JP7013301B2 - Al-containing ferritic stainless steel with excellent secondary workability and high-temperature oxidation resistance - Google Patents

Description

The present invention relates to an Al-containing ferritic stainless steel material having excellent secondary processability and high temperature oxidation resistance.

When the Al-containing ferritic stainless steel is heated to a high temperature, an oxide layer mainly composed of Al ₂ O ₃ is uniformly formed on the surface of the steel, so that it exhibits excellent high temperature oxidation resistance. On the other hand, when secondary processing is performed with a large amount of deformation, processing cracks may occur, which limits the use of processed products to which this stainless steel can be applied. Therefore, a method for improving the workability of Al-containing ferritic stainless steel has been proposed.

For example, in Patent Document 1, ferritic stainless steel that achieves both workability and oxidation resistance by reducing C and N, adding Ti, reducing the amount of Al, and optimizing the amount of Si added. Is disclosed. Patent Document 2 describes a ferritic stainless steel having improved secondary processing brittleness and Cr evaporative resistance by reducing the amount of Al and Si added, adding Nb and B in combination, and suppressing the addition of Ti. It has been disclosed.

However, in Patent Document 1, evaluation by secondary processing is not performed. In the secondary processing, since strict processing is required for stainless steel, processing cracks may occur due to rough skin and rigging.

Further, Patent Document 2 evaluates the secondary processing brittleness by performing a drop test at various temperatures using a primary drawing product that has been drawn and measuring the transition temperature. In the secondary processing, processing cracks may occur due to rough skin and rigging, and evaluation from such a viewpoint is also desired.

Japanese Unexamined Patent Publication No. 2004-307918 Japanese Unexamined Patent Publication No. 2012-21379

While the Al-containing ferritic stainless steel is excellent in high-temperature oxidation resistance, it becomes harder as the amount of Al added increases, so that its workability deteriorates. In particular, when performing secondary processing, cracks may occur due to rigging and rough skin, and it is inferior in secondary workability such as multi-stage drawing and bulge processing in steel pipe manufacturing. There has been a demand for steel materials having excellent workability, especially secondary workability.

Therefore, an object of the present invention is to provide an Al-containing ferritic stainless steel material having excellent secondary processability and high temperature oxidation resistance.

In view of the above problems, the present inventors have investigated the processability and high-temperature oxidation resistance of Al-containing ferritic stainless steel, and from the viewpoint of alloy composition and metallographic structure, while ensuring high-temperature oxidation resistance, secondary We have found that workability can be improved and have completed the present invention. Specifically, the present invention provides the following.

(1) In the present invention, in mass%, C: 0.025% or less, Si: 0.1 to 1.0%, Mn: 0.8% or less, P: 0.05% or less, S: 0. 01% or less, Ni: 0.5% or less, Cr: 16 to 24%, Al: 1.0 to 2.6%, N: 0.025% or less, Nb: 0.05 to 0.6%, B : Containing 0.0005 to 0.0060%, the balance is composed of Fe and unavoidable impurities, the average crystal grain size is 80 μm or less, and the secondary processability and high temperature resistance satisfy the following formula (1). It is an Al-containing ferrite-based stainless steel material having excellent oxidizing properties.
T _<111> / (T _<001> + T _<011> ) ≤ 2.5 Equation (1)
Here, T _<111> , T _<001> , and T _<011> in the formula (1) are obtained from the crystal orientations of <111>, <001>, and <011> in the cross section perpendicular to the plate thickness direction. The area ratio of the crystal grains distributed within 15 ° to the cross section is shown.

(2) In the present invention, Ti: 0.01 to 0.5%, V: 0.01 to 0.5%, Mo: 0.01 to 0.5%, Co: 0. The secondary processing according to (1), which comprises one or more selected from the group consisting of 01 to 0.5%, Zr: 0.01 to 0.5%, and Cu: 0.01 to 0.5%. It is an Al-containing ferritic stainless steel material with excellent properties and high temperature oxidation resistance.

(3) The present invention further comprises a group consisting of Hf: 0.001 to 0.05%, Sn: 0.001 to 0.05%, and rare earth elements: 0.001 to 0.05% in terms of mass%. The Al-containing ferritic stainless steel material having excellent secondary processability and high temperature oxidation resistance according to (1) or (2), which comprises one or more selected.

(4) The present invention is a processed product made of stainless steel made of the Al-containing ferritic stainless steel material according to any one of (1) to (3).

(5) The present invention is the processed product made of stainless steel according to (4), wherein the processed product is a steel pipe.

According to the present invention, when an Al-containing ferritic stainless steel material is secondarily processed, secondary processing cracks due to rigging and rough skin are suppressed, so that secondary processability is ensured while ensuring good high-temperature oxidation resistance. It is possible to provide an Al-containing ferritic stainless steel material having an improved quality. The Al-containing ferrite-based stainless steel material according to the present invention has workability and high-temperature oxidation resistance for all high-temperature combustion parts such as fuel cell reformers and pipes, automobile exhaust gas parts, burner combustion cylinders, chimneys, and thermoelectric heating elements. It is suitable for applications that require sex.

It is a figure for demonstrating the measurement about the formula (1).

Hereinafter, embodiments of the present invention will be described. The present invention is not limited to these descriptions.

As a result of research on the secondary workability of Al-containing ferritic stainless steel, the crystal grains are made finer and the crystal orientation is randomly distributed to form a metal structure, which is caused by processing defects such as rigging and rough skin. It was found that the secondary process cracking was suppressed and the secondary processability could be improved. Furthermore, it was found that high temperature oxidation resistance can be ensured by proper component balance. Further, in the component composition based on the addition of Nb, a predetermined crystal orientation can be obtained by combining the annealing conditions for distributing the Nb-based precipitates and cold rolling, so that secondary processing cracks due to rigging or the like can be prevented. Can be suppressed.

(Alloy component)
Hereinafter, the alloy components contained in the Al-containing ferritic stainless steel material according to the present embodiment and the reasons for their inclusion will be described. The% indication of the chemical composition means% by mass.

C is an element that improves high temperature strength. If the C content is excessively high, accelerated oxidation is likely to occur during high-temperature heating. Further, in the Al-containing ferritic stainless steel, the toughness of the slab and the hot coil deteriorates, and the manufacturability deteriorates. Therefore, the C content is preferably 0.025% or less.

Si has the effect of suppressing the formation of red scale in ferritic stainless steel. If the Si content is less than 0.1%, the suppressing effect is not sufficient. On the other hand, if the Si content is excessive, the toughness and workability are lowered. Therefore, the amount of Si added is preferably 0.1 to 1.0%.

Mn produces Mn-based oxides, inhibits the formation of a dense aluminum oxide layer, and lowers high-temperature oxidation resistance. Therefore, from the viewpoint of maintaining the high temperature oxidation resistance, the Mn content is preferably 0.8% or less, more preferably 0.5% or less.

P is an impurity inevitably contained in the steel and reduces the high temperature oxidation resistance and the toughness of the hot-rolled sheet. Therefore, the P content was limited to 0.05% or less.

S is an impurity inevitably contained in steel and reduces workability. Therefore, the S content was limited to 0.01% or less.

Cr is a necessary component for improving high temperature oxidation resistance. If the Cr content is less than 16%, high temperature oxidation resistance cannot be sufficiently obtained. On the other hand, if the Cr content is excessive, the toughness of the slab or hot coil is deteriorated. Therefore, the Cr content is preferably 16 to 24%. Since excessive addition deteriorates processability, the upper limit is more preferably 20%.

Al is a necessary component for forming a dense Al ₂ O ₃ film on the surface of steel and improving high temperature oxidation resistance. Further, in the initial stage of oxidation, the rapid formation of the aluminum oxide film has the effect of suppressing Cr evaporation from the steel surface. If the Al content is less than 1.0%, sufficient high temperature oxidation resistance cannot be obtained. On the other hand, if the Al content is excessive, the toughness of the slab or the hot coil is deteriorated, and the brittleness temperature during the secondary processing is increased, resulting in a decrease in workability. Therefore, the Al content is preferably 1.0 to 2.6%. More preferably, the upper limit is 2.0%.

N may combine with Al in steel to form AlN, which may be the starting point of accelerated oxidation. Therefore, from the viewpoint of maintaining high temperature oxidation resistance, the N content is preferably 0.025% or less.

Nb has an action of improving workability and toughness by binding to C and N in steel, which adversely affect workability and toughness, and fixing them as a compound. Further, the addition of Nb increases the high temperature strength of the steel. In addition, it promotes the formation of an aluminum oxide film and relaxes the stress generated in the process of growing the oxide film to prevent deformation of the material. If the Nb content is less than 0.05%, such an Nb addition effect cannot be sufficiently obtained. On the other hand, if the Nb content is excessive, the workability and toughness of the steel are lowered. Therefore, the Nb content is preferably 0.05 to 0.6%. More preferably, the lower limit is 0.10% and the upper limit is 0.30%.

B has the effect of preventing secondary process cracking during molding and improving the secondary processability. If the B content is less than 0.0005%, the effect is not sufficient. On the other hand, an excessive B content causes a decrease in toughness. Therefore, the B content is preferably 0.0005 to 0.0060%.

Further, if necessary, one or more selected from the group consisting of Ti, V, Mo, Co, Zr, and Cu may be added.

Ti and V are elements that fix solid solution C and N in steel as compounds and improve ductility and processability. It also has the effect of suppressing the precipitation of Cr carbides at the grain boundaries and improving the corrosion resistance. On the other hand, if it is added in an excessive amount, the processability is deteriorated. Therefore, the Ti content or V content is preferably 0.01 to 0.5%.

Mo has the effect of improving the high-temperature strength of steel, but when added in excess, it hardens the steel material and causes a decrease in toughness. Therefore, the Mo content is preferably 0.01 to 0.5%.

Zr has the effect of improving high-temperature oxidation resistance, but when added in excess, it hardens the steel material and causes a decrease in toughness. Therefore, the Zr content is preferably 0.01 to 0.5%.

While Cu increases the high-temperature strength of steel, if it is added in excess, it causes a decrease in high-temperature oxidation resistance and a decrease in hot workability. Therefore, the Cu content is preferably 0.01 to 0.5%.

Further, if necessary, one or more selected from the group consisting of Hf, Sn, and rare earth elements may be added.

Hf, Sn and rare earth elements have the effect of improving high temperature oxidation resistance. It stabilizes the aluminum oxide film formed on the surface of the steel, improves the adhesion between the matrix and the oxide film, and improves the high temperature oxidation resistance. On the other hand, if these elements are added in excess, the hot workability and toughness are deteriorated. In addition, inclusions that serve as a starting point for accelerated oxidation are generated, and high-temperature oxidation resistance is reduced. Therefore, it is preferable that Hf, Sn, or a rare earth element is contained in the range of 0.0001 to 0.05%, respectively.

(Average crystal grain size)
The Al-containing ferritic stainless steel material according to this embodiment preferably has an average crystal grain size of 80 μm or less. By refining the crystal grains, roughening of the surface surface after processing is suppressed, and secondary processability is improved. Therefore, the average crystal grain size is preferably 80 μm or less, more preferably 50 μm or less. On the other hand, if the crystal grains are excessively fine, they may be hardened and adversely affect workability. Therefore, the average crystal grain size may be 10 μm or more.

(Crystal orientation)
Further, the Al-containing ferrite-based stainless steel material according to the present embodiment is distributed within 15 ° from the crystal orientation of each of <111>, <001>, and <011> in the cross section perpendicular to the plate thickness direction. It is preferable that the following formula (1) is satisfied when the area ratio of the crystal grains to be formed in the cross section is represented by the reference numerals T _<111> , T _<001> , and T _<011> , respectively.
T _<111> / (T _<001> + T _<011> ) ≤ 2.5 ... Equation (1)

FIG. 1 shows a rolled steel plate 1. Based on the rolled surface 2 of the steel sheet 1, it is divided into a rolling direction 3, a rolling vertical direction 4, and a normal direction 5 with respect to the rolled surface 2. The "plate thickness direction" described in the present specification corresponds to the above-mentioned normal direction 5, and the "cross section perpendicular to the plate thickness direction" corresponds to the rolled surface 2 along the rolling direction. Since the above normal direction is abbreviated as ND, in the present specification, the above "cross section perpendicular to the plate thickness direction" may be referred to as "ND plane". Further, the rolling direction may be described as "RD direction", and the rolling direction may be described as "TD direction".

The left side of the formula (1) is based on the ratio of the crystal grains distributed within 15 ° from the crystal orientation of <111> to the cross section (rolled surface, ND surface) and the crystal orientations of <001> and <011>. The ratio with the ratio of the crystal grain distributed within 15 ° to the cross section is shown. The closer this ratio is to 1, the more the metal structure is that the individual crystal grains on the rolled surface are randomly distributed. When the formula (1) exceeds 2.5, the proportion of crystal grains oriented in a specific direction increases, and anisotropy becomes apparent in the mechanical properties of the crystal structure. As a result, the deformation during the molding process becomes non-uniform, causing processing defects such as rigging, and secondary processing cracks are likely to occur due to these defects. Therefore, it is preferable that the numerical value of the formula (1) is 2.5 or less.

(Number of distributions of Nb-based precipitates)
When 10 such fields of view (10 fields of view) are randomly selected and measured as one field of view in the range corresponding to 50 μm ² by an EDS (energy dispersion type X-ray spectroscopy) device on the ND plane, the field of view is within the field of view. Among the precipitates containing 20 mass% or more of Nb observed in the above, the precipitate having an average dimension of 0.5 μm or more calculated by “(maximum length + minimum length) / 2” was selected, and in the field of view. The total number of distributions of the Nb-based precipitates is calculated. The “number of distributions of Nb-based precipitates” described in the present specification means the total number of precipitates obtained by the above-mentioned measuring method. The Nb-based precipitates distributed in the steel structure suppress the progress of crystal grain growth during hot-rolled annealing and cold-rolled annealing in a specific direction, and enhance the uniformity of the crystal structure. Therefore, the occurrence of processing cracks due to processing defects such as rigging and rough skin is suppressed during the secondary processing. From this point of view, the number of distributions of Nb-based precipitates is preferably 3 or more per visual field and 40 or more per 10 visual fields.

(Production method)
The Al-containing ferritic stainless steel material according to the present embodiment can be manufactured by a known manufacturing process such as melting, casting, and rolling. After hot rolling the slab, it is manufactured by processes such as annealing, pickling, cold rolling, and final annealing. After cold rolling, if necessary, it may be manufactured by a process of intermediate annealing, pickling, final cold rolling, and final annealing. In addition, the shipped product may be pickled or polished.

Using the Al-containing ferritic stainless steel material according to the present embodiment, various molding processes are applied to the steel material in order to obtain a product having a desired shape, dimensions and the like. When molding, the processing process is divided into a plurality of times according to the processing form of the material to be processed and the degree of deformation. In the present specification, the first molding process is referred to as "primary processing", and the molding process for the primary processed product subjected to the primary processing is referred to as "secondary processing". In the case of molding using steel materials, the primary processed product is subjected to secondary processing such as drawing and bulge processing. In the secondary processing, in general, more severe deformation is given to the workpiece than in the primary processing, so that it can be said that processing cracks are likely to occur during the secondary processing.

(Use)
The Al-containing ferritic stainless steel material according to the present embodiment can be applied to materials of various processed products processed into a desired shape by various known methods such as rolling, forging, drawing and casting. For example, it is possible to provide processed products such as steel plates, strips, steel pipes, strips, shaped steels, steel bars, and wire rods.

Hereinafter, examples of the present invention will be described. The present invention is not limited to the following examples, and can be appropriately modified and carried out within the scope of the gist of the invention.

The test steel having the chemical components shown in Table 1 was melted in vacuum to cast a 100 kg ingot. The obtained ingot was agglomerated to prepare a slab having a thickness of 35 mm. After heating the slab to 1200 ° C., hot rolling was performed to obtain a hot-rolled plate having a plate thickness of 4 mm. Next, the hot-rolled plate was annealed at the hot-rolled annealing temperature shown in Table 1 and then pickled to obtain a hot-rolled annealed plate. The chemical composition shown in Table 1 is expressed in% by mass, and the balance is Fe and unavoidable impurities.

In order to reduce the plate thickness of a part of the obtained hot-rolled annealed plate, a plate material having a plate thickness of 2.5 mm and a plate thickness of 1.7 mm was produced by cutting. Next, these hot-rolled annealed plates were cold-rolled to produce cold-rolled plates having a plate thickness of 1 mm. Then, finish annealing was performed at the cold-rolled annealing temperature shown in Table 1, and pickling was performed to obtain a cold-rolled annealed plate. Table 2 shows the cold rolling ratios (cold rolling ratios) of the cold-rolled annealed plates with a plate thickness of 1 mm obtained from the hot-rolled annealed plates with a plate thickness of 4 mm, a plate thickness of 2.5 mm, and a plate thickness of 1.7 mm. As shown, it is 75%, 60% and 40%. The cold-rolled annealed plate was subjected to the following evaluation test.

(Average crystal grain size)
Using the above-mentioned cold-rolled annealed plate, it was cut perpendicular to the longitudinal direction, and its cross section was observed with an optical microscope at a magnification of 50 times. Five fields of view were randomly observed, and the average crystal grain size was measured according to the cutting method of JIS G0551.

(Area ratio related to crystal orientation: Equation (1))
A test piece cut out from the above cold-rolled annealed plate with dimensions of 15 mm in the RD direction × 10 mm in the TD direction was used. The crystal orientation of the crystal grains in the cross section (ND plane) perpendicular to the plate thickness direction of the test piece was measured by an EBSD device (backscatter electron diffractometer). Then, the area ratio of the crystal grains within 15 ° from the respective crystal orientations of <111>, <001> and <011> to each ND plane is calculated, and "T _<111> / (T) in the formula (1). _<001> + T _<011> ) ”was obtained. When measuring by EBSD, three visual fields were randomly selected on the ND surface of the test piece, and the total area of those visual fields was selected to be 9 mm ² or more.

(Number of distributions of Nb-based precipitates)
A test piece cut out from the above cold-rolled annealed plate with dimensions of 15 mm in the RD direction × 10 mm in the TD direction was used. Concentration analysis was performed on the ND surface of the test piece by an EDS (energy dispersive X-ray spectroscopy) device, and the distribution of precipitates containing 20 mass% or more of Nb was measured. At the time of measurement, 10 visual fields in the range corresponding to 50 μm ² were randomly selected. From the precipitates containing 20 mass% or more of the observed Nb, the precipitates having an average dimension of 0.5 μm or more calculated by “(maximum length + minimum length) / 2” were selected, and for 10 visual fields, The number of distributions of the precipitate in each field was measured. From the measured number of distributions, the minimum number of precipitates per field of view was determined, and the total number of precipitates per 10 fields of view was determined.

(Primary workability)
A test piece having a diameter of 68 mm was cut out from the above cold-rolled annealed plate. Using a mold having a punch diameter of 40 mm, the test piece was subjected to primary drawing at a drawing ratio of 1.7. The surface of the specimen after processing was visually observed to check for processing cracks. The case where no processing cracks occurred was evaluated as good (◯), and the case where processing cracks occurred was evaluated as unsuitable (×).

(Secondary workability)
Next, the test piece in which no processing cracks occurred in the above primary drawn product was subjected to secondary drawing using a punch diameter of 25 mm and a drawing ratio of 1.6. Since the drawing ratio in this secondary drawing is equivalent to about 2.7 based on the outer diameter of the above-mentioned cold-rolled annealed plate, it can be said that the processing conditions are stricter than those in the primary drawing. # 120 machine oil was used during the secondary drawing process. The surface of the test piece after the secondary processing was visually observed to check for processing cracks. The case where no processing cracks occurred was evaluated as good (◯), and the case where processing cracks occurred was evaluated as unsuitable (×). In addition, in the secondary processing, the surface texture of the test piece after processing was visually observed, and the degree of rough skin and rigging was also added to the evaluation of the secondary processability.

(High temperature oxidizing)
A test piece having a length of 35 mm and a width of 25 mm was cut out from the above-mentioned cold-rolled annealed plate, the surface thereof was dry-polished with # 400, and the cross section thereof was dry-polished with # 600. Then, the test piece was immersed in acetone for 5 minutes for ultrasonic cleaning, and then subjected to a high-temperature oxidizing property evaluation test. In the evaluation test, after the test piece was exposed to an air atmosphere of 1000 ° C. for 100 hours, the weight change before and after the test was measured, and the high temperature oxidizing property was evaluated based on the measured value. When the weight change was 2 mg / cm ² or less, it was judged to be good (◯), and when it exceeded 2 mg / cm ² , it was judged to be unsuitable (×).

(Hardness)
The hardness of the ND surface of the test piece was measured by a Vickers hardness test device (load 1 kg) using the test piece cut out from the above cold-rolled annealed plate to a size of 20 mm.

Table 2 shows the measurement results and evaluation results of the formula (1) regarding the average crystal grain size and crystal orientation, the number of distributions of Nb-based precipitates, the primary processability, the secondary processability, the high-temperature oxidizing property, and the hardness.

As shown in Tables 1 and 2, Steel No. 1 to No. The Al-containing ferritic stainless steel material of No. 16 is a steel material in which the numerical values of the formula (1) relating to the alloy composition, the average crystal grain size, and the crystal orientation are within the scope of the present invention, and has good secondary workability and high temperature oxidation resistance. Met.

On the other hand, Steel No. 17-No. In No. 26, any of the alloy composition, the average crystal grain size, and the numerical value of the formula (1) relating to the crystal orientation is outside the scope of the present invention, and therefore any of the primary processability, the secondary processability, and the high temperature oxidation resistance. Was defective.

Steel No. 17, Steel No. 22, Steel No. Since the average crystal grain size of No. 23 is more than 80 μm, rough skin is generated during the secondary processing, and processing cracks due to the rough skin are generated, so that the secondary processability is deteriorated. Steel No. 18, Steel No. 21, Steel No. In No. 25, since the numerical value of the formula (1) is more than 2.5, the degree of anisotropy of the metal structure is increased, rigging occurs during the secondary processing, and processing cracks due to the rigging occur. Secondary workability was reduced.

Steel No. No. 19 has a Cr content of more than 24%, and the steel No. 19 has a Cr content of more than 24%. Since the Al content of No. 20 is more than 2.6%, the steel material is hardened and the primary workability is lowered. The hardened point is the steel No. 19 and Steel No. The hardness (Hv) of 20 is steel No. 1 to No. It is shown to be higher than 16.

Furthermore, the steel No. 21, Steel No. 24-No. In No. 26, the Al content was less than 1.0%, or the Si content was less than 0.1%, so that the high temperature oxidation resistance was inferior.

In addition, steel No. 1 to No. 15, Steel No. In No. 26, the number of Nb-based precipitates was 40 or more, and the secondary processability was good.

1 Steel plate 2 Rolled surface (ND surface)
3 Rolling direction (RD direction)
4 Rolling perpendicular direction (TD direction)
5 normal direction (ND direction)

Claims (5)

By mass%, C: 0.025% or less, Si: 0.1 to 1.0%, Mn: 0.8% or less, P: 0.05% or less, S: 0.01% or less, Ni: 0 .5% or less, Cr: 16 to 24%, Al: 1.0 to 2.6%, N: 0.025% or less, Nb: 0.05 to 0.6%, B: 0.0005 to 0. It contains 0060% and the balance consists of Fe and unavoidable impurities.
The average crystal grain size is 80 μm or less,
An Al-containing ferritic stainless steel material that satisfies the following formula (1) and has excellent secondary workability and high-temperature oxidation resistance.
T _<111> / (T _<001> + T _<011> ) ≤ 2.5 Equation (1)
Here, T _<111> , T _<001> , and T _<011> in the formula (1) are obtained from the crystal orientations of <111>, <001>, and <011> in the cross section perpendicular to the plate thickness direction. The area ratio of the crystal grains distributed within 15 ° to the cross section is shown. Further, in terms of mass%, Ti: 0.01 to 0.5%, V: 0.01 to 0.5%, Mo: 0.01 to 0.5%, Co: 0.01 to 0.5%, The secondary processability and high temperature oxidation resistance according to claim 1, which comprises at least one selected from the group consisting of Zr: 0.01 to 0.5% and Cu: 0.01 to 0.5%. Excellent Al-containing ferritic stainless steel material. Further, one or more selected from the group consisting of Hf: 0.001 to 0.05%, Sn: 0.001 to 0.05%, and rare earth elements: 0.001 to 0.05% in terms of mass%. The Al-containing ferritic stainless steel material having excellent secondary processability and high temperature oxidation resistance according to claim 1 or 2, which is included. A processed product made of stainless steel made of the Al-containing ferritic stainless steel material according to any one of claims 1 to 3. The processed product made of stainless steel according to claim 4, wherein the processed product is a steel pipe.

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