JP4606113B2 - Austenitic stainless steel with high proportional limit stress and manufacturing method - Google Patents

Description

  The present invention relates to an austenitic stainless steel material having an increased proportional limit stress suitable for various blades such as doctor blades, fuel injection devices, pressure vessels, railway vehicles, and car antennas.

  Stainless steel materials used for the above applications are required to have high “proportional limit stress” in addition to basic characteristics such as corrosion resistance, workability, and weldability. The proportional limit stress is the maximum stress at which the so-called Hooke's law (proportional relationship) holds between stress and strain. When a stress exceeding the proportional limit is applied to a metal material, the material undergoes plastic deformation and does not return to its original shape even when the stress is removed. High proportional limit stress means that the maximum stress that can be applied to a member using the material is high. Particularly, in the case of a thin plate member that is used while being subjected to elastic deformation by directly applying an external force, such as various blades, the performance of the member greatly depends on the height of the proportional limit stress of the material.

Conventionally, austenitic stainless steel represented by SUS304 is widely used in various applications because of its excellent properties such as corrosion resistance, workability, and weldability. However, SUS304, which is a general-purpose steel type, cannot be said to have a very high proportional limit stress, and cannot sufficiently cope with the above uses including blades.
As ferritic stainless steels, SUS436L and SUS444 with improved corrosion resistance are used for various purposes, including SUS430, a general-purpose steel grade, but in terms of workability and weldability, it does not reach SUS304, There was a limit.

  On the other hand, as means for increasing the proportional limit stress, theoretically, there are a method of increasing the elastic modulus of a material by depositing a precipitate having a high elastic modulus and a method of controlling a texture using a single crystal. However, it is quite difficult to realize this industrially. In addition, there is an example in which the proportional limit stress is increased by using SUS301, which is a metastable austenitic stainless steel, by utilizing a processing-induced martensitic transformation that contributes to strength improvement (Non-patent Document 1). This method is considered industrially feasible. However, in order to cause the processing-induced martensitic transformation, cold strong processing is necessary, and this is accompanied by a significant decrease in ductility.

  Patent Documents 1 and 2 describe high Mn austenitic stainless steel in which a part of Ni is replaced with Mn.

JP-A-49-130309 JP-A-3-2357 Journal of the Japan Institute of Metals, Vol. 33, No. 5, p.511-515

As described above, as a stainless steel material with high proportional limit stress suitable for various blades, fuel injection devices, pressure vessels, railway vehicles, car antennas, etc., ferritic steel grades are applied in terms of workability and weldability. Is disadvantageous and we want to adopt austenitic steel grades. However, it is strongly desired to develop an element that suppresses the addition of expensive elements as much as possible and has characteristics (corrosion resistance, workability, weldability, etc.) equivalent to SUS304.
In order to meet such demands, the present invention is to develop and provide an austenitic stainless steel material with improved proportional limit stress by a method that does not involve much cost increase compared to SUS304.

  As a result of various studies, the inventors have found that the proportional limit stress can be remarkably improved in the metastable austenitic stainless steel material having the property that the ε phase is preferentially induced over the α ′ phase by applying stress. . In order to induce the ε phase preferentially, i) reduction of stacking fault energy, ii) adjustment of austenite stability, iii) a composition having a relatively high austenite stability, It turned out that adjustment was necessary. And it was confirmed that such a steel material is realizable in high Mn type steel. The present invention has been completed based on such findings.

That is, the above-mentioned purpose is mass%, C: 0.050% or less, Si: 0.20-4%, Mn: 3.0-15%, P: 0.045% or less, S: 0.005% Hereinafter, Cr: 12 to 20%, Ni: 3.0 to 6.5%, N: 0.3% or less, preferably O: 0.02% or less, Mo: 2% as necessary Cu: 3.5% or less, Ti: 0.5% or less, Nb: 0.5% or less, Al: 0.2% or less, B: 0.015% or less, REM (rare earth element): 0.3% 2% or less, Y: 0.2% or less, Ca: 0.1% or less, and Mg: 0.1% or less, including one or more of the balance Fe and unavoidable impurities, Md defined by formula (N): less than -150 to-20, SFE is defined by the following equation (2): -20 had the following composition, the matrix is "1 to 35 volume percent of ε-phase + 0 10 volume% α 'phase + balance γ phase Is achieved by an austenitic stainless steel with high proportional limit stress of the "a is metallic tissue. Among “steel materials”, “steel plates (including steel strips)” are particularly suitable targets.
Md (N) = 580-520C-2Si-16Mn-16Cr-23Ni-26Cu-300N-10Mo (1)
SFE = 2.2Ni-1.1Cr-13Si-12Mn + 6Cu + 32 (2)

  Here, the content of each element expressed in mass% is substituted for the element symbol in the formula (1) and formula (2). The γ phase is a parent austenite phase, and the α ′ phase is a so-called α ′ martensite phase. The ε phase is a phase having a structure in which the stacking order ABCABC... of the (111) closest packing surface of the γ phase having the fcc structure is changed to, for example, ABAB... be able to. The ε phase is sometimes called “ε martensite phase”.

A steel material of a high austenite stability in which Md (N) is less than −150 to −20 is obtained by subjecting a cold-rolled annealed material having the predetermined composition to a sub-zero treatment in a temperature range of 0 ° C. to −196 ° C. , And can be produced by a method of controlling the amount of ε phase to 1 to 35% by volume.

  According to the present invention, it becomes possible to provide an austenitic steel material in which the proportional limit stress, which is a stress at which plastic deformation starts, is remarkably improved as compared with SUS304, as shown in Examples described later. For this reason, the steel material of this invention brings about an improvement in a performance and reliability in various member uses used while being accompanied by elastic deformation including various blades. In addition, it is possible to reduce the thickness of the member. Furthermore, the steel material of the present invention is reduced in Ni content by adding Mn, and the dependence on Ni, which is expensive and the raw material price largely fluctuates, is reduced, so that the steel material cost is more stable than SUS304. It also helps reduce the burden on users.

  In the present invention, the proportional limit stress is remarkably improved as compared with SUS304 by using Mn-containing austenitic stainless steel. The mechanism of the proportional limit stress improvement is characterized in that an ε phase is induced at the start of plastic deformation. As described above, the ε phase is a kind of stacking fault that occurs on the close-packed surface of the parent phase γ, but the transformation from γ phase to ε phase involves volume shrinkage. This is in contrast to the general martensitic transformation from the γ phase to the α ′ phase accompanied by volume expansion.

  When stress is applied to a steel material, generally, Hooke's law is established until plastic deformation starts, and so-called elastic deformation is exhibited. If this elastic deformation is caused by tensile stress, the elastic deformation is accompanied by volume expansion. As the tensile stress is further increased, plastic deformation starts at the proportional limit stress. At that time, in the deformation mode due to the introduction of dislocations and in the deformation mode due to the generation of α ′ processing-induced martensite phase, volumetric expansion is accompanied, so that the transition from elastic deformation to plastic deformation occurs relatively easily. In SUS304 and many other metal materials, plastic deformation usually starts in this pattern.

  However, the situation is different in the case of a metastable austenitic stainless steel having the property that the transformation of γ phase → ε phase is more likely to occur than the transformation of γ phase → α ′ phase when stress is applied. That is, when an ε phase is generated under a tensile stress, the transformation is a shrinkage transformation, so that an internal stress acts in a direction in which plastic deformation is less likely to occur. It is considered that the proportional limit stress increases due to the action. In the present invention, the proportional limit stress is improved by such a mechanism.

  It was found that the property that the transformation from the γ phase to the ε phase is likely to occur can be imparted by optimizing the “chemical composition” and the “metal structure”. Hereinafter, matters for specifying the present invention will be described. “%” Indicating the content of each element means “% by mass” unless otherwise specified.

[Constituent elements]
C is an austenite forming element and is extremely effective for strengthening the ε phase. In order to sufficiently bring out the effect, for example, a C content of 0.020% or more is preferably secured. However, when the C content increases, Cr carbide precipitates at the grain boundaries during cooling after annealing, which causes a decrease in intergranular corrosion resistance and fatigue characteristics. In view of the industrial avoid this heat treatment conditions and the cooling rate of the control, C content shall be the 0.05% or less.

Si exhibits the effect of reducing the stacking fault energy of the γ phase and is extremely effective in promoting the generation of the ε phase. Si is a ferrite-based element, and hardens the α ′ martensite phase and the ε phase, and also solidifies in the γ phase to harden it, thereby increasing the strength. These To obtain effective action is that to ensure the Si content of more than e.g. 0.20%. It is more desirable to ensure 0.50% or more. However, when Si is added excessively, it becomes easy to induce hot cracking, and various problems arise in production. Therefore, the Si content is 4% or less.

Mn is a very important element in the present invention. That is, it is extremely effective in reducing the stacking fault energy of the γ phase and promoting the generation of the ε phase as in the case of Si. Moreover, even if added in a large amount compared to Si, there are few harmful effects. Therefore, in the present invention, Mn is positively added. Mn is an austenite-forming element and has an action of suppressing the formation of δ ferrite at high temperatures. Mn content is, depending on the added amount of Si, the present invention shall be the 3.0% or more. More preferably , the content is 4.5% or more. However, excessive addition of Mn causes a problem due to a decrease in molten metal temperature in steelmaking, so the Mn content is 15% or less, preferably 13.5% or less.

P is an element having a large solid solution strengthening ability, but it may adversely affect toughness, so it should be 0.045% or less.
S is an element which is not preferable from the viewpoint of occurrence of edge cutting in hot rolling, and should be reduced as much as possible. In the present invention, S is 0.005% or less.

  Cr is an essential element for ensuring the corrosion resistance of stainless steel, and in the present invention, 12% or more is required. However, the addition of a large amount of Cr causes a decrease in toughness and a decrease in workability due to the formation of δ ferrite phase. Further, when the Cr content is increased, the amount of austenite forming elements (C, N, Ni, Mn, Cu, etc.) added to generate the ε phase increases, which not only increases the cost of the steel material, It leads to excessive stabilization of the γ phase at room temperature, making it difficult to obtain high strength. Therefore, the Cr content is 20% or less.

Ni is an austenite forming element and has an action of suppressing the progress of corrosion in terms of corrosion resistance. In the present invention, the Ni content is 3.0% or more . On the other hand, Ni is an expensive element, and the price fluctuation due to the social situation is also increasing recently. For this reason, a large amount of Ni is not preferable because it causes an increase in cost and promotes destabilization of the steel price. In the present invention, the Ni content may be adjusted in the range of 6.5% or less, preferably 6.0% or less, in order to generate the ε phase appropriately by cooling or processing induction.

  N, like C, is an austenite-forming element and is extremely effective for strengthening the ε phase. For that purpose, for example, 0.003% or more, preferably 0.05% or more of N may be contained. Addition of a large amount of Mn increases the amount of solute N in the γ phase. However, considering problems such as blowhole generation and surface flaws during steelmaking, the allowable N content is approximately 0.3%. .

  Mo is an effective element that improves the corrosion resistance together with Cr, and can be added as necessary. However, if a large amount of Mo is added, δ ferrite is formed at a high temperature. Therefore, when Mo is added, it is preferable to perform the addition within a range of 2% or less.

  Cu is an austenite forming element, but has an action of suppressing the formation of the ε phase. For this reason, when adding Cu, it is desirable to carry out in 3.5% or less of range.

  Ti and Nb have the effect of suppressing grain growth during annealing and can be added singly or in combination as necessary. However, when added in a large amount, the surface flaws of the steelmaking slab become obvious. The above problems will arise. For this reason, when adding Ti or Nb, it is preferable to carry out within a range of 0.5% or less.

Al is an element effective for deoxidation at the time of steelmaking, and has an effect of drastically reducing A ₂ inclusions that adversely affect press formability. However, excessive addition of Al causes adverse effects such as an increase in surface defects. Therefore, when adding Al, it is desirable that the content is not more than 0.2%.

  B is an element effective for preventing the occurrence of edge cracks in the hot-rolled steel strip, which is considered to be caused by the difference in deformation resistance between the δ ferrite phase and the γ phase in the hot rolling temperature range. However, excessive addition of B deteriorates hot workability due to the formation of a low-melting boride, so when adding B, it is desirable to add within a range of 0.015% or less.

  REM (rare earth element), Y, Ca, and Mg are effective elements for improving hot workability and high-temperature oxidation resistance. Therefore, one or more of these elements should be added as necessary. Can do. The upper limit of the amount added is preferably 0.2% for REM and Y, and 0.1% for Ca and Mg.

  O forms oxide-based nonmetallic inclusions to lower the cleanliness of the steel, and may adversely affect press formability and bending workability. It is desirable to reduce the O content to 0.02% or less.

[Md (N) value]
Md (N) defined by the following formula (1) is an index of austenite stability.
Md (N) = 580-520C-2Si-16Mn-16Cr-23Ni-26Cu-300N-10Mo (1)
In the present invention, Md (N) is shall be the subject of austenitic steels in the range of less than -150 -20.

In steels with Md (N) lower than −150, it is difficult to generate a cooled ε phase even if sub-zero treatment is performed in the temperature range up to −196 ° C. (liquid nitrogen temperature). As will be described later, in a steel of which Md (N) is lower than −20, preliminarily generating an ε phase of 1% by volume or more preferentially causes the transformation from γ phase to ε phase at the start of plastic deformation. This is extremely advantageous. If Md (N) is lower than −150, a residual γ single-phase structure is basically obtained even when subzero treatment is performed. Therefore, the transformation of γ phase → α ′ phase occurs at the start of plastic deformation, and the proportional limit stress Improvement cannot be achieved .

On the other hand, when steel with Md (N) in the range of −150 or higher and lower than −20 (steel having a relatively high austenite stability) is subjected to subzero treatment in a temperature range up to −196 ° C., SFE described later. Combined with the optimization of the value, the ε phase can be generated. The presence of this ε phase brings about the property of preferentially causing the transformation of γ phase → ε phase, and contributes to the improvement of the proportional limit stress .

[SFE value]
The SFE value defined by the following equation (2) is an index of stacking fault energy in the γ phase.
SFE = 2.2Ni-1.1Cr-13Si-12Mn + 6Cu + 32 (2)
The stacking fault energy can be evaluated by measuring the amount of ε phase (including stacking faults) generated in the γ matrix in a sample to which a certain amount of strain is applied. Inventors evaluated the stacking fault energy by the following methods about the metastable austenitic stainless steel which has various component compositions.

  A cold rolled annealed steel sheet having a thickness of 1.0 mm is made by a process including hot rolling → annealing → cold rolling → annealing by a normal manufacturing method, and a JIS 13B tensile test piece parallel to the rolling direction is collected. A sample to which a tensile strain of true strain of 0.2 is applied at normal temperature is prepared. A cross section perpendicular to the tensile direction in the parallel part of this sample is observed with a transmission electron microscope, and 20 or more observation fields are randomly selected at a magnification of 30000 times. Whether or not an ε phase (including stacking faults) is observed in the field of view. Determine whether. Then, the ratio of visual fields in which the ε phase (including stacking faults) is recognized (hereinafter referred to as “stacking fault generation frequency”) is obtained out of the total number of viewing fields, and the stacking fault generation frequency is low (that is, stacking faults are not easily generated). It is evaluated that the stacking fault energy is high.

  As a result of such detailed investigation, it was found that the stacking fault energy of metastable austenitic stainless steel can be intentionally controlled by the chemical composition. Equation (2) gives the index. That is, Si, Mn, and Cr decrease the stacking fault energy, and Cu and Ni increase the stacking fault energy.

In the present invention, steel whose composition has been adjusted so that the SFE value is −20 or less is used. The SFE value by -20 or less, the Md (N) value is less than -150-20 steel as possible out to generate the proper amount of ε phase by subzero treatment at a temperature ranging -196 ° C. . When stress is applied to the steel material having this microstructure, the proportional limit stress is improved due to the property that the transformation of γ phase → ε phase preferentially occurs.

[Metal structure]
In the present invention, when a stress is applied to the steel material, the steel material is previously imparted with a property such that the transformation of γ phase → ε phase preferentially occurs over the transformation of γ phase → α ′ phase. For that purpose, in addition to the above-described chemical composition adjustment, it is necessary to adjust the metal structure in accordance with the Md (N) value.

  When Md (N) is less than −150 to −20, neither the ε phase nor the α ′ phase is generated in the cooling process after annealing, and the matrix is “γ single phase”. In this case, however, the transformation of γ phase → ε phase does not always preferentially occur even in the γ single phase. As a result of detailed studies, the inventors have found that the transformation from γ phase to ε phase occurs preferentially at the start of plastic deformation if the ε phase is present in advance. That is, if the SFE value is appropriate, sub-zero treatment is performed to obtain a metal structure of “1 to 35% by volume of ε phase + 0 to 10% by volume of α ′ phase + remainder γ phase”. It is possible to impart the property that this transformation occurs preferentially. In this case, the ε phase is more preferably 5% by volume or more. The α ′ phase may be 0% (not present).

[Proportional critical stress]
In applications such as various types of blades such as doctor blades, fuel injection devices, pressure vessels, railway vehicles, and car antennas, the proportional limit stress measured by the above-mentioned method is required to obtain a great effect by improving the performance of the member or reducing the thickness. desirably it is 230N / mm ² or more, very preferably a 250 N / mm ² or more. Such a high proportional limit stress can be realized by adjusting the chemical composition and the metal structure.

[Production method]
The austenitic stainless steel material having the above chemical composition and metal structure can be manufactured by using a normal manufacturing process according to SUS304. Specifically, steel having a predetermined composition is melted and continuously cast, and a continuous cast slab is hot-rolled at, for example, an extraction temperature of 1200 to 1230 ° C, a finish rolling temperature of 940 to 1000 ° C, and a winding temperature of 700 to 800 ° C. , Hot-rolled sheet annealing and pickling, cold rolling, and finishing annealing under conditions of 1050 to 1150 ° C. × soaking 0.5 to 2 minutes, water cooling or air cooling can be employed .

When M d (N) is less than −150 to −20, by subjecting this cold-rolled annealed material to a sub-zero treatment for holding for about 10 to 60 minutes in a refrigerant in a temperature range of 0 ° C. to −196 ° C., The desired structure can be controlled and a significant improvement in proportional limit stress is achieved.

  Steel having the chemical composition shown in Table 1 was melted, and hot-rolled sheets having a thickness of 3 mm were produced by hot rolling. Thereafter, it was cold-rolled to a thickness of 1.0 mm, subjected to finish annealing under conditions of 1050 to 1100 ° C. × soaking for 1 minute and air cooling, and pickled to produce a cold-rolled annealed steel plate. Steel S is SUS304.

Some of the obtained cold-rolled annealed steel sheets were further subjected to sub-zero treatment (those whose temperature is described in Table 2). The sub-zero treatment was performed by a method of immersing in a refrigerant and holding it for about 30 minutes, then taking it out and allowing the temperature to rise naturally to room temperature. The proportional limit stress was measured for the obtained specimen (cold-rolled annealed steel sheet or subzero-treated steel sheet). The test piece was a JIS No. 13B tensile test piece parallel to the rolling direction. A tensile test was performed at a strain rate of 4 × 10 ⁻⁴ , and the stress (0.01%) when plastically deformed from the elastic deformation region. % Offset stress) was determined and used as the proportional limit stress. The results are shown in Table 2.

  As can be seen from Tables 1 and 2, the examples of the present invention satisfying the chemical composition and metal structure defined in the present invention showed a marked improvement in proportional limit stress as compared with No. 15 SUS304.

On the other hand, Comparative Example No. 11 satisfies the chemical composition of the present invention, but when Md (N) is lower than −20, the subzero treatment was not performed, so that a γ single phase structure was formed, and the proportional limit stress was 230 N / It did not improve in mm ² or more. In No. 12, since the Cr content was high, a δ ferrite phase was present, and the proportional limit stress was remarkably improved, but the total elongation was lowered and sufficient workability could not be ensured. Since Nos. 13 and 15 had an SFE value higher than −20, an α ′ phase was induced at the start of plastic deformation, and the proportional limit stress was low. In No. 14, since Md (N) was too high, an α ′ phase was generated in the state of the cold-rolled annealed steel sheet, and the total elongation was low and the workability was poor. In addition, the α ′ phase was further induced at the start of plastic deformation, and the proportional limit stress was low.

Claims (5)

In mass%, C: 0.050% or less, Si: 0.20-4%, Mn: 3.0-15%, P: 0.045% or less, S: 0.005% or less, Cr: 12-12 20%, Ni: 3.0 to 6.5%, N: 0.3% or less, balance Fe and inevitable impurities, Md (N) defined by the following formula (1): −150 to less than −20, SFE defined by formula (2) below: Proportional limit having a composition of −20 or less and having a metal structure of “1 to 35 volume% ε phase + 0 to 10 volume% α ′ phase + balance γ phase” Austenitic stainless steel with high stress.
Md (N) = 580-520C-2Si-16Mn-16Cr-23Ni-26Cu-300N-10Mo (1)
SFE = 2.2Ni-1.1Cr-13Si-12Mn + 6Cu + 32 (2) In mass%, C: 0.050% or less, Si: 0.20-4%, Mn: 3.0-15%, P: 0.045% or less, S: 0.005% or less, Cr: 12-12 20%, Ni: 3.0 to 6.5%, N: 0.3% or less, Mo: 2% or less, Cu: 3.5% or less, Ti: 0.5% or less, Nb: 0 0.5% or less, Al: 0.2% or less, B: 0.015% or less, REM: 0.2% or less, Y: 0.2% or less, Ca: 0.1% or less, and Mg: 0.1 % Fe or inevitable impurities, Md (N) defined by the following formula (1): −150 to less than −20, SFE defined by the following formula (2) : An austenitic stainless steel material having a composition of -20 or less and having a metal structure of “1 to 35 volume% ε phase + 0 to 10 volume% α ′ phase + remainder γ phase” and having a high proportional limit stress.
Md (N) = 580-520C-2Si-16Mn-16Cr-23Ni-26Cu-300N-10Mo (1)
SFE = 2.2Ni-1.1Cr-13Si-12Mn + 6Cu + 32 (2) The austenitic stainless steel material having high proportional limit stress according to claim 1 or 2 , wherein O: 0.02% or less. Proportional limit stress is defined by 0.01% offset stress when subjected to a tensile test in the rolling direction at a strain rate of 4 × 10 ^-4 is according to any one of claims 1 to 3 in 230N / mm ² or more Austenitic stainless steel with high proportional limit stress. The austenitic stainless steel with high proportional limit stress according to claim 1 or 2 , wherein the amount of ε phase is controlled to 1 to 35% by volume by subjecting the cold-rolled annealed material to sub-zero treatment in a temperature range of 0 ° C to -196 ° C. Steel manufacturing method.