JPS6048584B2 - Ultra-low carbon/nitrogen ferrite stainless steel with excellent weld toughness and workability - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an ultra-low carbon/nitrogen ferritic stainless steel that has excellent welded part toughness and workability.

Although ferritic stainless steel has a remarkable advantage over expensive austenitic stainless steel that contains Ni in that it is less prone to stress corrosion cracking in various corrosive environments, it is inferior in weld toughness and workability, so it is naturally Its range of use is limited, and it has been mainly used for thin products where the toughness and workability of welded parts are not a problem.

Conventionally, in order to improve the toughness and workability of welded parts of ferritic stainless steel, stabilizing elements Nb and Ti have been added, or the amount of Mn has been increased by 1.0% - (weight%, hereinafter the same applies to percentages). ) or more, but it is known that C and N are each 200 pμ.
There is a limit to the improvement of the toughness of welded joints due to the high value of around m, and in addition, there are problems such as many surface flaws caused by Ti oxides in products with added Ti, which should be improved in practical terms. was left behind. On the other hand, in recent years, advances in steelmaking technology using VOD and other new equipment have enabled C and N to be reduced to 1
It has become possible to reduce the particle size to about 0 or 20 pμm, thereby improving the toughness and workability of the base material.

Furthermore, depending on the amount of (C+N) reduced in this way, Mn
It has also become clear that the toughness and workability of welded parts can be significantly improved by appropriately controlling the amount of Ti
Contains ferritic stainless steel. The inventors went further and conducted more extensive research into improving the properties of welded parts of ultra-low C, N ferritic stainless steel, and found that its toughness and workability were particularly good in the welding atmosphere. was found to be significantly affected by

In other words, in actual welding, there are many cases in which the weld cannot be completely sealed with an inert gas such as Ar. For example, even if the front side of the weld can be sealed, the back side must be welded while being exposed to the atmosphere. In some cases, nitrification occurs in the welded steel from the atmosphere, significantly deteriorating the toughness and workability of the welded part.Therefore, it would be of great practical significance if we could develop a steel that can overcome this problem. It can be said. In view of these circumstances, the inventors conducted various research and development efforts, and as a result, the inventors have developed an ultra-low carbon fiber that has extremely excellent weld toughness and workability even when welded in conditions partially exposed to the atmospheric environment.
This is a successful improvement of nitrogen ferritic stainless steel. This invention contains 0.20-1.5% A1 and 0.20-1.5% A1.
05-0.4% TiNO. MnO. 5% or less, Si
O. 5% or less, then. Crll. 5% to 20.0%, and in some cases MO4. It was found that the above-mentioned significant improvement in the toughness and workability of the welded zone was found in a ferritic stainless steel composition containing 0% or less, with the remainder consisting of iron and unavoidable impurities.

In this invention, MO is selectively used depending on the corrosive environment.Next, in this invention, the composition range of the alloy components is limited as described above, and its technical significance is as follows. It is.

4 First, carbon is an element that is harmful to the toughness, workability, and corrosion resistance of welded parts, and should be lowered (the lower the carbon content, the better these properties will be).

Figure 1 shows a 2-thick plate of 17% Cr stainless steel containing Ti and N with extremely low N, completely sealed with Ar on both the front and back sides.
When G-welded, the ductility-brittle transition temperature and 0.777Z7 by Charpy V-notch impact test of the welded part
The relationship between the Erichsen value and the amount of C for an Ar-sealed TIG welded part of the same steel plate with a thickness of 7I is shown. When C exceeds 0.010%, the ring transition temperature is -60℃ or higher and the Erichsen value is 8.
．． It becomes 57 TUn or less, and the toughness and workability deteriorate. According to conventional experience, in order for welds made using this type of ferritic stainless steel sheet (for example, by the TIG method) to have sufficient toughness, the ductility-brittle transition in the Charpy V-notch impact test must be It is desirable that the temperature is lower than about -60°C, and from the viewpoint of ensuring workability of the welded part during severe plastic working, 0.7Tn1
In the case of a plate of n thickness, the Erichsen value of the weld is 8.5T.
wt or more is required.

Therefore, when evaluated from these viewpoints, C is 0.01
It is necessary to set the upper limit to 0%, and it can be said that the lower it is, the better the toughness and workability of the welded part.

Furthermore, as shown in Figure 2, when N exceeds 0.010% in ultra-low C(7) Ti and Al-added 17% Cr steel, A
The ductility-brittle transition temperature of the r-seal weld increases rapidly to over -60°C, and the Erichsen value of the weld decreases to below 8.5, deteriorating the toughness and workability. C
Similarly, the upper limit is 0.010%.

The lower N is, the better these properties will be. Al is a particularly important component for achieving the effects of this invention.

Figure 3 shows ultra-low C, 17% Cr steel with NTi addition of 2.077
Z77! Thickness and 0.7Tf! : TIG of 111 thick steel plate
Ductility-brittle transition temperature and Erichsen value of weld zone and N
Shows the relationship with content. Completely Ar on both sides during welding
The toughness and workability of the welded part when sealed deteriorates when A1 exceeds 1.5%, but below 1.5% Al
It is almost unaffected by the size of the amount and is good.

On the other hand, the toughness and workability of the welded part of steel containing 0.2 to 1.50% N are the same when the front surface of the plate is sealed with Ar and the back side is exposed to a circular atmosphere. The ductility-ductility transition temperature is about -90°C, and the Erichsen value is about 10wn, which is almost the same. In comparison, when A1 is less than 0.2% or more than 1.5%, the ductile-ductility transition temperature is -60°C or higher, and the Erichsen value is 8.
It deteriorates below 5Twt. In this case, 0.2% or more of A
The reason for the improvement in toughness and workability due to the addition of A1 is probably due to the formation of a thin film of A1 oxide on the surface of the welded steel during melting, which prevents immersion from the atmosphere. Furthermore, the deterioration in toughness and workability due to addition of 1.5% Al or more is considered to be due to solid solution hardening of AI itself. When evaluated from this point of view, it is necessary to specify the lower limit of Al as 0.20% and the upper limit as 1.50%. By adding 0.20 to 1.50% A1 in this way, the various properties of the welded part when TIG welding is performed with the back side of the weld exposed to the atmosphere are comparable to those when the weld is completely sealed with Ar. is clear from Fig. 3, and this Al
Conventionally, Mn content exceeds at least 1.0% to ensure the toughness of the welded part, and therefore, it may be that the welding workability is reduced due to poor flow of the deposited copper during welding, or that the formability of the plate is affected. without the disadvantages that are inevitable with deterioration,
At a Mn content of 0.5% or less, significant improvements in weld toughness and formability can be achieved. MΠ is 0.5
%, there are disadvantages such as deterioration of formability of the plate as described above and reduction of welding workability due to melt flow chewing of the welded steel. In the invention, the upper limit of Mn is defined as 0.5%.

T1 and Nb are both effective components for improving the toughness, workability, and corrosion resistance of welded parts, and one of them is
The addition of a species or two is essential to this invention.

Figures 4 and 5 show 2.0TfUrL thickness and 0.7TfUrL thickness.
The relationship between the ductility-brittle transition temperature and Erichsen value of a TIG welded part of a TmTt thick steel plate and the amounts of Ti and Nb is shown. If T1 is less than 0.05% or exceeds 0.4%, the toughness and workability of the weld will deteriorate, while Nb will be less than 0.05% or more than 0.4%.
If it is less than 0.05% or more than 0.30%, the toughness and workability of the welded portion will deteriorate. Therefore, the upper limits of T1 and Nb amounts are set at 0.4% and 0.3%, respectively, and the lower limits are set at 0.05%, and it is desirable to add Ti and Nb alone or in combination within this range. . Si
is an element that is inevitably mixed in when refining stainless steel, and while increasing the amount of Si improves the flow of the melt in the welded steel, it slightly deteriorates the toughness of the welded part, but in order to achieve the purpose of this invention. It is sufficient to limit the content to 0.5% or less.

Cr is an essential basic component for imparting corrosion resistance and oxidation resistance to ferritic stainless steel.
%, both of the above properties deteriorate rapidly;
If it exceeds 0%, the toughness and workability of the welded part will deteriorate, so in the purpose of this invention, Cr
The amount range is limited to 11.5-20.0%.

Finally, MO is an effective element for improving corrosion resistance, particularly pitting corrosion resistance and crevice corrosion resistance.

Therefore, it is not necessary to add MO when used in a mildly corrosive environment, but when adapting to applications that are expected to be used in a severe corrosive environment containing C By adding MO, the effect of improving pitting corrosion resistance and crevice corrosion resistance is added to the above-mentioned effects of this invention. However, even if MO is added in an amount exceeding 4.0%, the effect is almost saturated and the cost increases. Therefore, the upper limit of MO is set at 4.0%, and an amount suitable for the usage environment can be added as needed within this range.In addition to the main components explained above, in this invention, copper composition is added as an impurity. P and S are the main elements contained in
are 0.040% each as in normal JIS
0.030% or less, and 0.5% or less of N1 mixed from steelmaking raw materials are allowed.

Steels having components 1 to 16 shown in Table 1 were melted in a 10k9 high frequency small melting furnace, and hot rolled under known conditions to produce hot rolled sheets with a thickness of 4wt.

Thereafter, it was annealed at 900°C x 5Tr$t, followed by cold rolling and final annealing (850°C x 37W1 air cooling) to produce a cold rolled annealed plate with -0.7TffXL and 2.0wL thickness.

2Twt thick cold rolled annealed board 90wrm wide x 30mm long
It was cut to a length of 0m771 and TIG welded using a butt joint.

The welding conditions in this case are welding current 85A) welding voltage IOV
) Speed 150Wf1/-, tungsten electrode 1.6T
wt. φ, shielding Ar gas 151'/T from torch
WL, and the back side was exposed to Ar gas 71/w$t and the back side was left in the atmosphere. JIS Z22O2 with a notch in the cross section of the welded steel to remove excess buildup from the weld after welding
A No. 4 impact test piece was prepared and subjected to a Charpy impact test. Also, Strauss by JIGGO575
Corrosion tests were also conducted. Separately, a cold rolled annealed plate with a thickness of 0.7Tm was cut into width 45Tr1:1rL x length 3 concavities, welding current 40A) welding voltage 8V) speed 350TW.
L/Wm, tungsten electrode 16mmφ, shield Ar gas from torch 1.51? /Wrln) The back side was welded under a shield with Ar gas 71/WL or under atmospheric conditions. An Erichsen test was conducted on this test piece. Further, the cold rolled annealed plate itself having a thickness of 0.77 nm was subjected to a tensile test using a JIS No. 3 B test piece and other formability tests. Table 2 is 2771771
Charpy impact test of TIG welded part of thick plate and St
The results of the Rauss corrosion test are shown.

The ductility-brittleness transition temperature of the TIG welded parts of comparative steels when welded with the back side exposed to the atmosphere is -35° to 20°.
℃, while that of the invention steel was low at -70℃ or less, which is almost the same as when the welded parts were sealed with Ar on both sides, and the toughness was extremely good. In addition, the invention copper TIG
All of the welds had good corrosion resistance, showing good intergranular corrosion resistance. Table 3 is 0.7T! r! The material properties of a 11-thick copper plate and the Erichsen value of its welded part are shown. The Erichsen value of comparative steel when welded with the back side exposed to the atmosphere is 6.0 to 7. Cymm was low, whereas that of the invented copper was 9.0 TWL or more, which is almost the same as when both sides of the welded part were sealed with Ar, and the workability was good.

In addition, the material properties of the thin steel sheet according to the present invention include extremely low amounts of C and N, so that the descending stress and tensile strength are approximately 23% each.
and 42k9/1, which is low, and the total elongation exceeds 40%, making it extremely soft.

Also, the r value, CCV, and Erichsen value are each 1.6,
The moldability is good at about 26.5 and 10.0 w1, and can sufficiently withstand severe plastic working. Thus, according to the present invention, the content of C and N in the steel is reduced, and AIO. 2-1.5%
is added, and one or two effective amounts of Tj and Nb are added.
By including seeds, special properties can be created during welding. It is possible to produce 11.5-20% Cr ferritic stainless steel that does not deteriorate the toughness and corrosion resistance of the welded part even when welded in the atmosphere without gas shielding.Additionally, if an appropriate amount of MO is added, corrosion resistance, especially porosity resistance, can be produced. The steel should have significantly improved corrosion resistance and crevice corrosion resistance. Therefore, it can be said that its industrial significance is extremely large.

[Brief explanation of the drawings] Figure 1 shows a 17% Cr steel plate containing ultra-low N, Al, and Tl (2.
A graph showing the impact characteristics of a TIG weld (Ar shield) of a TIG weld (0-thickness) and the relationship between the Erichsen value and C content of a TIG weld of a 0.7 wr 1n thick copper plate.
TIG of 17% Cr steel plate (2.0-thickness) containing Al and Ti
A graph showing the relationship between the Erichsen value and the N content of the welded part (Ar shield) and the TIG welded part of the same 0.7 TWL thick steel plate.
TI of 17% Cr steel plate containing Tl (2.0TK1Tt thickness)
Impact characteristics of G-welded part and TI of 0.7Twt thick steel plate
A graph showing the relationship between the Erichsen value of the G weld, the A1 content, and the welding atmosphere. Figure 4 shows the extremely low C, N, N content 1.
Impact characteristics of TIG welded part (Ar shield) of 7% Cr steel plate (2.0-thickness) and TIG of 0.7TW1-thick steel plate
There is a graph showing the relationship between the Erichsen value and Ti content of the weld, and Figure 5 shows the impact of a TIG weld (Ar shield) of a 17% Cr steel plate (2.0-thick) containing extremely low C, N, and Al. It is a graph showing the characteristics and the relationship between the Erichsen value and Nb content of the TIG welded part of the same 0.7 TWL thick steel plate.

Claims (1)

[Claims] 1 C: 0.010% by weight or less Si: 0.5% by weight or less Mn: 0.5% by weight or less Cr: 11.5-20.0% by weight Al: 0.20-1. 50 wt% and N: 0.010 wt% or less, and 0.05 to 0.40 wt% Ti and 0
．． An ultra-low carbon/nitrogen ferritic stainless steel containing one or two types of Nb in an amount of 05 to 0.30% by weight, with the remainder consisting of iron and unavoidable impurities, and has excellent welded part toughness and workability. 2 C: 0.010 wt% or less Si: 0.5 wt% or less Mn: 0.5 wt% or less Cr: 11.5 to 20.0 wt% Mo: 4.0 wt% or less Al: 0.20 to 1.50 wt% and N: 0.010 wt% or less, and 0.05 to 0.40 wt% Ti and 0
．． An ultra-low carbon/nitrogen ferritic stainless steel containing one or two types of Nb in an amount of 05 to 0.30% by weight, with the remainder consisting of iron and unavoidable impurities, and has excellent welded part toughness and workability.