description of the drawings]

Fig. 1 shows the NAZ and C + N contents of the HAZ at the time of the flattening test.
FIG. 2 is an explanatory view of a flattening test, FIG. 3 is an explanatory view of a push-spread test, and FIG.
It is a perspective view of the electric resistance welded tube test piece for SST.

──────────────────────────────────────────────────続 き Continuing on the front page (72) Makio Gunji, 1-1 1-1 Kawasaki-cho, Handa-city, Aichi Prefecture Kawasaki Steel Corporation Chita Works (72) Jun-ichi Karasawa 1-1-1, Kawasaki-cho, Handa-city, Aichi Prefecture Kawasaki (72) Inventor Takaaki Toyooka 1 Kawasaki-cho, Chiba-shi, Chiba Kawasaki Steel Corporation Research and Development Headquarters (56) References JP-A-63-1557837 (JP, A) (58) Survey Field (Int. Cl. ⁶ , DB name) C22C 38/00, 38/50

Claims (1)

(57) [Claims] [Claim 1] By weight%, C: 0.005 to 0.012%, N: 0.005 to
0.010%, C + N: 0.019% or less, Si: 0.45-0.7%, M
n: 0.35% or less, P: 0.025% or less, S: 0.005% or less, Cr: 10
Up to 12%, Ni: less than 0.5%, Al: 0.03% or less, Ti: 0.1 to 0.3%
An electric resistance welded pipe excellent in corrosion resistance, flat workability and push-out workability, characterized by comprising ferrite-based stainless steel containing Fe and unavoidable impurities.