JPS6187817A - Manufacture of heat resistant austenitic stainless steel - Google Patents

Abstract

PURPOSE:To manufacture an austenitic stainless steel having improved yield strength by subjecting a steel contg. specified percentages of C, Si, Mn, N, Ni, Cr and V to plastic deformation and heat treatment under specified conditions to make the grains fine. CONSTITUTION:A steel consisting of, by weight, <=0.15% C, <=2.0% Si, <=3.5% Mn, 0.02-0.5% N, 8-18% Ni, 15-22% Cr, 0.01-1.0% V and the balance essentially Fe is plastically deformed at >=20% rate of working and heat treated at 1,050-1,150 deg.C to make the grains fine. An austenitic stainless steel having grain size No.>=5 (JIS G 0551) is obtd.

Description

[Detailed Description of the Invention] [Technical Field of the Invention] The present invention relates to a method for producing austenitic stainless steel with excellent mechanical properties, and in particular, to improve the mechanical properties of nine-austenitic stainless steel by refining grains and improving mechanical properties. Regarding the manufacturing method.

[Technical background of the invention and its problems]

Austenitic stainless steel is often used in corrosive environments because of its excellent corrosion resistance. Furthermore, since the temperature dependence of mechanical properties is smaller than that of ferritic steel, it is said that the limit temperature for use can be made higher than that of ferritic steel. However, since austenitic stainless steel has a low yield strength, parts must have a thick wall structure to ensure a predetermined strength when actually used. As a result, the weight of the parts increases, making it difficult to transport and erect large parts, and at the same time, during heat treatment, the temperature distribution becomes uneven on the inner and outer surfaces of thick-walled parts, which can lead to thermal fatigue under conditions of repeated heating. is accelerated. Therefore, austenitic stainless steel improves mechanical properties and
It is necessary to avoid thick wall structures as much as possible.

Generally, precipitation strengthening, solid solution strengthening, hot and cold working, etc. are used to improve mechanical properties such as strength and yield strength of austenitic stainless steel. However, these strengthening methods have the following problems. That is, precipitation strengthening is not suitable for large members because heat treatment is complicated, and solid solution strengthening has not yet achieved sufficient strength. On the other hand, hot and cold working are effective in strengthening austenitic stainless steel because they make the grains finer and increase the strength through processing strain, but they are heated to high temperatures and are affected by heat during welding. Coarse crystal grains are formed in some areas, which causes a decrease in strength.

By the way, in order to improve thermal efficiency in thermal power plants that use oil or coal as fuel, there is a trend toward higher steam conditions at higher temperatures and higher pressures. A transition is being made to austenitic stainless steel, which has good high-temperature properties.

Additionally, for the same reason, the usage conditions for austenitic stainless steel, which is used in high-temperature environments in chemical plants and boilers, are becoming harsher. Despite these changes in the usage environment for steel, Existing austenitic stainless steels lack mechanical properties, especially yield strength, and there is a desire for austenitic stainless steels with even higher yield strength.

[Purpose of the invention]

The present invention was made in view of the above points, and is a method for manufacturing austenitic stainless steel with high strength without complex heat treatment. The object of the present invention is to provide a method for producing austenitic stainless steel having fine crystal grains, which refines the crystal grains and improves yield strength by combining processing and simple heat treatment.

[Summary of the invention]

The present invention uses less than 0.15% carbon by weight.

2.0% or less silicon, 3.0% or less manganese, 0
．． 02-0.3% nitrogen, 8-18% nickel, 1
Steel consisting of 5 to 22 inches of chromium, 0.601 to 1.0 inches of vanadium, the remainder substantially iron, and further 0.01 to 1.0 inches of vanadium;
1.0% niobium, 0.002-0.5% titanium, 0
．． 0005~0.01% of steel containing at least -1 kind of boron is plastically deformed at a processing rate of 20% or more, and then heat treated at a temperature of 1050~1150°C to refine the crystal grains. Method for producing austenitic stainless steel having grains 0 Here, the reasons for limiting the composition of the austenitic stainless steel according to the present invention will be explained.

Carbon (C): Carbon is more effective in stabilizing the austenite phase and strengthening stainless steel, but 0.15
If added in excess of
．． The content is set at 15%, but from the viewpoint of workability, it is desirable to set the content at 0.12% or less. Furthermore, in practical terms, it is 0.03 to 0.
08%, particularly 0.04 to 0.06.

Silicon (8i): Silicon acts as a deoxidizing agent during steel manufacturing and is a necessary element to improve the flowability during manufacturing, but since adding a large amount will impair toughness, the upper limit is set at 2.0 Let's do it. More preferably 0.1 to 1
．． 0%, especially 0.3-0.7%.

Manganese (Mn): Manganese is similar to silicon, tNfAQ
K acts as a deoxidizing agent and stabilizes the austenite phase as an austenite-forming element, but if added in a large amount, corrosion resistance such as oxidation resistance will be impaired, so the upper limit is set at 3.0%. The content is preferably 0.2 to 2.4%, but in practice it is preferably 0.5 to 1.5%.

Nitrogen (N): Nitrogen is necessary to stabilize the austenite phase and to form a solid solution in the austenite phase to improve yield strength. Also, along with vanadium, it has the effect of making crystal grains finer, so it must be contained at least 0.02%, but if added in excess, it will create pinholes and blowholes and form nitrides at grain boundaries, reducing toughness. Therefore, the upper limit is set to 0.3%. Considering strength, blowholes, etc., it is desirable to set the content to 0.08 to 0.35%, but considering industrial nitrogen control methods, it is desirable to set it to 0.11 to 0.2%.

Nickel (Nl): Nickel HaX -y-7L/X
It is an essential element for making steel tD m i & into austenite and requires at least 6%. However, if added in a large amount, the effect of making the crystal grains finer is weakened and the cost increases, so the upper limit is set at 20%. In order to effectively refine the crystal grains, it is recommended that the content be 8 to 15%, but in practice it is desirable that the content be 8.5 to 13.5%, especially 9.5 to 11.5%. ) Nichrome is 15% to increase strength at room temperature and high temperature, as well as improve corrosion resistance and oxidation resistance.
The upper limit is set at 22 modulus because adding a large amount forms a sigma phase and impairs toughness. Considering the balance with the amount of Ni and the amount of Cr required when adding nitrogen, the content is preferably 16 to 19.5%, and more practically 16 to 18.5%.

Vanadium (V): Vanadium is dissolved in the austenite phase to improve strength, and also interacts with nitrogen to make crystal grains finer and improve yield strength.
At least 1% is necessary, but since adding too much will cause segregation and impair workability, the upper limit is set at 1.0%. Considering mechanical properties and segregation, the range is preferably 0.03 to 0.5 inch, and more practically 0.05 to 0.35 inch.

Niobium (Nb): Niobium makes crystal grains finer in balance with the amount of nitrogen and vanadium. 0.0 to stabilize fine grains at high temperatures.
The content is required to be 1%, but if added in a large amount, the effect will be saturated and the processability will be impaired, so the upper limit is set at 1.0%. More preferably, it is 0.02 to 0.5%, but in practice it is 0.02 to 0.15%.

Titanium (Ti): Like niobium, titanium needs to be contained at 0.002% in order to make the crystal grains finer and improve yield strength in balance with nitrogen and vanadium crushing, but adding too much will cause segregation and reduce workability. Set the upper limit to 0 to avoid harm.
．． 5%. Furthermore, considering mechanical properties and segregation, 0.
It is desirable to set it to 0.02 to 0.3%, but in practice it is less than 0.
．． 02-0.15%.

Poron (B): Poron has the effect of strengthening grain boundaries, but if it is less than 0.010005, the effect is not sufficient and 0.01
Since addition exceeding % will impair weldability, the upper limit should be set at 0.0%.
The amount is preferably 0.001 to 0.000%. Further, in practical terms, it is desirable to set it to 0.003 to 0.0071. In addition, the reason why austenitic stainless steel with the above composition is subjected to plastic deformation at a working rate of 20% or more and then heat treated at a temperature of 1050 to 1150'O is explained below. Since the grain size number (JIS G 0551) of the crystal grains does not become 5 or more even after heat treatment for refinement, cold and hot working of 20% or more is necessary. In addition, after processing of 20% or more, 1050
Heat treatment at ~1150'0 means that even if heat treated at a temperature below 1050°C, a processed structure remains and if heat treated at a temperature exceeding 1150°○, the grain size number (JIS 0055
1) is less than 5 and it is no longer a fine grain, so 1050
The temperature should be in the range of ~1150°C.

〔Effect of the invention〕

According to the method for manufacturing austenitic stainless steel of the present invention, by combining the interaction of zero elements such as nitrogen and vanadium, hot and cold working, and simple heat treatment, the crystal grains are refined and the yield strength is increased. In addition, it is possible to suppress the formation of coarse crystal grains that cause strength reduction that occurs in areas heated to high temperatures such as the heat-affected zone during welding.

[Embodiments of the invention]

After melting samples having the chemical composition shown in Table 1 in a high frequency induction melting furnace, they were homogenized for 1100°03) hours and then cold rolled for 10 to 70 hours. Next, the JIS grain size number of the sample which was refined at a temperature of 900 to 1200 °0 for 2 hours was examined.The sample of Example 1 was cold rolled by 10 to 70% after casting, and then The grain size number of the sample refined for 2 hours at a temperature of 12oO<0>C was also examined. Comparative Example Sample 1 is commercially available SUS 316 and is subjected to the same homogenization treatment as the Example Sample.
Cold rolling and refinement treatment were performed. These results are the second
Table 3 shows the results.

Margin F: Chiku where the fiber structure remains It can be seen that an austenitic stainless steel having fine crystal grains, which cannot be obtained in Comparative Example 1, can be produced. In this case, as shown in Table 2, if the refinement temperature is less than 1050°C, the processed structure will not disappear, and if it exceeds 1150'O, the grain size number will become smaller than 5, indicating large grains. Further, as shown in Table 3, when the processing rate is less than 20%, the grain size number becomes smaller than 5 as well.

Next, the samples with compositions corresponding to Example 1.3.5 and Comparative Example 1 were melted again, and after homogenization treatment at 1100° for 020 hours, they were processed by about 50% by cold rolling to 1050°.
C. for 2 hours, JI8 grain size number and tensile test were conducted to determine 0.2% yield strength, which were designated as Examples 11, 12, 13 and Comparative Example 3, respectively.

The results are shown in Tables 4 and 5.

As shown in Table 5 below, the steel produced by the method for producing austenitic stainless steel according to the present invention is 5U made from conventional alloys.
It has finer crystal grains than S316 (Comparative Example 3), and as a result, the mechanical properties, especially the 0.2 inch proof stress, can be greatly improved.

As described above, the manufacturing method of the present invention exhibits excellent mechanical properties.
- It is possible to produce stenitic stainless steel, which can be used for parts that require high strength such as chemical plants and boilers, and is also suitable for manufacturing steel for turbine parts, especially casings. I can say that.

Claims (4)

[Claims] (1) 0.15% or less carbon by weight percent, 2.0
% or less silicon, 3.5% or less manganese, 0.02
~0.5% nitrogen, 8-18% nickel, 15-22
% of chromium, 0.01 to 1.0% of vanadium, and the remainder is substantially iron. After applying plastic deformation of 20% or more at a working rate, heat treatment is performed at a temperature of 1050 to 1150 °C to reduce crystal grains. A method for producing heat-resistant austenitic stainless steel, which is characterized by making the grains fine. (2) Grain size number of crystal grains of austenitic stainless steel (JIS G0551) in claim 1
A method for producing heat-resistant austenitic stainless steel, characterized in that is 5 or more. (3) 0.15% or less carbon by weight percent, 2.0
% or less silicon, 3.5% or less manganese, 0.02
~0.5% nitrogen, 8-18-nickel, 15-22
% chromium 0.01-1.0% vanadium, balance substantially iron and 0.01-1.0% niobium, 0.00
Steel containing at least one of 2 to 0.5% titanium and 0.0005 to 0.01% boron is subjected to plastic deformation at a processing rate of 20% or more and then heat treated at a temperature of 1050°C to 1150°C. A method for producing heat-resistant austenitic stainless steel characterized by reducing the crystal grains. (4) Grain size number of crystal grains of austenitic stainless steel (JIS G0551) in claim 3
A method for producing heat-resistant austenitic stainless steel, characterized in that is 5 or more.