JP7657026B2 - Cu-containing austenitic stainless steel castings - Google Patents

Description

The present invention relates to Cu-containing austenitic stainless steel castings.

Stainless steel has a wide range of applications due to its excellent corrosion resistance. In the manufacturing process of stainless steel, Cu is sometimes added to improve cold workability. Examples of technologies that improve cold workability by adding Cu include the following:

For example, Patent Document 1 discloses an austenitic stainless steel with excellent hydrogen embrittlement resistance that contains 0.10 to 3.00 mass% Cu as an optional element to improve cold workability.

Patent Document 2 discloses a high-strength nonmagnetic steel for fastening superconducting coils of linear motor cars, which is obtained by cold working stainless steel having a predetermined chemical composition at a working rate of 10 to 20%, followed by cold forging or header processing at a temperature range of 60 to 150°C, and then holding the steel at a temperature range of 300 ^to 550°C for 15 minutes or more, thereby obtaining a 0.2% yield strength of 650 N/mm2 ^or more, a tensile strength of 800 N/mm2 or more, and a magnetic permeability of 1.05 or less. The stainless steel used in the technology disclosed in Patent Document 2 contains 1.0 to 4.0 mass% Cu to reduce the work hardening index and suppress the occurrence of cracks during cold working.

Patent Document 3 also discloses a method for producing a Cu-containing stainless steel slab, which is characterized by strengthening the cooling of the crater end portion so that the grain size of the Cu alloy phase in the center of the slab becomes 50 μm or less when continuously casting stainless steel containing 1 to 5 mass % Cu. In the method for producing a Cu-containing stainless steel slab disclosed in Patent Document 3, the guideline for the specific water volume of the cooling water sprayed on the crater end portion is 0.2 to 0.5 L/(kg-steel).

JP 2016-14180 A Japanese Patent Application Publication No. 9-272954 JP 2000-225441 A

Cu improves cold workability but reduces hot workability. In the technology disclosed in Patent Documents 1 and 2, the upper limit of the Cu content is specified to suppress the deterioration of hot workability. However, internal cracks may occur during hot rolling due to macrosegregation of the cast slab.

In addition, the manufacturing method of Cu-containing stainless steel slabs disclosed in Patent Document 3 may cause internal cracks during hot rolling if the slab contains Cu and a large amount of Mn, which lowers the solid solubility limit of Cu. In addition, the method of controlling the specific water content of the crater end may not be sufficient to prevent internal cracks during hot rolling, depending on the casting conditions.

The present invention was made in consideration of the above problems, and the object of the present invention is to provide a Cu-containing austenitic stainless steel slab that can suppress the occurrence of internal cracks during hot working.

The inventors discovered that internal cracks that occur during hot rolling of Cu-containing austenitic stainless steel are caused by liquefaction of areas with high Cu concentration. They also discovered that liquefaction inside the slab is influenced not only by the Cu concentration, but also by the Mn concentration, which reduces the solubility of Cu, and the Ni concentration, which increases the solubility of Cu.

The inventors further discovered that even if there are many regions inside the cast piece that are prone to liquefaction, if the area of each of these regions is small, they will not become the starting point of internal cracks, but if the area of each region is large, that region will become the starting point of internal cracks.

The gist of the present invention, which has been completed based on the above findings, is as follows.
[1]
In mass percent,
C: 0.25% or less,
Si: 3.0% or less,
Mn: 0.1 to 20.0%,
P: 0.045% or less,
S: 0.0050% or less,
Cr: 10.0-25.0%,
Ni: 4.0 to 22.0%,
Mo: 5.0% or less,
Cu: 0.1 to 4.0%, and
N: 0.4% or less,
with the remainder being Fe and impurities,
The ratio of Cr _eq represented by the following formula (1) to Ni _eq represented by the following formula (2), Cr _eq /Ni _eq , is less than 1.48,
In the thickness direction center of the casting direction cross section at the width direction center, there is a region in which the α value represented by the following formula (3) exceeds 3.63,
The region has a circular equivalent diameter of 1.5 mm or less,
A Cu-containing austenitic stainless steel cast slab having a β value represented by the following formula (4) of 2.51 or less.
Cr _eq = Cr+(1.37×Mo)+(1.5×Si)…Formula (1)
Ni _eq = Ni+(0.31×Mn)+Cu+(22×C)+(14.2×N)…Formula (2)
α=[Cu]+(0.136×[Mn])−(0.2×[Ni])…Formula (3)
β=Cu+(0.0075×Mn ² )+(0.0404×Mn)−(0.09×Ni)…Formula (4)
In the formulas (1), (2), and (4), Cr, Mo, Si, Ni, Mn, Cu, C, and N represent the contents (mass%) of each element, and in the formula (3), [Cu], [Mn], and [Ni] represent the concentrations (mass%) of each element measured by an electron probe microanalyzer at the center in the thickness direction.
[2]
The Cu-containing austenitic stainless steel cast slab according to [1], wherein the equiaxed crystal ratio is more than 30%.
[3]
Instead of a part of Fe,
Al: 0.15% or less,
Ca: 0.005% or less; and
B: 0.005% or less,
The Cu-containing austenitic stainless steel cast slab according to [1] or [2], comprising one or more selected from the group consisting of:
[4]
Instead of a part of Fe,
Nb: 0.3% or less, or
Ti: 0.3% or less,
The Cu-containing austenitic stainless steel cast slab according to any one of [1] to [3], comprising at least one of the following:

As described above, the present invention makes it possible to provide a Cu-containing austenitic stainless steel slab that can suppress the occurrence of internal cracks during hot working.

<Cu-containing austenitic stainless steel cast slab>
The Cu-containing austenitic stainless steel cast slab according to this embodiment contains, by mass%, C: 0.25% or less, Si: 3.0% or less, Mn: 0.1 to 20.0%, P: 0.045% or less, S: 0.0050% or less, Cr: 10.0 to 25.0%, Ni: 4.0 to 22.0%, Mo: 5.0% or less, Cu: 0.1 to 4.0%, and N: 0.4% or less, with the balance being Fe and impurities, and has a ratio Cr _eq /Ni eq of Cr eq represented by the following formula (1) to Ni _eq represented by the following _formula (2 ₎ of less than 1.48, and has a circle equivalent diameter of a region where the α value represented by the following formula (3) exceeds 3.63 at the thickness direction center in the casting direction cross section at the width direction center being 1.5 mm or less, and a β value represented by the following formula (4) being 2.51 or less.
Cr _eq = Cr+(1.37×Mo)+(1.5×Si)…Formula (1)
Ni _eq = Ni+(0.31×Mn)+Cu+(22×C)+(14.2×N)…Formula (2)
α=[Cu]+(0.136×[Mn])−(0.2×[Ni])…Formula (3)
β=Cu+(0.0075×Mn ² )+(0.0404×Mn)−(0.09×Ni)…Formula (4)
In the above formulas (1), (2), and (4), Cr, Mo, Si, Ni, Mn, Cu, C, and N represent the contents (mass%) of each element, and in the above formula (3), [Cu], [Mn], and [Ni] represent the concentrations (mass%) of each element measured at the center in the thickness direction by an electron beam microanalyzer with a beam diameter of 50 μm.
The Cu-containing austenitic stainless steel cast slab according to this embodiment will be described in detail below.

[Chemical composition]
First, the chemical components of the Cu-containing austenitic stainless steel cast slab according to this embodiment will be described. Note that % indicating the components means mass %.

C (carbon): 0.25% or less Although C is an effective element for stabilizing the austenite phase, excessive C content may cause Cr carbides to precipitate, forming a Cr-deficient layer, and thus reducing corrosion resistance. For this reason, the C content is set to 0.25% or less. The C content is preferably 0.10% or less. On the other hand, C contributes to increasing the strength of the steel material by solid solution strengthening, and also contributes to improving hydrogen embrittlement resistance by suppressing the generation of processing-induced martensite phase through stabilization of the austenite phase. For this reason, the C content is preferably 0.01% or more. The C content is more preferably 0.015% or more.

Si (silicon): 3.0% or less Although Si is an element effective in stabilizing the austenite phase, excessive Si content promotes the formation of intermetallic compounds such as sigma phase, resulting in a decrease in hot workability and toughness. Therefore, the Si content is set to 3.0% or less. The Si content is preferably 1.0% or less, and more preferably 0.7% or less. On the other hand, Si stabilizes the austenite phase, suppressing the formation of processing-induced martensite phase and improving hydrogen embrittlement resistance, so the Si content is preferably 0.2% or more. The Si content is more preferably 0.4% or more.

Mn (Manganese): 0.1% to 20.0%
Mn is an element effective in stabilizing the austenite phase, but if an excessive amount of Mn is contained, the formation of coarse MnS inclusions is promoted, leading to a decrease in the ductility of the austenite phase. In addition, an excessive amount of Mn promotes the formation of nitrides. Therefore, the Mn content is set to 20.0% or less. The Mn content is preferably 10.0% or less. On the other hand, in order to stably obtain the austenite stabilization effect of Mn, the Mn content is set to 0.1% or more. The Mn content is preferably 1.0% or more, and more preferably more than 2.0%.

P (phosphorus): 0.045% or less P is contained as an impurity in the Cu-containing austenitic stainless steel slab according to this embodiment. P is an element that reduces hot workability. Therefore, in the Cu-containing austenitic stainless steel slab according to this embodiment, the P content is set to 0.045% or less. The P content is preferably 0.035% or less. Since it is preferable to reduce the P content as much as possible, no lower limit is set, but an extreme reduction leads to an increase in steelmaking costs. Therefore, the P content is preferably 0.008% or more.

S (sulfur): 0.0050% or less S is an element that segregates at the austenite grain boundaries when the slab is hot worked, weakening the bonding strength of the grain boundaries and inducing cracks during hot working. In order to prevent cracks during hot working, the S content is set to 0.0050% or less. The S content is preferably 0.0010% or less. Since it is preferable to reduce the S content as much as possible, no lower limit is set, but extreme reduction leads to an increase in steelmaking costs. Therefore, the S content is preferably 0.0001% or more.

Cr (chromium): 10.0 to 25.0%
Cr is an essential element for obtaining the corrosion resistance required for stainless steel. Cr also contributes to increasing the strength of austenitic stainless steel. To obtain the above effect, the Cr content is set to 10.0% or more. The Cr content is preferably 13.0% or more. On the other hand, if an excessive amount of Cr is contained, the grain boundary precipitation of Cr-based carbonitrides is promoted, and the corrosion resistance and toughness are reduced. For this reason, the Cr content is set to 25.0% or less. The Cr content is preferably 18.0% or less.

Ni (nickel): 4.0 to 22.0%
Ni forms intermetallic compounds together with Fe, Cr, Mo, or Si, and contributes to high strength, so the Ni content is set to 4.0% or more. Ni also contributes to improving the hydrogen embrittlement resistance of austenitic stainless steel. From the viewpoint of improving hydrogen embrittlement resistance, the Ni content is preferably 4.0% or more. By eliminating component segregation, these effects are further improved, so the Ni content is preferably 11.5% or more. On the other hand, an excessive amount of Ni causes an increase in material costs, so the Ni content is set to 22.0% or less. The Ni content is preferably 14.0% or less.

Mo (molybdenum): 5.0% or less Mo is an element that contributes to increasing the strength and corrosion resistance of austenitic stainless steel. However, the inclusion of Mo leads to an increase in alloy costs. Therefore, the Mo content is set to 5.0% or less. The Mo content is preferably 0.5% or less. On the other hand, Mo is an element that is inevitably mixed in from scrap raw materials. Since it is preferable to reduce the Mo content as much as possible, no lower limit is set, but excessive reduction of the Mo content leads to restrictions on the melting raw materials and increases the manufacturing cost. Therefore, in order to achieve both the above effects and manufacturability, the Mo content is preferably 0.05% or more.

Cu (copper): 0.10-4.0%
Cu is an element effective in stabilizing the austenite phase. Cu also contributes to increasing strength by Cu precipitation strengthening. Cu also improves the cold workability and hydrogen embrittlement resistance of the cast slab. On the other hand, if an excessive amount of Cu is contained, the strength of the austenite phase decreases, and the hot workability decreases. Therefore, the Cu content is set to 4.0% or less. The Cu content is preferably 3.55% or less. In order to obtain excellent cold workability and hydrogen embrittlement resistance, the Cu content is 0.10% or more. The Cu content is preferably 1.5% or more.

N (nitrogen): 0.4% or less N is an element effective for stabilizing the austenite phase, improving corrosion resistance, solid solution strengthening, and precipitation hardening of Cr-based nitrides, but if an excessive amount of N is contained, excessive production of Cr-based nitrides is promoted, which reduces the corrosion resistance, toughness, and hydrogen embrittlement resistance of the austenite phase. For this reason, the N content is set to 0.4% or less. The N content is more preferably 0.35% or less. On the other hand, in order to obtain the above-mentioned effects, the N content is preferably 0.01% or more. The N content is more preferably 0.04% or more.

The Cu-containing austenitic stainless steel cast slab according to this embodiment is composed of Fe and impurities in addition to the elements mentioned above, but preferably contains the following elements. The impurities referred to here mean components that are mixed in due to various factors in the raw materials such as ores and scraps and the manufacturing process when the Cu-containing austenitic stainless steel cast slab according to this embodiment is industrially manufactured, and are acceptable within a range that does not adversely affect the present invention. Examples of impurities include Pb, Sn, Zn, Bi, As, Sb, V, Ta, Co, W, Mg, and O (oxygen). The impurity contents are at levels that are acceptable, for example, Pb is 5 ppm or less, Sn is 0.03% or less, Zn is 10 ppm or less, Bi is 5 ppm or less, As is 100 ppm or less, Sb is 50 ppm or less, V is 0.3% or less, Ta is 0.02% or less, W is 0.3% or less, Co is 0.5% or less, Mg is 30 ppm or less, and O is 100 ppm or less.

The Cu-containing austenitic stainless steel slab according to this embodiment preferably contains the following elements in place of part of the Fe, as necessary.

One or more selected from the group consisting of Al (aluminum): 0.15% or less, Ca (calcium): 0.005% or less, and B (boron): 0.005% or less Al, Ca, and B are elements effective in deoxidizing and improving the hot workability and corrosion resistance of the cast slab. It is preferable that one or more elements selected from these are contained as necessary. However, if the above elements are contained in excess, it will lead to a significant increase in manufacturing costs. For this reason, it is preferable to set the Al content to 0.15% or less, the Ca content to 0.005% or less, and B: 0.005% or less. However, in order to fully obtain the deoxidizing effect, it is preferable to contain one or more of Al: 0.01% or more, Ca: 0.001% or more, and B: 0.001% or more. Note that Al may be contained as an impurity at 0.003% or less, and B and Ca may be contained as impurities at 0.0005% or less.

At least one of Nb: 0.3% or less and Ti: 0.3% or less Ti and Nb dissolve in the steel or precipitate as carbonitrides to increase the strength. Therefore, one or two selected from Nb and Ti may be contained as necessary. In this case, the Nb content and the Ti content are preferably 0.3% or less. From the viewpoint of obtaining the above effects of Nb or Ti, the Nb content and the Ti content are preferably 0.01% or more, more preferably 0.05% or more, and further preferably 0.1% or more.

In the Cu-containing austenitic stainless steel cast slab of the present invention, the remainder other than the elements mentioned above is Fe and impurities, but elements other than the elements mentioned above can also be included within a range that does not impair the effect of this embodiment.

[Cr _eq /Ni _eq : less than 1.48]
In the Cu-containing austenitic stainless steel cast strand according to this embodiment, while containing the above-mentioned chemical components, it is important that the ratio of Cr _eq to Ni _eq , Cr _eq /Ni _eq , calculated by the following formulas (1) and (2), is less than 1.48.

Cr _eq = Cr+(1.37×Mo)+(1.5×Si)…Formula (1)
Ni _eq = Ni+(0.31×Mn)+Cu+(22×C)+(14.2×N)…Formula (2)
In the formulas (1) and (2), Cr, Mo, Si, Ni, Mn, Cu, C, and N represent the contents (mass%) of each element in the cast slab.

If Cr _eq /Ni _eq is less than 1.48, the molten steel solidifies in austenite phase when cooled, and austenite phase single phase stainless steel is obtained. On the other hand, if Cr _eq /Ni _eq is 1.48 or more, the molten steel is cooled in a state of two phases, austenite phase and ferrite phase, and a stainless steel containing these two phases is obtained. Therefore, Cr _eq /Ni _eq is less than 1.48. Cr _eq /Ni _eq is preferably less than 1.35.

[The circle equivalent diameter of the region in which the α value of the thickness direction center in the casting direction cross section at the width direction center exceeds 3.63 is 1.5 mm or less]
In the Cu-containing austenitic stainless steel cast slab according to this embodiment, at the center in the thickness direction in the casting direction cross section at the center in the width direction, the equivalent circle diameter of a region where the α value exceeds 3.63 is 1.5 mm or less.

The α value is a value calculated by the following formula (3).
α=[Cu]+(0.136×[Mn])−(0.2×[Ni])…Formula (3)
In formula (3), [Cu], [Mn], and [Ni] are the concentrations (mass%) of each element measured at the center in the thickness direction by an electron probe microanalyzer (EPMA).

Mn is an element that makes Cu easier to liquefy, and Ni is an element that makes Cu more difficult to liquefy. Mn is an element that reduces the amount of Cu that can be dissolved in the gamma phase, and has the effect of promoting Cu precipitation and lowering the melting point of Cu. Therefore, Mn makes it easier for Cu to precipitate and liquefy. Ni has the effect of increasing the amount of Cu that can be dissolved in the gamma phase, and prevents the precipitation of Cu at a low melting point. Therefore, Ni makes it more difficult for liquefaction to occur due to Cu.

In regions where the α value exceeds 3.63, Cu is likely to liquefy. If the area of such regions becomes large, cracks are likely to occur. Therefore, the circle equivalent diameter of regions where the α value exceeds 3.63 is set to 1.5 mm or less. In the following, regions where the α value exceeds 3.63 and the circle equivalent diameter exceeds 1.5 mm are referred to as "semi-macro segregation regions."

With regard to the thickness direction center of the casting direction cross section at the width direction center, the casting direction cross section at the width direction center refers to the cross section when the slab is cut in the casting direction at a position W/2 from the width direction end of the slab, where W is the width of the slab. The thickness direction center refers to a rectangular region with a length in the thickness direction of the slab and a length in the casting direction of 25 mm, whose center is located at a position 1/2 of the thickness of the slab in that cross section. In the EPMA analysis, the thickness direction center of the casting direction cross section at the width direction center is analyzed.

The EPMA analysis is performed at 500 x 500 points in the above 25 mm square range with an acceleration voltage of 15 kV, an irradiation current of 3.25 x ¹⁰⁷ A, a beam diameter of 50 μm, a step of 50 μm, and an irradiation time of 40 msec per step. The EPMA analysis identifies the area where the α value is greater than 3.63. In the EPMA, the concentrations of Cu, Ni, and Mn are measured for each measurement point, so that the α value for each measurement point can be calculated. Therefore, the area obtained by connecting the measurement points where the α value is greater than 3.63 is regarded as the area where the α value is greater than 3.63. Note that when the measurement points where the α value is greater than 3.63 are connected, the measurement points where the α value is 3.63 or less may be included in the area obtained by connecting the measurement points where the α value is greater than 3.63. In such a case, the area where the measurement points where the α value is 3.63 or less are omitted is regarded as the area where the α value is greater than 3.63.
Then, the circle-equivalent diameter of the identified region is calculated. When the measurement points having an α value of more than 3.63 are connected, if a measurement point having an α value of 3.63 or less is included in the region obtained by connecting the measurement points having an α value of more than 3.63, the circle-equivalent diameter is calculated excluding the region having an α value of 3.63 or less.

[β value: 2.51 or less]
The Cu-containing austenitic stainless steel cast slab according to this embodiment has a β value calculated by the following formula (4) of 2.51 or less.

β=Cu+(0.0075×Mn ² )+(0.0404×Mn)−(0.09×Ni)…Formula (4)
In formula (4), Cu, Mn, and Ni represent the contents (mass%) of each element in the cast slab.

If the β value exceeds 2.51, semi-macro segregation will occur, and cracks will occur in this semi-macro segregation. Therefore, the β value should be 2.51 or less.

[Equiaxed crystal rate: over 30%]
The equiaxed crystal ratio of the Cu-containing austenitic stainless steel slab according to the present embodiment is preferably more than 30%. The equiaxed crystal ratio is the thickness of the equiaxed crystal portion relative to the thickness of the slab at a position halfway in the width direction of the plane (C cross section) obtained by cutting the slab along a plane perpendicular to the casting direction. The presence or absence of equiaxed crystals is determined from the macrostructure of the C cross section. The macrostructure is revealed by the hydrochloric acid method recommended for stainless steel in accordance with JIS G 0553:2019. In the hydrochloric acid method, the stainless steel is immersed in hydrochloric acid with a concentration of 20% by mass heated to 70°C to corrode it. The corrosion time (immersion time) is 40 minutes.

Equiaxed crystals are isotropic crystal structures that have no particular orientation and are observed in the center of the macrostructure of the C-section. Equiaxed crystals are clearly distinguished from chill crystals and columnar crystals that exist outside of them in the C-section.

If the equiaxed crystal ratio is more than 30%, the central segregation of Cu can be suppressed. Therefore, the equiaxed crystal ratio is preferably more than 30%. The equiaxed crystal ratio may be more than 30%, for example, 40% or more, or 50% or more.
On the other hand, the upper limit of the equiaxed crystal ratio is not particularly limited, and may be 80% or less, or 70% or less.

<Method for producing Cu-containing austenitic stainless steel slab>
Next, a method for producing the above-mentioned Cu-containing austenitic stainless steel cast slab will be described. The method for producing the Cu-containing austenitic stainless steel cast slab described below is merely one example, and other methods may be used for production.

The Cu-containing austenitic stainless steel slab according to this embodiment is produced by continuous casting. In detail, molten stainless steel having the above-mentioned chemical composition is produced by a known method, and the molten steel is supplied to a tundish. The molten steel contained in a ladle is supplied to the tundish, and the molten steel is supplied into the mold from a nozzle extending from the lower end of the tundish toward the mold. The molten steel is cooled in the mold (primary cooling), and the molten steel withdrawn from the mold is cooled by a secondary cooling zone arranged below the mold to produce a slab.

The withdrawal speed Vc of the molten steel from the mold is set to 0.4 to 0.6 m/min, and the superheat ΔT of the molten steel is set to 20 to 40°C. The superheat ΔT is the degree of superheat from the melting point of the steel, and is obtained by measuring the temperature of the molten steel in the tundish. If the superheat ΔT is 20 to 40°C, fine equiaxed grains are generated, and Cu segregation can be suppressed.

In the secondary cooling zone, spray cooling is performed by spraying water to cool the molten steel. The specific water amount, which is the amount of water per 1 kg of molten steel, is 3 to 5 L/(kg-steel), and spray cooling is performed until the temperature at the center of the thickness of the slab falls below 800°C. If the specific water amount is 3 to 5 L/(kg-steel), it is possible to reduce significant central segregation and increase the cooling rate to suppress the precipitation and growth of Cu, resulting in a slab that is advantageous in preventing Cu liquefaction. In addition, by continuing spray cooling until the temperature at the center of the thickness of the slab falls below 800°C, it is possible to suppress fine internal cracks that occur when the surface of the slab expands due to thermal recovery.

In the secondary cooling zone, a strand electromagnetic stirring device may be used to stir the molten steel inside the slab without contact. The strand electromagnetic stirring device is used to stir the molten steel in a portion of the slab where the solid fraction in the thickness direction cross section is 0.5 to 0.75. The stirring is performed by applying electromagnetic force to each of a pair of long sides in the thickness direction cross section of the slab so that the molten steel near each long side flows in the same direction. The electromagnetic force is applied in a repeated pattern of applying electromagnetic force for six seconds and removing the electromagnetic force for one second, for example. By applying the strand electromagnetic stirring device to the secondary cooling zone as described above, the equiaxed crystal fraction of the slab can be improved. As a result, Cu segregation can be suppressed.

The produced Cu-containing austenitic stainless steel cast slab has suppressed internal defects and is less likely to develop internal cracks during hot working, despite the inclusion of Cu. Furthermore, the Cu-containing austenitic stainless steel cast slab according to the present invention also has excellent cold workability due to the inclusion of Cu.
Furthermore, by using the Cu-containing austenitic stainless steel cast slab according to this embodiment, whose chemical composition is controlled within the above-mentioned range of chemical composition, it is possible to produce austenitic stainless steel that has excellent hydrogen embrittlement resistance and can be used in a high-pressure hydrogen environment, or a high-strength non-magnetic steel that has excellent corrosion resistance and can be used in applications where non-magnetism is required.

As an example, the above-mentioned manufacturing method for Cu-containing austenitic stainless steel slabs can achieve an equiaxed crystal ratio of 30% or more.

The following provides a detailed explanation of the embodiment of the present invention, with reference to examples. Note that the examples shown below are merely examples of the present invention, and the present invention is not limited to the examples below.

Using stainless steel materials having the chemical compositions shown in Tables 1 and 2, cast pieces having a thickness of 200 mm and a width of 1250 mm were produced under the following conditions.
- Withdrawal speed of molten steel from the mold Vc: 0.4 to 0.6 m/min
・Superheat degree ΔT: 20~40℃
・Secondary cooling specific water volume: 3-5L/(kg-steel)
・Electromagnetic stirring position: Part where the solid fraction in the thickness direction cross section of the slab is 0.5 to 0.75 ・Flow mode: Translational flow in which the molten steel near the long side flows in the same direction ・Electromagnetic force application pattern: Application for 6 seconds, removal for 1 second ・Secondary cooling temperature: Up to 800°C

For each of the cast pieces, the ratio of Cr _eq to Ni _eq (Cr _eq /Ni _{eq )} , the β value, and the equiaxed crystal rate were calculated, and the presence or absence of semi-macro segregation portions was confirmed.

Cr _eq is a value calculated by the following formula (1), and Ni _eq is a value calculated by the following formula (2).

Cr _eq = Cr+(1.37×Mo)+(1.5×Si)…Formula (1)
Ni _eq = Ni+(0.31×Mn)+Cu+(22×C)+(14.2×N)…Formula (2)
Here, in formulas (1) and (2), Cr, Mo, Si, Ni, Mn, Cu, C, and N represent the content (mass %) of each element.

The β value is calculated by the following formula (4).
β=Cu+(0.0075×Mn ² )+(0.0404×Mn)−(0.09×Ni)…Formula (4)
In formula (4), Cu, Mn, and Ni are the contents (mass%) of each element.

The presence or absence of semi-macro segregation was confirmed by the following method. First, the above-mentioned slab was cut in the casting direction of the slab at a position 625 mm from the width-direction end, and an analytical test piece was cut out so that the center was located at 1/2 the thickness of the slab on the cross section, the length in the thickness direction and the length in the casting direction were 30 mm, and the thickness (length in the width direction of the slab) was 10 mm. Therefore, the analysis surface was a surface at the center of the width direction of the slab, including the casting direction and the thickness direction of the slab. The analysis region was a 25 mm square region whose center was located at 1/2 the thickness of the slab on the above-mentioned analysis surface. The EPMA analysis was performed using an electron probe microanalyzer (JXA-8230) manufactured by JEOL Ltd., with an acceleration voltage of 15 kV, an irradiation current of 3.25×10 ⁷ A, a beam diameter of 50 μm, a step of 50 μm, and an irradiation time for one step of 40 msec. The Cu content [Cu], Mn content [Mn] and Ni content [Ni] were measured at 500 x 500 points in the center of the thickness direction. An area with an α value of more than 3.63 was identified by EPMA analysis. The circle equivalent diameter of the identified area was then calculated.

α=[Cu]+(0.136×[Mn])−(0.2×[Ni])…Formula (3)

When a region was identified in which the α value calculated by formula (3) exceeded 3.63 and the area of the region exceeded 1.5 mm, it was determined that a semi-macro segregation region was present.

The equiaxed crystal ratio of the sample was calculated as follows. That is, the sample was immersed in 20% by mass hydrochloric acid heated to 70°C for 40 minutes to reveal the macrostructure in accordance with JIS G 0553:2019. The thickness of the equiaxed crystal portion relative to the thickness of the slab at the center of the width of the C cross section was taken as the equiaxed crystal ratio.

Hot workability was evaluated by the following method. A test piece was taken from the prepared sample, with its center located 625 mm from the end in the width direction, and with a length in the casting direction and the length in the cast piece width direction of 200 mm. The thickness of the test piece was 200 mm. The test piece was hot rolled to evaluate the hot cracking tendency at the center of the thickness. In detail, the test piece was heated at 1200°C for 60 minutes, and then removed from the furnace and hot rolling was started. The reduction in area was 20% or more, and the finishing temperature of rolling was 1050°C or more.

When a crack occurred at the center of thickness during hot rolling, the test piece was opened in two plates, and hot rolling was stopped, the evaluation result was poor (×). In addition, the test piece after hot rolling was cut out at the rolling direction cross section at the width center, and the crack was revealed by PT (penetrant testing), and the crack length was measured. When a crack with a width direction length of 10 mm or more occurred from the cross section at the thickness center in the rolling direction cross section at the rolling direction cross section at the thickness center, it was judged as poor (×). When hot rolling was completed, a crack was revealed by PT on the rolling direction cross section at the width center of the test piece, and the crack length was measured, and when the length of the crack from the surface layer at the thickness center was less than 10 mm, it was judged as good (◯).
The results are shown in Table 3.

As shown in Tables 1 to 3, the Cu-containing austenitic stainless steel slabs according to the present invention prevented internal cracks during hot working.

Although the preferred embodiment of the present invention has been described in detail above, the present invention is not limited to such examples. It is clear that a person with ordinary knowledge in the technical field to which the present invention pertains can conceive of various modified or revised examples within the scope of the technical ideas described in the claims, and it is understood that these also naturally fall within the technical scope of the present invention.

Claims (4)

In mass percent,
C: 0.10% or less,
Si: 3.0% or less,
Mn: 0.1 to 20.0%,
P: 0.045% or less,
S: 0.0050% or less,
Cr: 10.0-25.0%,
Ni: 4.0 to 22.0%,
Mo: 5.0% or less,
Cu: 2.00 to 4.0%, and
N: 0.4% or less,
with the remainder being Fe and impurities,
The ratio of Cr _eq represented by the following formula (1) to Ni _eq represented by the following formula (2), Cr _eq /Ni _eq , is less than 1.48,
In the thickness direction center of the casting direction cross section at the width direction center, there is a region in which the α value represented by the following formula (3) exceeds 3.63,
The region has a circular equivalent diameter of 1.5 mm or less,
A Cu-containing austenitic stainless steel cast slab having a β value represented by the following formula (4) of 2.51 or less.
Cr _eq = Cr+(1.37×Mo)+(1.5×Si)…Formula (1)
Ni _eq = Ni+(0.31×Mn)+Cu+(22×C)+(14.2×N)…Formula (2)
α=[Cu]+(0.136×[Mn])−(0.2×[Ni])…Formula (3)
β=Cu+(0.0075×Mn ² )+(0.0404×Mn)−(0.09×Ni)…Formula (4)
In the formulas (1), (2), and (4), Cr, Mo, Si, Ni, Mn, Cu, C, and N represent the contents (mass%) of each element, and in the formula (3), [Cu], [Mn], and [Ni] represent the concentrations (mass%) of each element measured by an electron probe microanalyzer at the center in the thickness direction. The Cu-containing austenitic stainless steel cast piece according to claim 1, in which the equiaxed crystal ratio is greater than 30%. Instead of a part of Fe,
Al: 0.15% or less,
Ca: 0.005% or less; and
B: 0.005% or less,
The Cu-containing austenitic stainless steel cast slab according to claim 1 or 2, comprising one or more selected from the group consisting of: Instead of a part of Fe,
Nb: 0.3% or less, or
Ti: 0.3% or less,
The Cu-containing austenitic stainless steel cast slab according to any one of claims 1 to 3, comprising at least one of the following: