JPS5953343B2 - Non-magnetic stainless steel and its manufacturing method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an inexpensive non-magnetic stainless steel with excellent corrosion resistance and mechanical properties, and a method for producing the same.

In recent years, as plants such as chemical, power generation, ultra-low temperature, and nuclear power plants have become larger, the operating conditions have become more severe, and the electric currents and magnetic fields used have become enormous.
In addition to corrosion resistance and mechanical properties, there is also a strong demand for non-magnetic properties. SUS30, a typical type of austenitic stainless steel
No. 4 is widely used because it has excellent corrosion resistance, mechanical properties, and workability, and is nonmagnetic.

In addition, when welding work is involved, the intergranular corrosion resistance of the SUS304 is insufficient, and the SUS304, which has excellent intergranular corrosion resistance,
S304L is used. However, with these steels, there is a problem in that segregated delta-ferrite remains during solidification in large parts where the area reduction rate cannot be large. Furthermore, since SUS304 and SUS304L are metastable austenitic stainless steels, they also have the disadvantage that martensite is induced by cold working such as straightening, making them sensitive to magnetism. In this way, SUS304 and SUS304L have insufficient non-magnetism and are not suitable for large piping parts or large structural members, including welding. There has been a demand for the development of an inexpensive non-magnetic stainless steel that has intergranular corrosion resistance and general corrosion resistance. The present invention has been developed in response to such demands and provides an inexpensive nonmagnetic stainless steel with excellent corrosion resistance, workability, and mechanical properties.

Note that the term "nonmagnetic" in the present invention refers to a magnetic permeability of 1.03 or less. In general, magnetization in austenitic stainless steel is caused by segregated delta ferrite during solidification and martensite induced by cold working.

Therefore, if non-magnetism is generally required, Ni
Efforts have been made to increase the content to stabilize the austenite phase. However, increasing the amount of Ni leads to an increase in cost and a decrease in strength, which is not preferable. On the other hand,
SUS3 like SUS3O4LN for low cost
If O4L contains a large amount of N, the stability of the austenite phase will increase, but intergranular corrosion resistance will deteriorate, and furthermore,
This is not preferable because the strength increases more than necessary and the workability and machinability decrease.

Therefore, as a result of repeated research to overcome the drawbacks of conventional steel, the present inventors found that, firstly, the addition of an appropriate amount of N provides mechanical properties and workability comparable to SUS3O4, and excellent intergranular corrosion resistance comparable to SUS3O4L. Get sex at the same time.

Moreover, it helps improve the stability of the austenite phase.
Second, with regard to segregated delta ferrite, we have found that the cheapest solution is to add an appropriate amount of N, limit the amount of Si, and, if necessary, heat it at a high temperature for a long time. In this case, the amount of Ni in the steel of the present invention is 9.6 to 10.
About 6% is sufficient. Third, for deformation-induced martensite, an appropriate amount of N is added and the amount of Mn is increased to achieve a Ni equivalent of T25. , O or more, we have discovered that we can solve the problem without impairing the various performances of the steel of the present invention. .

, ゛nadaima, 81 is obtained by J4 in the following formula.
C+N) +Ni+0.35Si+1.05Mn+0.
65Cr Furthermore, the steel of the present invention has a Ni content of 9.6 to 10.6.
% and its content is small, so the steel ingot or continuously cast piece contains 2 to 6% of segregated delta ferrite.

If the plating pressure ratio exceeds 80, there is no problem with the machining process as L disappears. The segregated delta ferrite remains. The second invention is for eliminating residual segregated delta ferrite in large structural members, and is characterized by heating at high temperature for a long time.

This point will be explained in detail below.

In order to reduce the amount of segregated delta ferrite to 2% or less during solidification, it is sufficient to increase the Ni content, but this increases the material cost, resulting in an expensive non-magnetic stainless steel as a whole.

The present invention obtains non-magnetism with the same amount of Ni as SUS3O4, and in order to decompose and eliminate the 2-6% segregated delta ferrite remaining in the steel ingot or continuous cast piece, 110%
It was discovered that reducing the amount of delta ferrite to 0.2% or less by heating and holding in a high temperature range of 0 to 1280°C for 6 to 15 hours is most effective in terms of both material cost and manufacturing cost. It is something that

In the most desirable embodiment, the amount of segregated delta ferrite is 3 to 5%, and 12
This is a method of heating and holding at a temperature of 00 to 1280°C for 7 to 12 hours to decompose and eliminate.

In addition, when the amount of delta ferrite is relatively high, such as 5 to 6%, if the delta ferrite is kept at 1200 to 1280 °C for about 15 hours before rolling, the delta ferrite will become spheroidized and locally in an equilibrium state, and no more Decomposition of delta ferrite hardly progresses, but once rolled, delta ferrite transforms from a spherical shape to a long, thin string, and begins to decompose rapidly upon reheating. As described above, in the present invention, even when the amount of residual delta ferrite is large, heating and holding at 1200 to 1280°C for about 6 to 15 hours is performed before rolling, and if necessary, the reheating conditions for solid solution treatment are changed to 1.
This problem can be solved by two-step heating at 100-1200°C for 1-3 hours.

Furthermore, Figure 1 shows the amount of residual delta ferrite in the steel ingot and the Ni
This shows the relationship with balance, and from now on, in order to maintain the amount of delta ferrite in a steel ingot from 2 to 6%, Ni
It turns out that the balance should be within the range of -0.5 to 1.5.

Note that the Ni balance is determined by the following equation. 30C+20N+Ni-1.3Cr-231+13
．． 5 In addition, the relationship between product cost and residual delta ferrite amount in the steel ingot is summarized as shown in FIG. 2.

The region 1 in the figure is the range where non-magnetism can be guaranteed by normal manufacturing methods, and the region (11) in the figure is the range where non-magnetism can be guaranteed by long-term heating according to the present invention. This region is a range in which it is difficult to decompose and eliminate the amount of ferrite to 0.2% or less even with the heating method of the present invention. That is,
The greater the amount of segregated delta ferrite during solidification of a steel ingot or continuously cast piece, the lower the material cost becomes, but if the amount of segregated delta ferrite exceeds 6%, the amount of ferrite cannot be reduced to 0.2% or less even after long-term heating. It becomes difficult to decompose and disappear, and the product cost increases. Therefore, in order to obtain non-magnetic stainless steel at a low cost, the best method is to use a material in which 2 to 6% of the segregated delta ferrite remains and decompose and eliminate it during the manufacturing process, taking into account the material cost and manufacturing cost. It turns out to be a good idea. The reasons for limiting the composition of the steel of the present invention will be explained below.

Like N, C is a strong austenite phase stabilizing element, but on the other hand, C lowers corrosion resistance, especially intergranular corrosion resistance, so the upper limit was set at 0.03%.

Although Si is a necessary element as a deoxidizing agent, it is also a strong ferrite-forming element, so its upper limit was set at 0.50%. Note that it is preferably 0.40% or less. Mn is an element useful as a deoxidizing agent and a desulfurizing agent, and also improves the hot workability of steel.

In the present invention, 1 is the main element that most stabilizes the austenite phase at room temperature and increases the Ni equivalent.
．． 00% or more content is required. However, if the content exceeds 3%, the corrosion resistance of the steel of the present invention will be impaired, so the upper limit was set at 3.00%. Cr is a basic element necessary for obtaining excellent corrosion resistance of austenitic stainless steel.
The content must be 18% or more.

However, since Cr is also a strong ferrite-forming element, the upper limit was set at 19%. Ni is a basic element of austenitic stainless steel, and is an element that provides excellent corrosion resistance, cold hardening properties, and nonmagnetism.

Since the purpose of the present invention is to obtain an inexpensive nonmagnetic steel, the use of expensive Ni was kept to the minimum necessary to ensure nonmagnetism, and its content was set at 9.6%. Also, 1
If the content exceeds 0.6%, the amount of residual delta ferrite in the steel ingot or continuously cast piece will be 2% or less, making it impossible to take advantage of the features of the present invention, so the upper limit was set at 10.6%. N is a strong austenite-forming element that suppresses the formation of segregated delta ferrite and deformation-induced martensite.
It is an element necessary to obtain non-magnetism at low cost.

Furthermore, N has a large solid solution strengthening ability, and as in the present invention, low C
It is also an element that compensates for the lack of strength without impairing the intergranular corrosion resistance of austenitic stainless steel, and in order to exhibit these properties, it must be contained in an amount of 0.06% or more. However, if the content exceeds 0.09%, susceptibility to intergranular corrosion increases, so the upper limit was set at 0.09%. Note that the content is preferably 0.06 to 0.07%. Next, the characteristics of the steel of the present invention will be clarified through examples in comparison with conventional steel.

Table 1 shows the chemical composition of these test steels.

In Table 1, A1 to A4 steels are conventional steels, and A1 is SUS.
3O4, A2, and A3 are SUS3O4L, and A2 has a Ni content close to the lower limit of the specification, and A3 has a Ni content close to the upper limit of the specification.

Further, A4 is SUS3O4L<7) with an increased amount of Ni. B1 to B5 steels are steels of the present invention.

Table 2 shows the strength, corrosion resistance, and nonmagnetism of the A1 to A4 steels and B1 to B5 steels subjected to the solid solution heat treatment shown in Table 1.

Regarding strength, yield strength, tensile strength, and elongation were measured using JIS No. 4 test pieces.

Regarding corrosion resistance, sulfuric acid resistance, pitting corrosion resistance, and intergranular corrosion resistance were measured, and sulfuric acid resistance indicates the corrosion loss when immersed in a boiling 5% H, SO4 aqueous solution for 6 hours.
Pitting corrosion resistance is measured in 4% FeCl3 aqueous solution at 40℃ for 24 hours.
This shows the corrosion loss when immersed, and the intergranular corrosion resistance is determined by performing a copper sulfate monosulfate corrosion test according to JISGO575 after sensitization treatment at 650°C for 2 hours, and then showing the presence or absence of cracking in a bending test. It is.

For non-magnetic properties, after ingot making, (1) was heated to 1265℃ for 3 hours, 38φ (rolling pressure ratio 158).
(11) is heated at 1265℃ for 3 hours and then rolled to 60φ, 100φ, and 200φ (rolling ratio is 71 for 60φ, 26 for 100φ, and 6 for 200φ), (I) is 1265℃ After heating for 10 hours, 60φ,
Rolled to 100φ and 200φ, (IV) is (I)
After rolling, it was subjected to solid solution heat treatment for 2 hours at 1150℃, (V) is the one that was subjected to 30% cold compression after the treatment in (), ○ indicates magnetic permeability (μ) 1.03 or less, The × mark is (
μ) exceeds 1.03.

As is known from Table 2, A1 steel, which is a conventional steel, has excellent strength, but is inferior in intergranular corrosion resistance and non-magnetism.

A2 steel, which has a lower C content and an increased Ni content than A1 steel, has excellent corrosion resistance, but lacks strength and cannot obtain nonmagnetic properties. A3 steel, which is obtained by further increasing the Ni content of A2 steel, has excellent nonmagnetism and corrosion resistance, but has low strength like A2 steel. Although A4 steel contains a large amount of Ni (14.46%), it can be made non-magnetic by normal rolling, but it is inferior in strength and intergranular corrosion resistance. In contrast, B1 to B5 steels, which are the steels of the present invention, are SUS3
By using O4L as a pace, containing an appropriate amount of N, increasing the amount of Mn, and regulating the amount of Si, N
The i balance is set to -0.5 to 1.5, and the Ni equivalent is set to 25.
．． By making it more than 0, the strength is 2
2.5kg/m! l or more, tensile strength 56kg/Mii
, the elongation is 54% or more, which is excellent, and the corrosion resistance is sulfuric acid resistance, 75 to 89 g/m...Hr, and pitting corrosion resistance is 6.
．． 0 to 6.9g/MhHr, the corrosion loss is small and excellent, and SUS3O4L has excellent intergranular corrosion resistance.
2. Similar to A3 steel, there is no cracking.

For non-magnetic properties, Ni content is 9.68-10.51%
Therefore, non-magnetism can be guaranteed for products with a rolling pressure ratio of (1) exceeding 80, but when the rolling pressure ratio of (11) is below 80, the normal 3H sub-cut heat at 1265°C is obtained before rolling. Although heating cannot guarantee non-magnetism, (11
1) rolled after heating at 1265℃ for 10 hours is B.
Non-magnetism can be guaranteed for all steels except B5 steel, and B5 steel, which has a Ni balance of -0.3 due to its low Ni and N content, is subjected to solid solution heat treatment for 2 hours at 1150°C after rolling. B
Non-magnetism can be guaranteed for any of the steels No. 1 to B5, and non-magnetism can also be guaranteed at low cost with low Ni. From now on, the steel of the present invention will have a strength equal to or higher than that of SUS3O4, and will also have corrosion resistance compared to SUS3O4.
It can be seen that it is equivalent to or higher than L, and is an excellent non-magnetic stainless steel.

As mentioned above, in order to obtain an inexpensive non-magnetic steel, the steel of the present invention requires the use of expensive Ni to be kept to the minimum required amount, containing an appropriate amount of N, increasing the amount of Mn, and regulating the amount of Si. By adjusting the Ni balance to -0.5 to 1.5 and Ni
With an equivalent weight of 25.0% or more, we have succeeded in obtaining non-magnetic properties without deteriorating strength and corrosion resistance. Furthermore, even when segregated delta ferrite remains in large structural members, it can be heated for a long time even at high temperatures. It can be disassembled and destroyed by doing so, and has high practicality as a large piping member or large structural member, including welding work.

[Brief explanation of the drawing]

Claims (1)

[Claims] 1. C0.03% or less, Si 0.50% or less, Mn 1.00 to 3.00%, Ni 9.6 to 10.6% by weight.
%, Cr18~19%, N0.06~0.09%, the balance consists of Fe and impurity elements, the Ni balance is -0.5~1.5, and the Ni equivalent is 25.0
A non-magnetic stainless steel characterized by: 2 Weight ratio: C 0.03% or less, Si 0.50% or less, Mn 1.00-3.00%, Ni 9.6-10.6
%, Cr18~19%, N0.06~0.09%, the balance consists of Fe and impurity elements, the Ni balance is -0.5~1.5, and the Ni equivalent is 25.0
Steel ingots or continuous cast pieces with a weight of 1100 to 1280 or more
A method for producing non-magnetic stainless steel, characterized in that the magnetic permeability is reduced to 1.03 or less in hot working at a heating pressure ratio of 80 or less after holding at a temperature of 6 to 15 hours.