JP2016044332A - Stainless steel for low temperature application - Google Patents

Abstract

PROBLEM TO BE SOLVED: To industrially stably supply an inexpensive and stable austenite (A)-based stainless steel applied to products such a steel sheet, a steel strip and a steel tube used at low temperature of room temperature or lower.SOLUTION: There is provided a stainless steel for low temperature application containing, by mass%, C:0.01% to 0.20%, Si:1.0% or less, Mn:3.0% or less, Cr:10.0% to 20.0%, Ni:5.0% to 10.0%, N:0.01% to 0.30% and the balance Fe with impurities and having the contents of above described components satisfying following formula, structure of an austenite single phase, average crystal particle diameter of 10 μm or less and generating no martensite phase when cooling to room temperature or lower. -273>1032-42Cr-61Ni-33Mn-28Si-1000C-2860N.SELECTED DRAWING: None

Description

  The present invention relates to an inexpensive and stable austenitic (A) stainless steel used at a low temperature below room temperature, and the steel is intended for products such as steel plates, steel strips, steel pipes, and more particularly for superconducting magnets. Ideal for structural materials.

  Conventional applications of stainless steel used at low temperatures below room temperature, especially at cryogenic temperatures, include structural materials for superconducting magnets used at liquid helium and liquid nitrogen temperatures, liquefied natural gas (LNG) piping, tanks, Examples include rocket containers that use liquid hydrogen as fuel.

  Recently, due to environmental problems, the so-called “hydrogen society” represented by the products of stationary fuel cells and fuel cell vehicles has been studied, and its application to infrastructure such as liquefied hydrogen bases and pipelines has been expanded. Expected.

  The properties generally required for structural materials used at cryogenic temperatures are that they do not cause brittle fracture for safety reasons, high strength, and are non-magnetic when used as structural materials for superconducting magnets. Is mentioned.

  A-series stainless steel is generally applied to cryogenic applications as described above in order to maintain ductility up to extremely low temperatures. On the other hand, from the point of high strength required for structural materials, addition of N is known as a generally effective means, and SUS304LN and SUS316LN are defined in the JIS (JIS-G-4315) standard. (See Patent Documents 1 to 4).

  On the other hand, A-based stainless steel is classified as a rare metal as an A-stabilizing element and contains a large amount of Ni, which is an expensive alloy element, so that there is a problem that the material is valuable and expensive. However, a simple decrease in Ni lowers the stability of A, generates a second phase including a processing-induced martensite (M) transformation, etc., and particularly during cooling to a very low temperature, M phase or ferrite ( F) A phase is generated.

  Since these phases are magnetized, there is a problem that they cannot be used for superconducting magnet structural materials. In addition, there is a problem that low-temperature embrittlement of materials becomes a concern. In the present invention, the M phase refers to an α ′ phase having a body-centered tetragonal lattice structure close to the F phase.

  For this reason, the addition of C and N, which are inexpensive and easily available, and are the most powerful A stable elements, is generally studied. In particular, N is a promising element that is contained in a large amount in the atmosphere and has a small environmental load (see Patent Documents 5 to 8). However, most of the studies on the addition of N are for the processing-induced M transformation at room temperature, and the thermal stability in cooling to an extremely low temperature has not been sufficiently studied.

For example, Ms, which is an index of thermal A stability in cooling, and Md30, which is an index of A stability against processing-induced transformation, is represented by Non-Patent Document 1 by the following formula (a) or (b). Yes. In both equations, the coefficients of N and C are the same. This is the same value because N and C contributions to Ms will be almost the same, but details of the actual contribution to Ms are not necessarily clarified.
Ms (° C.) = 1032-42Cr-61Ni-33Mn-28Si
-1667 (C + N) (a)
Md30 (° C) = 413-462 (C + N) -9.2Si-8.1Mn
-13.7Cr-9.5Ni-18.5Mo (b)

  The above formulas (a) and (b) are empirical formulas in which the A stability is calculated from the components. Ms is the temperature (° C.) at which the M transformation starts upon cooling, and Md30 is a tensile strength of 0.3. This is the temperature (° C.) at which 50% M transformation occurs when true strain is applied. The amount of each element is mass%.

  Furthermore, as a factor for which high strength is expected, refinement of crystal grains (fine graining) can be mentioned. With regard to this as well, studies are being conducted for the purpose of improving various room temperature characteristics. For example, Patent Documents 9 and 10 disclose stainless steel for gaskets that has been improved in strength without significantly reducing the workability by refining crystal grains like other strengthening factors. Stainless steel for etching in which a chemically corroded portion becomes smooth is disclosed.

  JIS (G-4315) standard SUS301, SUS301L, SUS304, SUS304L, and SUS304LN equivalent stainless steels described in these are classified as metastable A-based stainless steels and cause processing-induced M transformation. Excellent relationship.

  Furthermore, it is known that by subjecting the M phase that has undergone processing-induced transformation to a heat treatment at a relatively low temperature, it undergoes reverse transformation to the A parent phase, and a fine grain structure with a crystal grain size of several μm or less can be obtained through these transformations. It has been. However, with regard to this fine graining, the object of study is mostly the processing-induced M transformation at room temperature, and the thermal stability during cooling to cryogenic temperatures has not been sufficiently studied.

  In the description of Non-Patent Document 2, the influence of the crystal grain size on Md30, which is an index of A stability against the processing-induced transformation, is only taken into account. In other words, in the field of cryogenic applications below room temperature, the effects of C and N on the thermal A stability, which is also greatly related to the characteristics, and the influence of the crystal grain size have not been sufficiently studied. It is.

JP 60-009862 A JP 60-013063 A JP-A-61-270356 Japanese Patent Laid-Open No. 02-097649 Japanese Patent No. 5091732 Japanese Patent No. 5091733 JP 2014-019925 A Japanese Patent No. 4327030 Japanese Patent No. 3068861 Japanese Patent No. 4019630 Japanese Patent No. 3562492 International Publication No. 2014/030607

Stainless Steel Handbook (3rd Edition, Stainless Steel Association), p. 114 Stainless Steel Handbook (3rd Edition, Stainless Steel Association), p. 115

  In view of the current situation in the field of cryogenic use below room temperature, the present invention targets products such as steel sheets, steel strips, steel pipes, etc., used at low temperatures below room temperature, and is inexpensive and stable austenite ( A) An object of the present invention is to supply industrially stable stainless steel, and to provide stainless steel for low-temperature applications that solves the problem.

  The present inventors have studied in detail the effects of C and N on the thermal A stability of A-based stainless steel and the influence of crystal grain refinement (fine graining), and clarified their effects. . As a result of the study, it was found that when cooling to an extremely low temperature, even when stress in the elastic deformation region was applied, the processing-induced M transformation was suppressed and the A phase was stabilized.

  Based on the above knowledge, the present invention has been continually researched and developed, and after undergoing trial production with small ingots with components adjusted and trial production of actual equipment, it is an inexpensive and stable A-based stainless steel used at low temperatures below room temperature, especially at extremely low temperatures. It was made with the aim of putting steel into practical use.

That is, it has been confirmed that the influence of C and N on the thermal A stability is significantly larger than that of other alloy elements, and further, it is clear that the influence of N shows about three times that of C. I made it. In particular, the following formula (described later) was obtained as a formula indicating the degree of A stabilization by the effective utilization of N.
-273> 1032-42Cr-61Ni-33Mn-28Si
-1000C-2860N

  On the other hand, atomization also improves A stability and is added to the effect of addition of C and N. The effect becomes clear when the particle diameter is 10 μm or less.

  As a result, when the present inventors have cooled the material satisfying the above components and the crystal grain size to an extremely low temperature, the M transformation is suppressed even when stress in the elastic deformation region is applied. It was confirmed that the A stability of the material was improved and the safety was also improved.

  The present invention has been made based on these findings, and the gist thereof is as follows.

(1) In mass%,
C: 0.01% or more, 0.20% or less,
Si: 1.0% or less,
Mn: 3.0% or less,
Cr: 10.0% or more, 20.0% or less,
Ni: 5.0% or more and 10.0% or less,
N: 0.01% or more and 0.30% or less, the balance being Fe and inevitable impurities, and the content of the above element satisfies the following formula (1),
A stainless steel for low-temperature applications, characterized in that the structure is an austenite single-phase structure, the average crystal grain size is 10 μm or less, and no martensite phase is formed upon cooling to room temperature or lower.
-273> 1032-42Cr-61Ni-33Mn-28Si
-1000C-2860N (1)

  (2) The composition according to (1), further comprising at least one of Nb: 0.5% or less, Ti: 0.5% or less, and V: 0.5% or less in mass%. Stainless steel for low temperature applications.

  (3) The stainless steel for low-temperature applications according to (1) or (2), wherein a martensite phase is not generated when a stress in an elastic deformation region is further applied upon cooling to the room temperature or lower.

  ADVANTAGE OF THE INVENTION According to this invention, the cheap and stable austenite (A) type stainless steel applied to products, such as a steel plate, a steel strip, and a steel pipe used at low temperature below room temperature, can be supplied industrially stably. .

The stainless steel for low-temperature applications of the present invention (hereinafter sometimes referred to as “the present invention steel”)
% By mass
C: 0.01% or more, 0.20% or less,
Si: 1.0% or less,
Mn: 3.0% or less,
Cr: 10.0% or more, 20.0% or less,
Ni: 5.0% or more and 10.0% or less,
N: 0.01% or more and 0.30% or less, the balance being Fe and inevitable impurities, and the content of the above element satisfies the following formula (1),
The structure is an austenite single phase structure, the average crystal grain size is 10 μm or less, and the martensite phase is not generated upon cooling to room temperature or lower.
-273> 1032-42Cr-61Ni-33Mn-28Si
-1000C-2860N (1)

  First, the reasons for limiting the component composition of the steel of the present invention will be described. Hereinafter, “%” means mass%.

C: 0.01% or more, 0.20% or less,
C, together with N (described later), is a strong γ-stabilizing element and a solid solution strengthening element. In order to obtain the effect of addition, 0.01% or more is added. Preferably it is 0.02% or more. However, excessive addition leads to precipitation of a large amount of carbides, and neither the required strength nor A stability can be obtained, so the upper limit is made 0.20%. Preferably it is 0.18% or less.

Si: 1.0% or less Si is an element that functions as a deoxidizer during melting and is an F stabilizing element. However, if added excessively, coarse inclusions are generated, workability deteriorates, and phase A becomes unstable, so the upper limit is made 1.0%. Preferably it is 0.9% or less. The lower limit is not particularly defined, but 0.05% or more is preferable in order to reliably obtain the deoxidation effect.

Mn: 3.0% or less Mn is a relatively inexpensive and effective A-stabilized alloy element. However, if added excessively, coarse inclusions are generated and workability deteriorates, so the upper limit is made 3.0%. Preferably it is 2.6% or less. The lower limit is not particularly defined, but is preferably 0.1% or more from the viewpoint of reliable stabilization of the A phase.

Cr: 10.0% or more, 20.0% or less Cr is a basic element of stainless steel, and is an element for obtaining effective corrosion resistance. To obtain the effect of addition, 10.0% or more is added. Preferably it is 10.5% or more. However, Cr is an F stabilizing element, and if added excessively, the A phase becomes unstable, and the possibility of forming a compound with C and N increases, so the upper limit is made 20.0%. Preferably it is 19.4% or less.

Ni: 5.0% to 10.0% Ni is the most powerful A-stabilized alloy element. Including the addition of C and N, 5.0% or more is added to stabilize the A phase to room temperature. Preferably it is 5.4% or more. However, as described above, since it is an expensive and rare alloy element and it is desirable to reduce it as much as possible, the upper limit is made 10.0%. Preferably it is 9.8% or less.

N: 0.01% to 0.30% N is the most powerful γ-stabilizing element and an effective interstitial solid solution strengthening element. To obtain the effect of addition, the content is made 0.01% or more. Preferably it is 0.02% or more. However, excessive addition causes precipitation of nitrides, and neither the required strength nor A stability can be obtained, so the upper limit is made 0.30%. Preferably it is 0.28% or less.

  In the steel of the present invention, it is further necessary that the content of the above element satisfies the following formula (1). The following formula (1) is a relational expression obtained by examining the influence of each element on the A stability during cooling to room temperature or lower.

  When the contents of C, Si, Mn, Cr, Ni, and N satisfy the composition range of each element and the following formula (1), the A phase may exist stably up to 0 K (about −273 ° C.). found. Furthermore, as a result of examining the difference in the A phase stabilization effect for C and N, which are particularly effective for stabilizing the A phase, N has an effect about three times that of C (in the following formula (1), N Was found to be 2860 and C was 1000).

  Based on these findings, it has been found that there is a component composition that satisfies -273> Ms and the conventional formula (a) exceeds -273 ((a)>-273> (1)). .

According to this component composition, for example, the addition amount of Ni, which is an expensive and rare alloy element, can be suppressed as much as possible, and inexpensive and stable A-based stainless steel can be industrially stably supplied.
-273> 1032-42Cr-61Ni-33Mn-28Si
-1000C-2860N (1)

  In general, use at a low temperature of room temperature or lower means liquid helium temperature (about −269 ° C., about 4K), liquid hydrogen temperature (about −253 ° C., about 20K), liquefied natural gas ( It refers to use at about -162 ° C, about 111K) and liquid nitrogen temperature (about -196 ° C, about -77K). In the steel of the present invention, the low temperature below room temperature refers to a low temperature below the liquid nitrogen temperature.

  In addition to the above components, the steel of the present invention may contain at least one of Nb: 0.5% or less, Ti: 0.5% or less, and V: 0.5% or less.

Nb: 0.5% or less, Ti: 0.5% or less, V: 0.5% or less Nb, Ti, and V are compounds that bind to C and N and suppress the growth of crystal grains by a pinning effect. It is an element to be formed. However, if any element exceeds 0.5%, a coarse compound is formed, and there is a high possibility that the formation of the A phase becomes unstable, the workability is deteriorated, and the coarse compound is a starting point of destruction. Therefore, the upper limit of each is 0.5%. Preferably, both are 0.4% or less.

  Although a minimum is not specifically limited, 0.01% or more of Nb, Ti, and V is preferable at the point which ensures an addition effect reliably.

  Furthermore, in the steel of the present invention, an austenite single phase structure is used, and the average crystal grain size of the structure is set to 10 μm or less. This is because the refinement of the structure contributes to the improvement of the stability of the thermal A phase simultaneously with the improvement of the strength of the steel, and is added to the effect of the above formula (1). As a result, the stability of the A phase is remarkably improved, and the A phase is stabilized against the application of stress within the elastic deformation region.

  If the average crystal grain size exceeds 10 μm, the strength of the steel and the A phase stability are not sufficiently improved, so the average crystal grain size is set to 10 μm or less. Preferably it is 8 micrometers or less.

  The average crystal grain size is preferably as small as possible, but the lower limit cannot be clearly defined because it largely depends on rolling conditions and / or heat treatment conditions.

  Next, examples of the present invention will be described. The conditions in the examples are one example of conditions used for confirming the feasibility and effects of the present invention, and the present invention is based on this one example of conditions. It is not limited. The present invention can adopt various conditions as long as the object of the present invention is achieved without departing from the gist of the present invention.

Example 1
Table 1 shows the composition of the test steel (small ingot or actual ingot) hot rolled into a hot rolled sheet with a thickness of around 4 mm. After heat treatment, the sheet was cold rolled with a processing rate of 90%. A cold-rolled sheet (prototype material) having a thickness of 0.4 mm was used. This cold-rolled sheet was subjected to a final heat treatment held at 800 to 1100 ° C. for 1 minute for the purpose of adjusting the crystal grain size.

Next, the characteristics of the prototype material were investigated. For property studies, cooling was performed to liquid helium temperature. 0.2% proof stress at room temperature of the inventive example No.A3 is 312N / mm ^2, imparting tension during cooling, was loaded the approximately 30% of 100 N / mm ² (constant).

  The application of tension is considered to promote the M transformation during cooling. In the actual use, in the case of a superconducting magnet, such as the weight of a jig including itself, the influence of the magnetic force in fixing is taken into consideration.

  Regarding the investigation method, the structure was observed by using an optical microscope after embedding, polishing, and corroding a vertical section in the rolling direction, and a photograph was taken at an average site. Next, the number of crystal grains contained in the photograph was measured, and the average crystal grain size per one was measured. The particle size (radius) was calculated as the equivalent circle diameter. The amount of A phase was calculated by measuring the proportion of the amount of M phase or F phase generated by the change in magnetic permeability using a vibrating sample magnetometer (VSM) and calculating the difference in the proportion.

  Table 2 shows the structure of the specimen and the amount of A phase before and after cooling to a cryogenic temperature.

  Invention Example No. It can be confirmed that A1 to H2 maintain the A single phase structure before and after cooling and exhibit high A stability. Invention Example No. B1 and Comparative Example No. B2, Invention Example No. F1 and Comparative Example No. When F4 is compared, it can be confirmed that the stability of A is improved by the fine graining.

  Invention Example No. From the results of C1 and D1, it can be confirmed that the A single-phase structure is maintained after cooling at a crystal grain size of 10 μm or less. In the invention examples, it can be confirmed that the A single-phase structure is maintained even when a tension is applied during cooling.

  Furthermore, Invention Example No. 1 satisfying M (a)>-273> M (1). In A1, A2 and A3, Ni can be lowered to 5.45%. In F1, F2 and F3, Ni could be lowered to 5.52%.

  On the other hand, Comparative Example No. outside the range of the component composition of the present invention. In I1 to N2, none of the A single-phase structures can be maintained after cooling. In particular, Comparative Example No. 1 having the highest M value corresponding to the equation (1) and the lowest A stability. I1 cannot maintain the A single phase structure even in the stage before cooling. Comparative Examples No. J1 and J2, and Comparative Example No. From the results of M2 and M3, it is considered that the amount of phase A after cooling is decreased by applying tension and promotes the M transformation.

  As described above, according to the present invention, an inexpensive and stable austenite (A) stainless steel applied to products such as steel sheets, steel strips, and steel pipes used at a low temperature below room temperature is industrially stable. Can be supplied. Therefore, the present invention has high applicability in the stainless steel manufacturing / utilizing industry.

Claims (3)

% By mass
C: 0.01% or more, 0.20% or less,
Si: 1.0% or less,
Mn: 3.0% or less,
Cr: 10.0% or more, 20.0% or less,
Ni: 5.0% or more and 10.0% or less,
N: 0.01% or more and 0.30% or less, the balance being Fe and inevitable impurities, and the content of the above element satisfies the following formula (1),
A stainless steel for low-temperature applications, characterized in that the structure is an austenite single-phase structure, the average crystal grain size is 10 μm or less, and no martensite phase is formed upon cooling to room temperature or lower.
-273> 1032-42Cr-61Ni-33Mn-28Si
-1000C-2860N (1)   Furthermore, it contains at least 1 sort (s) of Nb: 0.5% or less, Ti: 0.5% or less, and V: 0.5% or less by the mass%, The low temperature use object of Claim 1 characterized by the above-mentioned. Stainless steel.   The stainless steel for low-temperature applications according to claim 1 or 2, wherein a martensite phase is not generated when a stress in an elastic deformation region is further applied upon cooling to the room temperature or lower.