JP5014915B2 - Ni-saving austenitic stainless steel - Google Patents

Description

  The present invention suppresses Ni, which is essential for austenitic stainless steel, to the minimum necessary content, and does not impair the surface properties, and is excellent in workability, time crack resistance, corrosion resistance, and stress corrosion crack resistance. Related to stainless steel.

  Austenitic stainless steel represented by SUS304 has been widely used for various purposes such as Western tableware, pans, kitchenware, building materials, and home appliances because of its excellent workability and corrosion resistance. In addition, these applications often require not only processability and corrosion resistance but also design properties, and the western white color unique to stainless steel is indispensable as an industrial product material in terms of interior and exterior.

  However, due to the recent soaring Ni raw material, SUS301 containing more than 6% of Ni, SUS304 containing 8% or more, etc. have been used in some cases that cannot be applied in terms of cost. In order to cope with this, in recent years, steels based on 200 series stainless steel (Patent Documents 1 to 6) are being provided as substitutes for so-called 300 series stainless steel. These contain a large amount of Mn as an austenite forming element in place of Ni (mostly about 3% by mass or more) to save the Ni content. In addition, there is an example in which Ni is saved without containing much Mn (Patent Documents 7 and 8).

Japanese Patent Publication No. 2-29048 JP-A-6-271995 JP 2006-219751 A JP 2006-219743 A JP 2006-111932 A Japanese Patent Publication No. 6-86645 Japanese Patent Publication No. 60-33186 JP 2005-154890 A

  When manufacturing high-Mn austenitic stainless steel with a Mn content exceeding 2.5 mass%, harmful Mn oxide fine particles are generated during steelmaking and refining, and measures are required from the viewpoint of environmental conservation. There is a case. In addition, when stainless steel is recycled, it has been conventionally treated as 300 series scrap if it is non-magnetic. However, high-Mn steel is also non-magnetic, so there are few valuable scraps and Ni containing a large amount of Ni. It is difficult to separate the scrap containing a large amount of Mn, and there is concern that the scrap market may be confused. Furthermore, since the Mn content is high, the surface quality is deteriorated, and it is likely to cause a load increase in annealing pickling and a coloring trouble in bright annealing. In this case, despite the reduction of Ni, the effect is offset in the total cost due to the decrease in productivity.

  On the other hand, with the Ni-reduced steel technology that suppresses Mn as in Patent Documents 7 and 8, as a result, cost is reduced at the expense of either corrosion resistance or workability, which is an excellent feature of 300 series stainless steel. Yes. Specifically, in the steel of Patent Document 7, the austenite phase is excessively unstable in drawing and the like, and time cracking is likely to occur after processing. For this reason, sufficient processing degree cannot be earned. The steel of Patent Document 8 is disadvantageous in terms of corrosion resistance because of its low Cr content, and according to the study by the present inventors, it is disadvantageous in obtaining sufficient ductility because of the small amount of dissolved C and N. I found out.

  The present invention is excellent in recyclability, has no problem of productivity reduction due to surface quality, and has a Ni-saving type that has good workability, time crack resistance, corrosion resistance, and stress corrosion crack resistance in terms of material characteristics. The aim is to provide austenitic stainless steel.

The above-mentioned problems are in mass%, C: exceeding 0.05% and satisfying the following formula (3), Si: 1% or less, Mn: 0.5-2.5%, Ni: 3-6%, Cr : More than 16 to 25% or less, Cu: 0.8 to 4%, N: a range satisfying the following formula (3), balance Fe and unavoidable impurities, defined by the following formula (1), austenite stability index Ni-saving austenite system with excellent workability, corrosion resistance, stress corrosion cracking resistance, and surface properties having a composition in which Md30 is −50 to 10 and the stacking fault difficulty index SFE defined by the following formula (2) is 5 or more Achieved with stainless steel.
Md30 = 551-462 (C + N) -9.2Si-8.1Mn-29 (Ni + Cu) -13.7Cr (1)
SFE = 2.2Ni + 6Cu-1.1Cr-13Si-1.2Mn + 32 (2)
0.15 ≦ C + N ≦ 0.3 (3)

  Here, the value of the content of each element expressed in mass% is substituted for the element symbol in the above formulas (1) to (3).

  According to the present invention, while suppressing the Mn content to 2.5% by mass or less, it is excellent in recyclability, has no problem of productivity reduction due to surface quality, and has good corrosion resistance, workability, and stress corrosion cracking resistance. A Ni-saving austenitic stainless steel having the characteristics is provided. This steel can be applied to various applications as a substitute for 300 series stainless steel. Therefore, the present invention can cope with the soaring Ni raw material in terms of cost and quality.

  The present inventors diligently studied to achieve the above-mentioned problems in the austenitic stainless steel in which the Ni content is suppressed to 6% or less, and have obtained the following knowledge.

[Processability, Formability]
In the austenitic stainless steel, the so-called processing-induced transformation plasticity (TRIP) phenomenon is obtained in which the austenite phase is transformed into a hard processing-induced martensite (α ') phase by processing strain, and the strain is dispersed and uniform strain is obtained. Are known. According to the inventors' research, in the present steel with a reduced Ni content, it contains C and N, which are solid solution strengthening elements that influence the generation of α ′ phase due to processing strain and the strength of α ′ phase. It became clear that the amount was deeply involved in the improvement of ductility by TRIP. That is, when the Md30 value of the formula (1), which is an index of austenite stability, is −50 or more, C is more than 0.05% by mass, and (C + N) content is 0.15% by mass or more. Thus, an α ′ phase having an appropriate strength effective for improving ductility is appropriately generated.

[Time cracking resistance]
Conventionally, it is known that austenitic stainless steel represented by SUS304 causes a delayed fracture phenomenon called time cracking when left in a room temperature atmosphere for several hours to several days after deep drawing. The same phenomenon is observed in austenitic stainless steel with reduced Ni. However, the inventors have found that when the Md30 value defined in the formula (1) is 10 or less, the time cracking can be remarkably suppressed without excessively generating the α ′ phase.

[Corrosion resistance]
Pitting potential, which is one of the evaluation indexes of corrosion resistance, generally depends on the Cr content. If the pitting corrosion potential is simply improved, the lower limit of the Cr content may be set high even in the Ni-saving austenitic stainless steel. However, in the present invention, it is necessary to ensure good workability and time cracking resistance, and after balancing with other alloy components, the lower limit of the Cr content is defined as 16% by mass as described later. ing.

[Stress corrosion cracking resistance]
In austenitic stainless steel, stress corrosion cracking often becomes a problem in the processed part and welded part. The inventors have studied various means for imparting stress corrosion cracking resistance to Ni-saving austenitic stainless steel. As a result, it was found that the stress corrosion cracking behavior depends on the stacking fault energy. And as a result of detailed experiments, when the stacking fault difficulty index SFE value defined by the formula (2) is 5 or more, in a processing mode in which stress corrosion cracking becomes a problem, generation of stacking faults can be suppressed, and excellent It has been found that it has resistance to stress corrosion cracking.

[Surface quality]
Stainless steel requires designability in many applications in addition to corrosion resistance and workability. Therefore, it is demanded that a homogeneous surface can be produced efficiently, which in turn greatly contributes to a reduction in production cost. In an AP (annealing pickling) line that is 2D finished by continuous pickling after continuous atmospheric annealing, the homogeneity of the scale produced during annealing and the peelability of the scale by pickling are important factors in determining the manufacturability Factors. The inventors have controlled the Mn content to 2.5% by mass or less in the steel in which Ni is reduced to 6% by mass or less so that the scale removal in the AP line can be performed under the same operating conditions as the conventional SUS304. It has been found that it can be carried out as well as SUS304. This avoids an increase in manufacturing cost associated with an increase in Mn content.

  When Mn content exceeds 2.5 mass%, the load in pickling increases and cost reduction becomes difficult. The mechanism of the load increase in the pickling is considered as follows. In ordinary austenitic stainless steel typified by SUS304, Cr oxide is uniformly generated on the surface of the steel plate during annealing, and pickling is performed efficiently. However, in steels with Ni reduced to 6% or less, when the Mn content exceeds 2.5% by mass, Cr oxide is easily generated during air annealing, and Cr-Mn composite oxide is easily generated nonuniformly. Thus, unevenness occurs in pickling properties between the site where the Cr oxide is generated and the site where the Cr—Mn composite oxide is generated. That is, the pickling property becomes non-uniform. In order to obtain good pickled skin under such circumstances, longer pickling is required, resulting in an increase in load in pickling and a decrease in productivity. As a result of various studies, it has been found that such a problem can be avoided if the Mn content is less than 2.5% by mass.

  In addition, an increase in the Mn content causes a decrease in productivity even in bright annealing. That is, blue-yellow coloration tends to occur when annealing is performed in a reducing atmosphere mainly composed of hydrogen. This is considered as follows. Usually, as the Mn content increases, the proportion of Mn in the steel sheet surface increases. Since Mn is more easily oxidized than Cr in the annealing temperature range, a Mn oxide film is formed on the steel plate surface when the reducing power is relatively low (when the dew point is relatively high) in a bright annealing gas atmosphere. It is easy to do and is often colored yellowish. By setting the Mn content to 2.5% by mass or less, such coloring due to the Mn oxide film can be avoided, and a surface quality equivalent to SUS304 can be provided without reducing the production efficiency.

Hereinafter, the alloy components contained in the steel of the present invention will be described.
[Alloy components]
C and N are useful elements for strengthening the solution-induced martensite (α ′) phase by solid solution. In the steel of the present invention, the total content of C and N (hereinafter sometimes referred to as “(C + N) content”) is 0.15 mass so that sufficient ductility due to TRIP is expressed when the α ′ phase is generated. % Or more. Further, for C, it is important to secure a content exceeding 0.05 mass% in order to stably obtain a remarkable ductility improving effect. If the contents of C and N are too large, the content becomes excessively hard and becomes a factor that hinders workability. For this reason, it is desirable that the (C + N) content is regulated to 0.3% by mass or less. Regarding the individual contents of C and N, C is preferably adjusted in the range of 0.15% by mass or less, and more preferably in the range of 0.1% by mass or less. N is regulated to 0.25% by mass or less, but may be usually adjusted in a range of about 0.04 to 0.2% by mass.

  Mn is a useful austenite-forming element that is less expensive than Ni and can replace the function of Ni. In order to utilize the function in the present invention, it is necessary to secure a Mn content of 0.5% or more. On the other hand, if the Mn content is excessive, environmental conservation problems in the steelmaking process are likely to occur. Moreover, it becomes a factor which causes the productivity fall (previously mentioned) resulting from the surface property. For this reason, the Mn content is limited to 2.5% by mass or less, preferably less than 2.5% by mass.

  Ni is an essential element for austenitic stainless steel, but in the present invention, component design is performed to keep the Ni content as low as possible from the viewpoint of cost reduction. Specifically, the Ni content can be reduced to 6% by mass or less. You may set to less than 6 mass%. However, in order to realize a component balance that combines manufacturability, workability, and corrosion resistance within the above Mn content range, a Ni content of 3% by mass or more must be ensured.

  Cr is an essential element for forming a passive film that ensures the corrosion resistance of stainless steel. When the Cr content is 16% by mass or less, the corrosion resistance required for the conventional austenitic stainless steel to be substituted for the present invention may not be sufficiently secured. However, since Cr is a ferrite-forming element, excessive addition of Cr leads to the formation of a large amount of δ-ferrite phase at a high temperature range, and is a factor that impairs hot workability. As a result of various studies, in the present invention, Cr can be contained up to 25% by mass. Therefore, Cr content is prescribed | regulated to more than 16-25 mass%.

  Si is an element useful for deoxidation in steelmaking, but if the Si content increases, the steel becomes hard and the workability is impaired. Further, since Si is a ferrite-forming element, excessive addition causes a large amount of δ ferrite phase to be generated at a high temperature range, thereby inhibiting hot workability. Accordingly, the Si content is limited to 1% by mass or less.

  Cu is an element that suppresses work hardening due to the formation of a work-induced martensite phase and contributes to softening of austenitic stainless steel. In addition, since Cu is an austenite forming element, the degree of freedom in setting the Ni content is increased as the Cu content is increased, and the component design with Ni suppressed is facilitated. Furthermore, Cu is an extremely effective element for increasing the SFE value, and greatly contributes to the improvement of the stress corrosion cracking resistance by suppressing the generation of stacking faults. In order to obtain these effects sufficiently, it is necessary to ensure a Cu content of 0.8% by mass or more. However, a large amount of Cu exceeding 4% by mass tends to hinder hot workability. For this reason, Cu content is prescribed | regulated to 0.8-4 mass%.

P and S are mixed as unavoidable impurities, but generally P is allowed up to 0.045% by mass and S is allowed up to 0.03% by mass.
The steel of the present invention can be melted by a general stainless steel making process. Then, according to the manufacturing method of a general austenitic stainless steel plate, it can be set as the cold-rolled annealing steel plate of board thickness 0.1-3.5mm, for example.

  An austenitic stainless steel having the composition shown in Table 1 was melted to obtain a continuous cast slab, and then hot rolled at an extraction temperature of 1230 ° C. to produce a hot rolled steel strip having a thickness of 3 mm. After annealing the hot-rolled steel strip at 1100 ° C. × 1 minute soaking, it is cold rolled into a cold-rolled steel strip having a thickness of 1 mm and annealed at 1050 ° C. × 1 minute soaking and pickling. A steel wash strip was prepared.

  JIS 13B test pieces were cut out from steels C, F, G having an Md30 value of about −50 to −40 and steel O having an Md30 value of −16, respectively, and a tensile test parallel to the rolling direction was performed. The distance between the scores was 50 mm, the tensile speed was 40 mm / min, and the distance between the scores was measured by attaching the specimens after fracture to obtain the elongation at break. In order to use it as an alternative to a conventional general austenitic stainless steel sheet that is not Ni-saving, a good workability with an elongation at break of 40% or more is desired in the annealed austenitic stainless steel sheet having this thickness. . The results are shown in Table 2.

  As can be seen from Table 2, the present invention having a C content of more than 0.05% and a (C + N) content of 0.15% by mass or more exhibited a high elongation at break of 40% or more. On the other hand, the elongation at break of Steel F having a (C + N) content of less than 0.15% by mass was as low as 35%. Further, steel O having a low C content of less than 0.05% by mass could not achieve a breaking elongation of 40% or more despite the (C + N) content being 0.15% by mass or more.

From the annealed pickled steel strips of steels A to E and M manufactured in Example 1, disk-shaped blanks having outer diameters of various stages are cut out every 2 mm within the outer diameter of φ76 to 84 mm, and deep drawing tester A cup-shaped drawing process was applied. Using a die having an inner diameter Dd of φ43 mm, a corner curvature rd of 4 mm, and a punch having an outer diameter Dp of φ40 mm and a corner curvature rp of 3 mm, a lubricant having a viscosity of 60 mm ² / s is applied to the surface in contact with the blank die, The blank was processed and molded under the condition of a pressure of 5 tons.

The molded product was allowed to stand for 24 hours in air at room temperature, and then the presence or absence of cracks at the edge of the molded product cup was determined, and the time crack limit drawing ratio of each steel was determined by the following equation (4).
[Time crack limit drawing ratio] = Db _max / Dp (4)
Here, Db _max is the maximum blank diameter (mm) that did not cause time cracking, and Dp is the punch outer diameter (mm). In order to use it as an alternative to a conventional general austenitic stainless steel sheet that is not Ni-saving, it is desired that the time crack limit drawing ratio is 2.0 or more. Table 3 shows the results.

  As can be seen from Table 3, when the Md30 value is 10 or less, good time cracking resistance with a time crack limit drawing ratio of 2.0 or more was realized.

The pitting potential of steels B, H, and I produced in Example 1 was measured according to JIS G0577. The test solution was a 3.5% NaCl aqueous solution, the temperature was 30 ° C., and the potential was increased from a natural potential by a potentiostat at a sweep rate of 0.33 mV / sec, and the corrosion current was 100 mA / cm ² or more in the passive region. The resulting potential (mV vs. S. C. E) was taken as the pitting potential. In order to use it as an alternative to a conventional general austenitic stainless steel sheet that is not Ni-saving, corrosion resistance with a pitting potential of 200 mV or more is desired. The results are shown in Table 4.

  As can be seen from Table 4, a good pitting potential was obtained for those containing 16 mass% or more of Cr.

  A blank having an outer diameter of φ80 mm was cut out from the steels B, J, K, and N manufactured in Example 1, and a drawn cup was produced using a deep drawing tester having a punch diameter of φ40 mm. The cup edge was cut at a height of 15 mm from the cup bottom with a precision cutting machine and subjected to a 42% magnesium chloride boiling test defined in JIS G0576. In order to use it as a substitute for a conventional general austenitic stainless steel sheet that is not Ni-saving, stress corrosion cracking resistance is desired in which cracking does not occur even after 24 hours or more in the boiling test. The results are shown in Table 5.

  As can be seen from Table 5, in the examples of the present invention having an SFE value of 5 or more, cracks did not occur even after 24 hours or more and good stress corrosion cracking resistance was exhibited.

  A 50 mm square cut plate was collected from the cold-rolled steel plates (steel strips before annealing) of steels B, C, I, J, and L manufactured in Example 1, and an annealing experiment was performed in an air atmosphere in a laboratory. It was. Each sample was heated for 60 seconds in an air annealing furnace set at 1100 ° C., and then immediately sandwiched between water-cooled copper plates and rapidly cooled. Immerse this in a mixed acid aqueous solution of 2% by mass hydrofluoric acid and 10% by mass nitric acid in 60 ° C., and the time until the scale is removed and the surface becomes homogeneous (this is called “pickling time”). It was measured. The results are shown in Table 6.

  As can be seen from Table 6, the examples of the present invention having a Mn content of 2.5% by mass or less have a pickling time of 30 seconds or less, and have a special line speed reduction in a general continuous annealing pickling line. It was judged that good pickling could be carried out without it.

  In the same manner as in Example 5, 50 mm square cut plates were collected from the cold rolled steel sheets of steels B, C, I, J, and L, and here, bright annealing experiments were performed in the laboratory. The sample is held in a quartz tube continuously supplied with 100% hydrogen gas with a dew point adjusted in the range of −40 ° C. to −60 ° C., and the entire quartz tube is placed in a heating furnace at 1100 ° C. and heated for 60 seconds. An experiment was conducted in which the entire quartz tube was quickly removed from the furnace and allowed to cool. The presence or absence of coloring was evaluated by visually observing the surface of the sample after cooling. For each steel, the upper limit dew point at which no coloration occurred (herein referred to as “dew point upper limit temperature”) was determined. In order to use it as an alternative to a conventional general austenitic stainless steel sheet that is not Ni-saving, it is desirable that it has good coloration resistance with a dew point upper limit temperature of −50 ° C. or higher in this bright annealing experiment. It is. The results are shown in Table 7.

  As can be seen from Table 7, the examples of the present invention having a Mn content of 2.5% by mass or less have coloring resistance sufficiently clearing the “dew point upper limit temperature; −50 ° C. or higher”, and bright annealing. A decrease in productivity is also avoided.

Claims (1)

In mass%, C: exceeding 0.05% and satisfying the following formula (3), Si: 1% or less, Mn: 0.5 to 2.5%, Ni: 3 to 6%, Cr: exceeding 16 25% or less, Cu: 0.8 to 4%, N: a range satisfying the following formula (3), balance Fe and inevitable impurities, and an austenite stability index Md30 defined by the following formula (1) is −50 Ni-saving austenite excellent in workability, time cracking resistance, corrosion resistance, stress corrosion cracking resistance, and surface properties having a composition in which the stacking fault difficulty index SFE defined by the following formula (2) is 5 or more Stainless steel.
Md30 = 551-462 (C + N) -9.2Si-8.1Mn-29 (Ni + Cu) -13.7Cr (1)
SFE = 2.2Ni + 6Cu-1.1Cr-13Si-1.2Mn + 32 (2)
0.15 ≦ C + N ≦ 0.3 (3)