JPS6360809B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for producing high-strength austenitic stainless steel with excellent corrosion resistance. In recent years, the depletion of oil resources has come to be seen as a problem, and against this backdrop, in order to expand and secure stable energy resources, offshore oil fields are being developed, and onshore oil fields are being developed. So-called sour gas and sour oil, which contain acidic gases such as hydrogen sulfide and carbon dioxide gas, have been mined until now. Materials and equipment related to the production of such energy resources inevitably come into contact with chlorides and acid gases, and therefore the steel materials used in materials and equipment in such fields must first have corrosion resistance. In addition, strength requirements are becoming more severe as wells become deeper, and low-temperature toughness is also required for use in cold regions. Ni-based alloys, Ti-based alloys, Co-based alloys, etc. are known as materials that can meet these demands.
These alloys are too expensive compared to conventional conventional low alloy steels. In addition, for example, austenitic stainless steel has been known as a relatively inexpensive material, but currently it has a poor corrosion resistance-strength balance and is also poor in chloride corrosion resistance. On the other hand, although martensitic stainless steel has almost satisfactory strength, it has the drawback of being extremely susceptible to sulfide stress corrosion cracking. In general, methods for improving the yield strength of austenitic stainless steel include solid solution strengthening with C and N, as well as precipitation strengthening and working strengthening, but precipitation strengthening and working strengthening have a negative impact on corrosion resistance. If the amount of C is increased, Cr carbides will be generated and the corrosion resistance of the steel will be deteriorated, and if the amount of N is increased, defects will easily occur during the production of steel ingots. Therefore, as a result of intensive research to solve the above-mentioned problems, the inventors of the present invention have regulated the upper limit of the C and N content in austenitic stainless steel, and have determined that the addition of these elements will reduce the corrosion resistance of the steel. While solid solution strengthening is achieved while preventing the occurrence of defects during ingot production, by making V coexist with elements such as Cr and Ni, the formation of Cr carbides is suppressed, and furthermore, the dispersion and precipitation of V carbonitrides are suppressed. As a result, the yield strength is improved, and thus both the yield strength and corrosion resistance of the steel are improved.Furthermore, by heat-treating such steel according to predetermined conditions, a steel with further improved machinability such as elongation and drawing is produced. The present invention was based on the discovery that it can be obtained. Therefore, an object of the present invention is to provide a high-strength austenitic stainless steel with excellent corrosion resistance, particularly in an environment containing chlorides and sulfides, and high strength. The purpose of the present invention is to provide a method for producing austenitic stainless steel that has excellent machinability. The first method for producing a highly corrosion-resistant, high-strength austenitic stainless steel according to the present invention consists of C 0.05 to 0.15%, Si 0.10 to 0.50%, Mn 0.5 to 5.0%, Cr 18 to 25%, Ni 6 to 10% by weight. %, Mo 2 to 4%, V 0.05 to 0.25%, N 0.15 to 0.45%, the balance being iron and unavoidable impurities. Austenitic stainless steel is solution treated at a temperature of 1030 to 1100°C and then rapidly cooled. , characterized by tempering treatment at a temperature of 250-500℃. Moreover, the second is C 0.05-0.15%, Si 0.10-0.50%, Mn 0.5-5.0%, Cr 18-25%, Ni 6-10%, Mo 2-4%, V 0.05-0.25. %, and N 0.15-0.45%, at least one selected from Nb 0.05-0.50% and Ti 0.01-0.50%, the balance being iron and unavoidable impurities. After solid solution treatment, it is rapidly cooled, and then
Characterized by tempering treatment at a temperature of ℃. First, the reason for limiting the components in the austenitic stainless steel according to the present invention will be explained. C is an austenite stabilizing element, and at the same time is an interstitial solid solution strengthening element that is effective in improving the strength of steel, including its yield strength. In the present invention, Ni
0.05 to precipitate fine carbonitrides in coexistence with V and V to improve the yield strength and toughness of steel.
It is necessary to add more than 0.15%
When it exceeds Cr, Cr carbide is formed and corrosion resistance is reduced. Therefore, the C content is set to 0.05 to 0.15%. It is necessary to add 0.10% or more of Si as a deoxidizing agent for steel, but when added in excess, it increases the susceptibility to weld cracking and may also cause cracking during hot rolling, so the upper limit should be set. It shall be 0.50%. Like Si, Mn is not only necessary as a deoxidizing agent for steel, but also increases the amount of solid solution of N, stabilizes austenite, and improves weld cracking resistance by 0.5%.
It is necessary to add the above. However, 5%
If it exceeds this, problems such as loss of hot workability will occur. Therefore, the Mn content is preferably in the range of 0.5 to 5%, and particularly preferably in the range of 2.0 to 4.0% from the viewpoint of improving weld cracking resistance. In the present invention, Cr is an essential element for improving the corrosion resistance of steel, and is also an element necessary for increasing the solid solubility limit of N. However, excessive addition is undesirable because it disrupts the balance between austenite and ferrite, and in order to maintain the properties of the steel of the present invention, it becomes necessary to add a large amount of expensive Ni, etc. The amount should be 18-25%. Ni is an essential element for improving corrosion resistance and mechanical properties when balanced with Cr etc.
For this purpose, it is necessary to add more than 6.0%, but on the other hand, when adding excessively to Cr, corrosion resistance deteriorates, so the upper limit is set at 10%. Mo is an essential element for the corrosion resistance of steel, especially for preventing crevice corrosion and pitting corrosion.For this purpose, it is necessary to add 2% or more, but even if it is added in excess, the effect of improving corrosion resistance tends to be saturated. The upper limit will be set at 4%, as this will further increase the product price. In the present invention, in order to improve the strength, toughness, and corrosion resistance of steel in a well-balanced manner, V is particularly added to Cr.
In addition to suppressing the formation of carbides and improving corrosion resistance,
In order to improve the yield strength through the dispersed precipitation of V carbonitrides, it is necessary to add at least 0.05%.
However, when added in excess, it promotes the formation of ferrite, disrupts the balance between austenite and ferrite, and deteriorates corrosion resistance. Therefore, the upper limit is set at 0.25%. Like C, N is an austenite-forming element, and improves the yield strength of steel through solid solution.
It has the effect of forming fine carbonitrides and improving toughness. In order to effectively express this effect,
It is necessary to add 0.15% or more, but if the content decreases excessively, it will cause problems during the production of steel ingots, so the upper limit is set at 0.45%. In addition to the above-mentioned elements, at least one element selected from Nb and Ti can be added to the austenitic stainless steel according to the present invention. Nb is known as an element that forms carbides and stabilizes C, but in steel with a high N content, it forms fine carbonitrides and improves both yield strength and toughness. In order to express such an effect, it is necessary to add 0.05% or more,
When added in excess, it deteriorates weldability, and since Nb is an element that forms stable carbonitrides, it leads to a decrease in the amount of solid solution C and N, which in turn decreases the yield strength and causes huge Forms carbonitrides that significantly impair toughness. Therefore, the upper limit
It shall be 0.50%. Like Nb, Ti is also an element that forms very stable carbonitrides, and when added in the range of 0.01 to 0.50%, it improves the yield strength of steel. Since it causes a decrease in toughness, its content is regulated as described above. In the method of the present invention, steel having the above chemical composition is solution treated at a temperature of 1030 to 1100°C, then rapidly cooled with water or oil cooling, and then
Tempering treatment to a temperature of 250-500℃. In the method of the present invention, by performing the solution treatment in the above temperature range, it is possible to achieve not only solid solution of Cr carbides, but also softening due to recrystallization, improvement in corrosion resistance, removal of internal stress, etc. However, when the solution treatment temperature is lower than 1030°C, the solution treatment of Cr carbides is insufficient, and undissolved Cr carbides remain in the steel, which has a detrimental effect on the corrosion resistance of the steel. On the other hand, when solution treatment is performed by heating to a high temperature exceeding 1100° C., carbides such as V, Nb, and Ti are also dissolved, leading to deterioration of yield strength. A particularly preferable solution treatment temperature range is 1040 to 1080°C. The solution heating time needs to be increased in proportion to the thickness of the steel material, but generally it may be set at a rate of 1 hour per 25 mm thickness of the steel material. Cooling of steel materials that have undergone solution heating requires rapid cooling faster than oil cooling, that is, cooling with an average cooling rate of approximately 0.2°C/sec or more, especially in the temperature range of 900 to 500°C. Since Cr carbides tend to precipitate at grain boundaries, it is preferable to rapidly cool the steel in this temperature range to ensure excellent corrosion resistance. According to the method of the present invention, after the solution treatment and rapid cooling, the steel is tempered at a temperature of 250 to 500°C to remove thermal strain in the steel, thereby improving not only corrosion resistance and yield strength. In particular, elongation and reduction of area can be further improved. That is, by tempering to the above-mentioned temperature range, the hardening effect caused by the rapid cooling can be alleviated, the thermal strain caused by the solution treatment can be removed, and the toughness can be improved. If the tempering temperature is less than 250°C, thermal strain cannot be sufficiently removed, so there is a risk that the stress corrosion cracking resistance of the steel will decrease. On the other hand, when the tempering temperature is a high temperature exceeding 500°C, both corrosion resistance and mechanical properties deteriorate due to grain boundary precipitation of Cr carbides. A more preferable tempering temperature range is 350 to 500°C, and an especially preferable temperature range is 400 to 500°C. Conventionally, it is known that austenitic stainless steel has the best corrosion resistance when solution treated, and that subsequent heating leads to precipitation of Cr carbides and deteriorates corrosion resistance. However, according to the method of the present invention, by tempering in the temperature range where low-temperature sensitization phenomena are conventionally thought to occur, elongation, reduction, etc. can be reduced without impairing the excellent corrosion resistance of steel. Mechanical properties can be improved. As described above, according to the method for producing austenitic stainless steel of the present invention, the upper limit of the content of C and N is regulated to prevent a decrease in corrosion resistance due to the addition of these substances and the occurrence of defects during the production of steel ingots. While aiming at solid solution strengthening, by coexisting elements such as Cr and Ni with V, the formation of Cr carbides is suppressed,
By aiming at dispersion strengthening of V carbonitrides and balancing solid solution strengthening and precipitation strengthening, we can obtain austenitic stainless steel that has excellent corrosion resistance in acidic environments and has excellent yield strength. By tempering such stainless steel at a temperature range that is conventionally said to deteriorate corrosion resistance and mechanical properties, it is possible to further improve mechanical properties such as elongation and drawing while ensuring excellent corrosion resistance. It can be done. The present invention will be explained below with reference to Examples. Examples Table 1 Table 2 shows the mechanical properties when the steel shown in steel code A was heated to various temperatures and subjected to solution treatment. After solution treatment at a temperature of 1030℃ or higher, 450
It is shown that tempering at a temperature of 0.degree. C. particularly improves elongation. Table 2 also shows the results of stress corrosion cracking tests for each steel. It is recognized that corrosion resistance is similarly improved by solid solution treatment at a temperature of 1030°C or higher. Next, Table 3 shows the mechanical properties and stress corrosion cracking test results of steels with steel codes A, B, and C in Table 1, which were solution-treated, water-cooled, and then tempered to various temperatures. show. However, for steel A, 1040
After solution treatment at a temperature of ℃, water cooling and steel B and

【table】

【table】

[Table] For C, after solid solution treatment at a temperature of 1050°C, it was water-cooled and tempered at 450°C. According to the method of the invention,
It is clear that by rapid cooling after solution treatment and then tempering at a predetermined temperature range, the elongation of the steel is significantly improved while maintaining its excellent corrosion resistance. In the present invention, it is particularly important that C, Si, Ni, and N, which contribute to improving the yield strength, satisfy the following relationship in order to improve both the yield strength and corrosion resistance of the austenitic stainless steel. 37≦100C%+20Si%+Ni%+60N%≦49 When the value of the above formula is larger than 49, corrosion resistance deteriorates, while when it is smaller than 37, the balance between corrosion resistance and yield strength is lost, resulting in austenitic stainless steel having excellent properties. This is because steel tends to be difficult to obtain. In the corrosion test, stress was applied to the test piece by U-bending, and after immersing it in NACE solution (5% salt solution + 0.5% acetic acid + 1 atm hydrogen sulfide gas saturation) for one month,
In addition to measuring the corrosion rate, the presence or absence of cracking, pitting corrosion, and crevice corrosion was determined using a microscope (100x magnification).

Claims (1)

[Claims] 1% by weight: C 0.05-0.15%, Si 0.10-0.50%, Mn 0.5-5.0%, Cr 18-25%, Ni more than 6.0% up to 10%, Mo 2-4%, Austenitic stainless steel consisting of V 0.05-0.25%, N 0.15-0.45%, balance iron and unavoidable impurities is solution treated at a temperature of 1030-1100°C, then rapidly cooled, and then cooled to a temperature of 250-500°C. A method for producing highly corrosion-resistant, high-strength austenitic stainless steel, which is characterized by subjecting it to a tempering treatment. 2% by weight: C 0.05-0.15%, Si 0.10-0.50%, Mn 0.5-5.0%, Cr 18-25%, Ni over 6.0% up to 10%, Mo 2-4%, V 0.05-0.25%, and N 0.15-0.45%, at least one selected from Nb 0.05-0.50% and Ti 0.01-0.50%, the balance iron and unavoidable impurities. A method for producing a highly corrosion-resistant, high-strength austenitic stainless steel, which comprises solution treatment followed by rapid cooling and then tempering at a temperature of 250 to 500°C.