JPS58104161A - Ferritic stainless steel with superior rust resistance, oxidation resistance and weldability - Google Patents

Abstract

PURPOSE:To obtain a ferritic stainless steel with superior rust resistance, oxidation resistance and weldability by adding specified amounts of C, Si, Mu, Ni, Cr, N and Ti to a steel. CONSTITUTION:The composition of a ferritic stainless steel is composed of, by weight, <0.03% C, 1-5% Si, <2% Mu, 0.5-3%Ni, 12-20% Cr, <0.02% Ni, 0.1-1% Ti and the balance Fe with inevitable impurities while satisfying Ti> 10X(C+N). Thus, by combinedly adding Ni and Si, the rust resistance is remarkably enhanced by the synergistic effect. By adding Si, the strength at high temp. as well as the oxidation resistance is extremely enhanced. The combinedly added Si and Ni prevent cracking and spotlike defects during welding.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a ferritic stainless steel that has excellent induction resistance, oxidation resistance, and weldability.

In recent years, there has been a tendency for equipment using austenitic stainless steel to be replaced by equipment using ferritic stainless steel due to the VA trend, and even among ferritic stainless steels, there has been a demand for cheaper steel types.

Conventional ferritic stainless steels are generally inferior to austenitic stainless steels in terms of workability such as press forming, toughness of welds, and high-temperature strength, but AO
Although it has become possible to achieve high purity using outside-furnace refining methods such as the D method or the VOD method, and it has become possible to significantly improve workability and weld toughness, the above characteristics However, it is still not as good as austenitic stainless steel, so a design change will be required when switching to ferritic stainless steel.

Furthermore, among ferritic stainless steels, low-chromium steels, which are inexpensive, have a low amount of Or, and therefore have insufficient resistance to induction and oxidation compared to austenitic stainless steels.

For example, automobile exhaust gas materials have been changed from a thermal lifter type to a catalytic type, and as a result, low chromium ferritic stainless steels such as BUB4I10L or 8U84709 are used. The above steel types cannot be used due to their oxidation resistance and high temperature strength. In addition, high-frequency welded pipes are used for exhaust pipes, but oxides tend to get caught up in the welded parts during the pipe-making process, and on the other hand, the toughness of the welded parts is not sufficient, and this also causes problems due to bulge or pend processing. Cracks sometimes occurred at welded parts. For this reason, argon gas sealing or post-heat treatment is performed during pipe manufacturing, but all of the above methods are extremely difficult operations, and it is desired to improve the oxidation resistance of the material and the toughness of the welded joint.

8U8DA30 is used for the interior materials of Maku buildings, and Kyu cosmetic pipes are often used, but cracks may occur at the welded parts even after simple bending, and the induction resistance of the welded parts is is also not enough.

In addition, austenitic stainless steel has traditionally been used for burner materials for gas and oil appliances due to its press formability, but due to improvements in manufacturing technology in recent years, 8UI34I30 and other steels are increasingly being used.

However, since the equipment has a high calorific value, the burner nozzle is 100%
808 in terms of oxidation resistance, which is heated to high temperatures above ℃
Austenitic stainless steels such as 301B are still used.

An object of the present invention is to provide a ferritic stainless steel that eliminates and improves the above-mentioned drawbacks of conventional ferritic stainless steels and has excellent oxidation resistance, high-temperature strength, weld zone toughness, and induction resistance. The above object can be achieved by providing the steel according to the claims.

That is, the present invention is less than 00.03 mo, 81 /, 0-
! , 0$, Mn Ko, 0-F, Ni O,
1~3.0'ls, Or/-~ko0chi, yo,
o, r1o, i to i, o, and T1 is more than IO times the sum of C and N, and the balance is Fe and unavoidable impurities. This relates to ferritic stainless steel with excellent properties.

Next, the present invention will be explained in detail.

The inventor of the present invention has conducted a detailed study on the influence of the composition of the various defects of conventional ferritic stainless steel, and has discovered the following fact: The induction resistance of ferritic stainless steel is N1. When 81 is added in combination, the synergistic effect results in a remarkable improvement.

f) Addition of 61 greatly improves not only oxidation resistance but also high temperature strength.

J) Conventional ferritic stainless steel has the drawback of easily generating cracks and point defects when processing the welded part of high-frequency welded pipes, but by adding 81 and N1 in combination, the above cracks and points At the same time, defects can be prevented.

The present inventor completed the present invention based on the above novel findings 1) to 3).

The reason for limiting the component composition in the present invention will be explained.

C impairs the induction resistance and weld toughness, so the less the better, but if the amount exceeds o, o, yIs, the above properties deteriorate, so C must be less than o, o, ys.

Sl is an element that improves oxidation resistance, works synergistically with N1 to improve induction resistance, and has the effect of preventing point defects that may occur when processing high-frequency welded pipes.
If 1 is less than /, 0-, the above characteristics will not be exhibited; on the other hand! , If it is more than 0-, the toughness of the weld will deteriorate, so 81
must be within the range of 8 o −j* o o .

The smaller the amount of Mn, the better in terms of induction resistance, but some amount remains for steelmaking purposes. Mn #'i, 2.0'%
If the amount is more, the induction resistance will deteriorate, so it is necessary to make it less than 011.

N1 is an element that contributes to improving the induction resistance and weld toughness by being contained together with sl, but when N1 is
, sqb, both of the above characteristics are poor, while 3.01
If the Ni content is more than 4.0%, an austenite phase will be formed at high temperatures and both of the above properties will be degraded, so the Ni content must be within the range of O, S to 3.0%.

Or is a main element in ferritic stainless steel that is essential from the viewpoint of resistance to induction, but if Cr is less than 1, the resistance to induction is insufficient, while if it is more than Jls, the toughness of the weld will deteriorate. Since it deteriorates, it is necessary to keep it within the range of Cr/Co to Cr.

From the viewpoint of induction resistance and the toughness of the welded part, it is preferable that N has a low value.If N is more than 0.02, both of the above properties will deteriorate, so N should be less than θ, θ.

T1 is an element that combines with C and N and contributes to improving the toughness and induction resistance of the weld zone, but T1 is 0. /
% and less than 10 times the sum of C and N, both of the above properties deteriorate; on the other hand, when it exceeds O4, brittle sigma phase and Laves phase are likely to occur, and the toughness of welds in particular deteriorates. Therefore, Ti needs to be in the range of 0.1 to 1.0% and at least 10 times the sum of C and N.

Next, the present invention will be explained using experimental data.

It is made of ferritic stainless steel having the composition shown in Table 1. /, -91111 thickness rotary B surface finished plate material was used as a test material, and its induction resistance, oxidation resistance, and high temperature strength were investigated. '1 "The induction resistance was evaluated by conducting a salt spray test in accordance with JIS Z Co. 37/ and using a rating number of 10 (no rusting) from the degree of rusting/(full surface rusting). Also, the oxidation resistance was evaluated by weight gain after 100 hours of oxidation in air at 900°C.

/J The influence of addition of Sl and N1 on the oxidation resistance and induction resistance of Cr-Ti steel was investigated, and the results are shown in FIG. From the figure, it can be seen that No. 81 exhibits a remarkable effect on improving oxidation resistance. The reason why this effect is improved is that dense 5102 is formed in the inner layer of the scale, which prevents the progress of oxidation, but in order to obtain sufficient oxidation resistance, the Sl content must be at least 0-. I understand.

On the other hand, although N1 contributes to improving the induction resistance, its contribution rate is small when used alone, and it is not possible to obtain an induction resistance comparable to that of SU '130, which is the most common among ferritic stainless steels. However, it was newly discovered that when N1 is added in combination with Sl, a synergistic effect is exhibited, and when N1o, sc4 or more is included, induction resistance equivalent to or higher than SUB Da 30 can be obtained. On the other hand, N1 is large, that is, 3
．． If the amount is higher than 04, the oxidation resistance will deteriorate rapidly.

The cause of this is the rounding that generates an austenite phase with a low Or and a high coefficient of thermal expansion at high temperatures, and furthermore, this austenite phase transforms into a brittle martensite phase at room temperature, causing deterioration of the toughness of welds etc. Therefore, N1, which causes rapid deterioration of oxidation resistance, is 81. It is determined in relation to Or, but in the steel of the present invention, it is more than 3.0%. Therefore, N1 needs to be more than O and S in terms of resistance to induction, and less than 3.theta. O
Since it is a ferrite-forming element such as r and Ti,
Also in terms of measuring the quantitative balance of the austenite- or ferrite-forming elements, N1 needs to be less than y, os.

The results of investigating the relationship between yield strength, tensile strength, and temperature for temperature control 79 of the present invention and comparison steel/#6 DE are shown in Figure 1. 5i
O(lliikon:: 1,i,□.Yu□−□・.

It can be seen from the figure that the steel of the present invention has the characteristics necessary for use in high-temperature applications such as automobile exhaust gas systems, because it has high oxidation resistance and high oxidation resistance.

Several types of steel shown in Table 1 are subjected to high-frequency welding1, jt
A tube of 7φXJ was manufactured, and the toughness of the welded part was examined by a flat test, and the induction resistance of the welded part was mainly investigated by a salt spray test. High-frequency welding failed without argon sealing or post-heat treatment□.

In addition to cracks, point defects may occur when a weld is processed.In a flattening test, a 'rP with a length of 1 TrL was flattened to a close contact, and the number of point defects in the weld was evaluated.

The cumulative length of cracks was investigated. In addition, the rusting state of the welded parts was mainly investigated in the salt spray test. .

8υ841/θI, , 8U81I09 and SU
Since B da 30 has low 81 and N1, cracks and point defects occur together after the flattening test, and 8US da 10L and 8U81I30 do not contain T1, so the welds are susceptible to rusting due to intergranular corrosion.

A more detailed investigation of the point defects caused by the above-mentioned flattening test revealed that Or-
It was discovered that a huge oxide of 81-Mn was not discharged during subsequent upsetting and remained in the welded area, and that this area opened during processing, resulting in point defects. To counter this defect, there are two methods: one is to suppress oxidation as much as possible using a special Ar seal during pipe making, and the other is to increase the amount of discharge by strengthening the upset, but both are unfavorable in terms of operability or yield, especially the latter. Since the toughness of the weld bead deteriorates, post-heat treatment is required.

According to the present invention, by adding an appropriate amount of Sl, the formation of giant oxides during the overheating process of welding is suppressed, the fluidity of molten steel is increased, and discharge in upsetting is facilitated, and furthermore, N
The toughness of the weld bead can be improved by adding 1 complex.

On the other hand, in the comparison steel A6t with low S1, spot defects occurred in the welded part in the flattening test, and the comparison steel with low N1/1
In No. 66, cracking occurred. On the other hand, in the steel of the present invention, since Sl and N1 are appropriately added, no defects occur in the flattening test, and C and N
Is an appropriate amount of T1 added? The induction resistance of the welded part is also sufficient. However, if the amount of 81 is too large, the toughness deteriorates, and cracks occur even when N1 is added in combination, as seen in Comparative Steel A/C. .

Next, the fields of application of the steel of the present invention will be explained.

The operating temperature of catalyst containers or exhaust pipes for automobile exhaust gas systems increases as the size of the vehicle to which they are installed increases, so it has been difficult to cope with these temperatures using the conventionally used 80S, 10L or ErUSQ09. By using the steel of the present invention, it is possible to sufficiently cope with such high-temperature environments. High-frequency welded pipes are used in a very large amount in the exhaust system, and the steel of the present invention can easily produce high-frequency welded pipes that have welded parts with sufficient workability. Therefore, the steel of the present invention can be stably supplied for use as a material for producing high-frequency welded pipes of excellent quality.

SU8'130 has conventionally been used for decorative pipes, etc., which are interior materials for buildings, but there was a problem with the induction resistance of welded parts. Ferritic stainless steel with good oxidation resistance has been in demand as a burner material for gas and oil appliances, and the steel of the present invention has good resistance to welds and has excellent oxidation resistance. is better than 5UsL7θ and can be manufactured at low cost, making it VA-oriented.

[Brief explanation of the drawing]

Figure 1 is a diagram showing the influence of Sl and Ni loading on the induction resistance and oxidation resistance of /30r steel containing T1,
FIG. 1 is a diagram showing the relationship between the yield strength and tensile strength of comparative steel scrap 9 and steel scrap 3 of the present invention and test temperature. Patent applicant Nippon Yakin Kogyo Co., Ltd. Patent attorney Mura 1) Politics 2nd figure J! #Ultra (℃)

Claims (1)

[Claims] L 00. ．． 03 below, 81 /, 0-s, 0
fla, Mnko, θ width or less, ) Ji O,! r ~
3.0 Section, Or /J ~ Koochi. NO, 0 or less, T10. /~/, 016,
And T1 is more than 10 times the sum of C and N. e
and resistance to unavoidable impurities. Nine-ferritic stainless steel with excellent oxidation resistance and weldability.