JP5063024B2 - Method of casting alloy steel containing Cr and Ni - Google Patents

Description

INDUSTRIAL APPLICABILITY The present invention can prevent or reduce pinholes generated in the surface part of a slab during casting of alloy steel containing Cr and Ni , and surface flaws generated during processing such as rolling and forging due to this. The present invention relates to a casting method.

  Conventionally, when stainless steel or high alloy steel is cast and then rolled or forged in the lump process, streaky surface defects extending linearly in the rolling direction may occur. When such wrinkles occur, the wrinkles remain or expand in subsequent product rolling and cannot be shipped as a product. Therefore, it is necessary to carry out care at the stage of the billet billet, resulting in reduced yield and maintenance costs associated with this care. Was a problem.

  Recently, with the aim of increasing the size of industrial equipment and structures such as chemical plants and offshore structures, there is an increasing demand for improving the strength of structural stainless steel. Nitrogen (N) is added accordingly. In addition, a steel type whose strength is improved by its solid solution strengthening action is known. For example, austenitic stainless steel such as SUS304N2, SUS304LN, SUS316N, and SUS316LN. Patent Document 1 contains 0.18 to 0.25% by mass of nitrogen, which is a solid solution strengthening component, for the purpose of application under harsh environmental conditions, enhances strength and corrosion resistance, and performs hot working. In order to improve the properties, a high nitrogen stainless steel is disclosed in which the B and O contents are defined, and the Nb content is defined from the balance between improving the strength and ensuring hot workability.

  As mentioned above, high alloy steels and stainless steels have been developed with high nitrogen concentrations for various reasons, but some of these steel types contain excessive amounts of gas components such as nitrogen. Therefore, these gas components may exceed the solubility during solidification to generate pinholes. If such a pinhole is present, the pinhole may be enlarged during rolling, which may cause surface defects such as billets, coils, and products after rolling.

  Stainless steel is often used as an exterior material for various buildings and transportations without taking advantage of its excellent corrosion resistance and beauty, but it is not suitable for coating or plating. If there is, there is a problem in that it remains as a streak after rolling and impairs surface gloss and the like. On the other hand, as in Patent Document 2, the casting conditions such as the discharge flow rate from the immersion nozzle, the mold cross-sectional area and the discharge nozzle angle of the immersion nozzle, and the rolling conditions such as the furnace time during hot rolling are specified. Have been disclosed. However, as disclosed in Patent Document 2, surface flaws that can be prevented by optimizing the casting conditions such as the molten steel supply conditions into the mold and the slab heating conditions are minute floats floating in the molten steel in the mold. It is a pinhole based on bubbles. Therefore, even if the technique disclosed in Patent Document 2 is applied, there is a problem that no reduction effect can be obtained for the generation of pinholes that are generated longer than the solubility as the solidification progresses.

  As described above, although many techniques for improving and improving steel quality by focusing on the product characteristics and surface properties of high alloy steel and stainless steel have been disclosed, nitrogen and hydrogen dissolved in molten steel (H) There are no reports on pinholes generated during solidification due to the above, or improvement of surface defects generated during hot working due to pinholes.

JP-A-6-293942 (Claims and paragraph [0016]) JP 2003-10946 (Claims and paragraph [0016])

As mentioned above, the cause of surface defects that occur in rolling or forging of stainless steel or high alloy steel after casting is that the gas in the steel becomes longer than the solubility as the solidification progresses. There is a problem that a reduction effect cannot be obtained by the conventional generation prevention technology. The present invention has been made in view of the above problems, and the problem is likely to occur in the slab surface layer when casting alloy steel containing Cr and Ni such as stainless steel or high alloy steel. The object of the present invention is to prevent the generation of the pinholes and to prevent the generation of streak-like surface defects extending in the processing direction that are likely to occur on the billet surface during rolling or forging in the lump process.

  In order to solve the above-mentioned problems, the present inventors examined the actual condition and generation mechanism of linearly-extending surface defects generated by rolling or forging after casting of stainless steel or high alloy steel, We studied the method of preventing the generation of pinholes in the slab surface layer, which is the cause of generation, and the generation of wrinkles on the billet surface, and obtained the following knowledge (a) to (d) to complete the present invention. .

  (A) The occurrence of surface flaws such as billets varies depending on the steel type, and the results of investigations on the solidification behavior of steel and the solubility of nitrogen (N) and hydrogen (H) in steel by equilibrium calculation based on thermodynamics In addition, problems such as generation of surface defects occur only in a steel type that once generates a δ phase in the solidification process and has a nitrogen content exceeding the solubility of nitrogen in the δ phase.

  (B) A pinhole is formed in the surface layer portion of the slab of the steel type in which the problem (a) occurs. This pinhole extends longer than 10 mm substantially perpendicularly to the surface of the slab and exhibits a very characteristic shape.

  (C) Despite the occurrence of defects such as flaws in steel types with a high nitrogen content, the generation of pinholes at the slab stage can be prevented by reducing the H content to less than 10 ppm. it can.

  (D) By reducing the sulfur (S) content in the steel to less than 20 ppm, it is possible to reliably prevent the formation of billet surface flaws.

The present invention has been completed based on the above findings, and the gist thereof is a casting method of alloy steel containing Cr and Ni shown in the following (1) and ( 2 ).

(1) Containing Cr and Ni, the value obtained by dividing Cr equivalent represented by the following formula (1) by Ni equivalent represented by the following formula (2) in mass% is 1.5 or more, and solidification once generated the δ phase in the process, a method of casting an alloy steel having a nitrogen content of greater than nitrogen solubility in the δ phase,
The hydrogen content in the molten steel is less than 10 ppm, and the sulfur content in the molten steel is 7 ppm or less ,
A method for casting an alloy steel containing Cr and Ni, comprising producing a slab in which a pinhole extending approximately 10 mm or more substantially perpendicular to the surface is not formed.
Cr equivalent = Cr + 1.37 × Mo + 1.5 × Si + 2 × Nb + 3 × Ti (1)
Ni equivalent = Ni + 22 × C + 0.31 × Mn + 14.2 × N + Cu (2)
Here, the element symbols in the formulas (1) and (2) represent the content (mass%) of each element in the steel.

( 2 ) The method for casting alloy steel containing Cr and Ni according to (1 ) above, wherein the casting method is a continuous casting method.

  In the present invention, “high alloy steel” means steel having a total content of alloy elements such as Cr and Ni of about 40% by mass or more.

  “Steel that once generates δ phase during the solidification process” means a steel that once crystallizes the ferrite (δ) phase from the melt during the solidification process. This corresponds to steel having a value divided by about 1.5 or more. Here, the Cr equivalent and the Ni equivalent can be obtained by, for example, the following formulas (1) and (2).

Cr equivalent = Cr + 1.37 × Mo + 1.5 × Si + 2 × Nb + 3 × Ti (1)
Ni equivalent = Ni + 22 × C + 0.31 × Mn + 14.2 × N + Cu (2)
Here, the element symbols in the formulas (1) and (2) represent the content (mass%) of each element in the steel.

  In the following description, “% by mass” for the component composition of steel is also expressed as “%”.

According to the method of the present invention, the hydrogen content in the molten steel of the alloy steel containing Cr and Ni having a nitrogen content exceeding the nitrogen solubility in the δ phase once generated in the solidification process is less than 10 ppm. By adjusting and casting, it is possible to prevent the generation of pinholes that are likely to occur in the slab surface layer. Furthermore, it is possible to reduce the generation of linear wrinkles that are likely to occur on the surface of billets or the like during processing such as rolling or forging due to the generation of pinholes. In addition to this, by making the sulfur content less than 20 ppm, it is possible to further reduce the occurrence of surface flaws on the product and improve the surface quality.

As described above, the present invention provides a method for producing hydrogen in molten steel when casting an alloy steel containing Cr and Ni, which once generates a δ phase in the solidification process and has a nitrogen content exceeding the nitrogen solubility in the δ phase. This is a method for casting an alloy steel containing Cr and Ni, which is cast by adjusting the content to less than 10 ppm or in addition to adjusting the sulfur content in the molten steel to less than 20 ppm. Below, the casting method of this invention is demonstrated in detail.

(1) Generation mechanism of surface flaws The present inventors investigated the generation state of surface flaws on a billet of alloy steel containing Cr and Ni and examined the generation mechanism.

  As a result, it was found that there is a dependence on the steel type in the occurrence of defects, and there are steel types that do not generate surface defects at all and steel types that generate surface defects at a high rate even under the same production conditions. . Furthermore, assuming that there is a possibility that defects at the slab stage may be the starting point of surface defects, the surface properties of the slab were investigated. It became clear that a pinhole was generated in the part. It has also been found that the pinholes generated in these steel types have a needle-like shape extending longer than 10 mm substantially perpendicular to the surface of the slab.

  In general, the shape of the pinhole generated in the slab is affected by solidification, so that it is generally spherical or nearly spherical, although there are some irregularities. On the other hand, it is confirmed that the pinholes generated in the steel types generated with a high ratio of surface defects as described above have a needle-like characteristic shape extending substantially perpendicular to the slab surface. It was done.

  Usually, the pinhole is generated by trapping bubbles already existing in the molten steel in the solidified shell, and in this case, the pinhole has a spherical shape or a shape close to a spherical shape depending on the surface tension of the molten steel. On the other hand, the pinhole in the shape of a long needle extends, the dissolved gas in the molten steel exceeds the saturation solubility on the solid phase side as solidification progresses, and is discharged to the liquid phase side to form bubbles, It is considered that the bubbles are elongated while the dissolved gas is discharged along with the progress of the solidification interface, and this is trapped in the solid phase to form a needle-like shape. Since the molten steel static pressure acts at the center of the slab during solidification, the generation of bubbles is suppressed.

  Therefore, using a commercially available thermodynamic equilibrium calculation program (program name “Fact-Sage”), the solidification behavior of the steel and the solubility of nitrogen and hydrogen, which are gas components in the steel considered to be caused by the occurrence of pinholes, were determined. investigated. Table 1 shows the component composition of 9% Cr-containing steel as an example of the examination results and SUS310 steel containing 20% Ni and 25% Cr.

  FIG. 1 is a diagram showing calculation results of the phase transformation behavior in the solidification process of 9% Cr-containing steel and the solubility of nitrogen and hydrogen. FIG. 1 (a) shows the relationship between the temperature and the abundance ratio of each phase. (B) shows the relationship between temperature and nitrogen solubility in each phase, and (c) shows the relationship between temperature and hydrogen solubility in each phase. Also. FIG. 2 is a diagram showing calculation results of the phase transformation behavior in the solidification process of SUS310 steel and the solubility of nitrogen and hydrogen. FIG. 2 (a) shows the relationship between the temperature and the abundance of each phase. (B) shows the relationship between temperature and nitrogen solubility in each phase, and (c) shows the relationship between temperature and hydrogen solubility in each phase.

  As shown in the results of FIG. 1, in the 9% Cr-containing steel, a δ phase is once generated during the solidification process and then transformed into a γ phase. Both nitrogen solubility and hydrogen solubility in the δ phase are significantly lower than the solubility in the liquid phase or the γ phase, and its minimum value is about 290 ppm. In 9% Cr-containing steels, the basic components are almost the same, and there are steel types containing about 200 ppm of nitrogen and steel types containing about 400 to 500 ppm of nitrogen, depending on the application, but about 400 to 500 ppm of nitrogen. The problem of pinholes and billet surface flaws occurs only in steel types that contain. This is thought to be due to the fact that in a steel type with a high nitrogen content, once the δ phase is generated, the nitrogen content in the steel exceeds the nitrogen solubility in the δ phase.

  On the other hand, as shown in the results of FIG. 2, in SUS310 steel, a γ phase is generated directly from the liquid phase and solidifies. The nitrogen solubility in the γ phase is lower than the nitrogen solubility in the liquid phase, but is about 1900 ppm immediately after the start of solidification. Therefore, although steel contains about 1000 ppm of nitrogen, the nitrogen solubility is not exceeded, and the problem of pinhole and billet surface flaws does not occur.

  Nitrogen solubility is greatly affected and changed by differences in steel types such as Cr content, but no significant difference is observed in hydrogen solubility. As a result of similar investigations with various steel types, all of the steel types where surface defects occur at a high rate are those in which the δ phase is once generated during the solidification process, and have a nitrogen content exceeding the nitrogen solubility in the δ phase. It turned out to have.

(2) Appropriate content of hydrogen and sulfur for reducing surface soot generation The effect of nitrogen, hydrogen and sulfur content on the generation of billet surface soot was investigated. FIG. 3 is a diagram showing the relationship between the surface flaw occurrence state of a partially rolled billet produced from a continuous cast slab of 9% Cr-containing steel and the nitrogen content and hydrogen content in the steel. The billet surface flaw occurrence status is 0 for no-care, 1 for light-care, 2 for heavy-care, and 3 for non-care, for each billet. Are averaged as “Billette average surface wrinkle index” and shown in the figure.

  According to the results of the figure, as described above, the nitrogen content in the steel is roughly classified into those exceeding the nitrogen solubility of the δ phase and those not exceeding according to the component specifications, and the nitrogen content is 420 to In the case of a steel type that is about 520 ppm and exceeds the δ-phase nitrogen solubility (about 290 ppm) and has a high hydrogen content, the soot generation rate is increased. That is, even if it is a steel type once the δ phase is generated and the nitrogen content exceeds the nitrogen solubility in the δ phase, billet surface flaws may or may not occur. It was found to be in the hydrogen content.

  As described above, by reducing the hydrogen content to less than 10 ppm, the generation of pinholes at the slab stage can be prevented, and the billet surface flaw can be greatly reduced. Furthermore, if the hydrogen content is reduced to less than 8 ppm, the generation of pinholes and billet surface flaws can be prevented more reliably.

  As described above, the reason why the generation of billet surface flaws can be reduced by reducing the hydrogen content even if the steel content exceeds the nitrogen solubility in the δ phase is presumed as follows.

  That is, when the content of nitrogen or hydrogen in steel exceeds their solubility, hydrogen is considered to form bubbles more easily than nitrogen. When a δ phase is generated during the solidification process and the hydrogen content exceeds its solubility, hydrogen bubbles are generated. Under the situation where hydrogen bubbles already exist, nitrogen is easily released into the hydrogen bubbles and easily grows in a needle shape as the solidification progresses. On the other hand, even if only nitrogen exceeds the solubility in the δ phase, there is no space (bubbles) for accepting the release of nitrogen gas, so pinholes are not easily formed, and therefore surface flaws are generated. It does not lead to. Further, even if only a high concentration of hydrogen is contained in a steel type having a low nitrogen content, the hydrogen content is originally as low as about 10 ppm, so that it does not grow into a pinhole having a problem size.

  For the above reason, when the hydrogen content is less than 10 ppm, since there are few spaces (bubbles) from which nitrogen gas is easily released, the generation of pinholes is suppressed, and as a result, billet surface wrinkles are also reduced. Inferred.

  The nitrogen content varies in the range of 420 to 520 ppm, but no particular correlation was observed between the nitrogen content in this range and the occurrence of billet surface flaws.

  Although the hydrogen content in steel can be reduced by vacuum treatment, it requires strengthening of the degree of vacuum and extension of the vacuum treatment time, which are factors in inhibiting productivity. Therefore, rather than these, flux materials such as lime, which is a source of hydrogen in steel, moisture management of tundish powder, mold powder, etc., sufficient drying of moisture used in refractory construction such as tundish and ladle Various refining or casting such as removal, improvement of refractory construction method, and operation management to avoid injecting steel grades that are likely to cause pinholes into tundish lined with new refractory immediately after construction It is practical to adopt methods for optimizing the conditions for reducing the hydrogen concentration, and these methods can reduce the hydrogen content in the molten steel to less than 8 ppm.

  As described above, billet surface defects were greatly reduced by reducing the hydrogen content in the steel, but the degree of care still varied, so this point was investigated in more detail. . As a result of statistically treating various factors such as the composition of steel, casting conditions, and ingot rolling conditions on the occurrence rate of billet surface flaws, and examining the effects of each, the results of the sulfur content in steel The involvement was found to be significant.

  FIG. 4 is a diagram showing the influence of the hydrogen content and the sulfur content in steel on the surface flaw generation rate of a block-rolled billet manufactured using a continuous cast slab of 9% Cr-containing steel.

  From the results shown in the figure, the surface soot generation rate can be reduced by setting the hydrogen content to less than 10 ppm, and further the surface soot generation rate can be further reduced by making the sulfur content less than 20 ppm. It has been found that the billet surface quality can be improved reliably. That is, by reducing the sulfur concentration in the steel, it is possible to more reliably prevent the formation of linear soot. This is considered to be due to the fact that the surface tension of the molten steel is increased and bubbles are less likely to be generated by reducing the content of sulfur having a surface active action.

The method for preventing surface flaws of alloy steels containing Cr and Ni according to the present invention is highly effective not only for the surface flaws of billets but also for steel materials of other shapes such as plate materials and shaped steels.

In order to confirm the effect of the casting method of the alloy steel containing Cr and Ni of the present invention, the following test was conducted and the result was evaluated.

(1) Test method Molten steel obtained by melting scrap or the like using an 80-ton electric furnace or blowing blast furnace hot metal with a converter is degassed with AOD and VOD, and some steel types Then, secondary refining was performed using the ladle refining method in which alloy components were added by wire supply and gas bubbling was performed, and the mother molten steel of five basic steel types was melted.

  The molten steel was supplied to a vertical continuous casting machine and cast at a casting speed of 0.4 m / min to produce a slab having a thickness of 280 mm and a width of 600 mm. This slab is transported to a lump factory, heated to about 1200 ° C in a soaking furnace, then rolled into a round billet with a diameter of 190 to 300 mm, and the occurrence of wrinkles on the billet surface is visually examined. did. For some steel types, a slab having a thickness of 390 mm and a width of 700 mm is manufactured by casting at a casting speed of 0.4 m / min using a curved continuous casting machine, and then, similarly, the block rolling is performed. However, there was no particular effect on billet surface flaws due to differences in the type of continuous casting machine.

  Table 2 shows the main component composition, solidification morphology, nitrogen solubility in the solidification process determined by thermodynamic equilibrium calculation, and the minimum value of the hydrogen solubility of the five basic steel types examined.

The commercially available calculation program was used for the thermodynamic equilibrium calculation. Equilibrium calculation at each temperature was performed in a temperature range of 1200 to 1600 ° C. including the solidification process in an atmosphere of 1.013 × 10 ⁵ Pa (1 atm) to obtain nitrogen solubility and hydrogen solubility in each phase.

  Table 2 also shows the presence / absence of billet surface flaws in the normal operation process. There are also steel types that contain Al, Ti, Ca, V, B, etc. in addition to the components in the table, but they are omitted because they do not affect the occurrence of billet surface flaws due to pinholes. did.

(2) Effect Confirmation Test of Nitrogen Solubility in δ Phase and Nitrogen Content in Molten Steel on Surface Flaw Generation As described above, steel type number 1 containing about 400 to 500 ppm of nitrogen in 9% Cr-containing steel depending on the application And a steel type No. 2 containing about 200 ppm of nitrogen. Since the basic component systems are almost the same, a δ phase is once generated in the solidification process, and the nitrogen solubility is about 290 ppm. However, the problem of pinholes and billet surface flaws occurs only in the steel type No. 1 containing about 400 to 500 ppm of nitrogen.

  Therefore, as a result of taking a sample of the pinhole generating portion from the surface layer portion of the slab and observing the cross section, the pinhole has a diameter of about several hundred μm and a length of 20 to 30 mm in a direction substantially perpendicular to the slab surface. It has been found that it has grown to a long extent. As described above, generally, a pinhole generated by trapping bubbles present in molten steel in a solidified shell has a spherical shape or a nearly spherical shape. On the other hand, the needle-like pinhole extended long as in the above observation results, the dissolved gas in the steel exceeding the saturation solubility with the progress of solidification is discharged to the liquid phase side to form bubbles, As the solidification interface progresses, the bubbles grow into an elongated shape, which indicates that they have formed a needle shape while being trapped by the solid phase.

  Further, in the steel type No. 3, the δ phase once forms during the solidification process, and the nitrogen content in the steel exceeds the nitrogen solubility of the δ phase. Also in this steel type, pinholes having a shape similar to that of steel type number 1 may be generated, and billet surface defects may occur.

  On the other hand, the steel grades Nos. 4 and 5 having a value obtained by dividing the Cr equivalent by the Ni equivalent of about 1.5 or less both generate a γ phase directly from the liquid phase, and the δ phase in the solidification process. Do not generate. For this reason, although the nitrogen concentration in steel is high, the solubility of nitrogen is not exceeded during the solidification process, and pinholes and billet surface flaws are not generated.

In addition, the same casting test was performed on various stainless steels and high alloy steels as alloy steels containing Cr and Ni . However, in all steel types where surface defects occur, a δ phase is once generated during the solidification process. And the nitrogen content exceeds the nitrogen solubility in the δ phase. Based on the above results, the present invention is directed to alloy steels containing Cr and Ni that once generate a δ phase in the solidification process and have a nitrogen content exceeding the nitrogen solubility in the δ phase.

(3) Effect confirmation test of hydrogen content rate and sulfur content rate For the steel type No. 1 and No. 3 steel types shown in Table 2 above, a test to investigate in more detail the relationship between the composition of molten steel and the state of occurrence of soot went.

  Table 3 shows the results of investigation of the content of hydrogen, nitrogen and sulfur in the steel types No. 1 and No. 3 and the pinhole generation status on the slab surface and the billet surface flaw generation status.

  Here, the contents of hydrogen, nitrogen and sulfur in the steel types No. 1 and No. 3 were adjusted by the following method. That is, hydrogen strengthens the moisture management of refractories and fluxes that come into contact with molten steel, such as reducing the amount of moisture contained in the CaO source input during refining, and sufficiently drying the refractory spray material used for tundish. Was reduced. Nitrogen was changed by the amount of nitrogen gas blown in ladle refining, and sulfur was changed by controlling the refining conditions in AOD and VOD.

  Further, in the same table, the pinhole was ground with a grinder for about 1 m in the longitudinal direction of the slab, and the scale was removed. “None” in the evaluation column means that no pinhole was found in the above grinding range, “decimal” means that there were less than 10 pinholes in the same range, and “Many” means that 10 or more existed in the same range.

  The value in the billet surface defect index column is 0 for each billet, 1 for light care, 2 for heavy care, 3 for non-care, and averaging these values for each heat of casting. Means a normalized value.

Test Nos. 1 to 5 use the steel type No. 1 steel type, that is, the steel type whose minimum nitrogen solubility is about 290 ppm as shown in Table 2 and whose nitrogen content exceeds the minimum nitrogen solubility, and hydrogen in the molten steel. This is a test for a reference example of the present invention using a molten steel having a content of less than 10 ppm, and test numbers 6 and 7 are tests for a comparative example using a molten steel having a hydrogen content of more than 10 ppm in the molten steel. is there.

In test numbers 1 to 5 of the reference examples of the present invention in which the hydrogen content was less than 10 ppm, the occurrence of pinholes was reduced, billet surface wrinkles were improved, and the billet surface wrinkle index showed a low value. In addition, among the test numbers 1 to 5 of the reference example of the present invention , in test numbers 1 to 3 in which the sulfur content is less than 20 ppm, the surface defects are further improved. In particular, in test number 1 in which the hydrogen content was reduced to less than 8 ppm, the condition of surface flaws was improved to such an extent that almost no maintenance was required.

  On the other hand, in test numbers 6 and 7, which are tests for a comparative example using a steel type of steel type No. 1 and a molten steel having a hydrogen content in the molten steel exceeding 10 ppm, many pinholes were generated. As a result, the billet was inferior surface properties with a large number of surface defects, and a large amount of uncareable material exceeding 20% was generated.

  Test Nos. 8 and 9 use the steel type No. 3, ie, the steel type whose minimum nitrogen solubility is about 1000 ppm as shown in Table 2 above and whose nitrogen content greatly exceeds the minimum nitrogen solubility. This is a test for an example of the present invention using a molten steel having a hydrogen content of less than 10 ppm, and test number 10 is a test for a comparative example using a molten steel having a hydrogen content in the molten steel exceeding 10 ppm.

  In test numbers 8 and 9, which are examples of the present invention, the nitrogen content in the steel is 2600-2900 ppm, which greatly exceeds the minimum nitrogen solubility in the δ phase, but the hydrogen content is less than 10 ppm, In both cases, the occurrence of pinholes is reduced, and the billet surface wrinkles are greatly improved. On the other hand, in test number 10 using molten steel having a hydrogen content of more than 10 ppm, a large number of pinholes were generated, and the billet had an inferior surface property with many surface defects.

According to the method of the present invention, the hydrogen content in the molten steel of the alloy steel containing Cr and Ni having a nitrogen content exceeding the nitrogen solubility in the δ phase once generated in the solidification process is less than 10 ppm. As a result, it is possible to prevent the generation of pinholes that are likely to occur in the slab surface layer. Furthermore, it is possible to reduce the generation of linear wrinkles that are likely to occur on the billet surface during processing such as rolling or forging due to the generation of pinholes. In addition, by making the sulfur content less than 20 ppm, the generation of surface defects can be further reduced, and the surface quality can be further improved. Therefore, the casting method of the present invention contains Cr and Ni such as stainless steel or high alloy steel as a casting method that can reduce the maintenance cost in the lump process and improve the quality and yield of billets and the like. This technology can be widely applied in the field of casting of alloy steel .

It is a figure which shows the phase transformation behavior in the solidification process of 9% Cr containing steel, and the calculation result of the solubility of nitrogen and hydrogen, The figure (a) shows the relationship between temperature and the abundance ratio of each phase. b) shows the relationship between temperature and nitrogen solubility in each phase, and FIG. 10 (c) shows the relationship between temperature and hydrogen solubility in each phase. It is a figure which shows the phase transformation behavior in the solidification process of SUS310 steel, and the calculation result of the solubility of nitrogen and hydrogen, The figure (a) shows the relationship between temperature and the presence rate of each phase, The figure (b) FIG. 4C shows the relationship between temperature and nitrogen solubility in each phase, and FIG. 5C shows the relationship between temperature and hydrogen solubility in each phase. It is a figure which shows the relationship between the surface flaw generation | occurrence | production state of the split rolling billet manufactured from the continuous casting slab of 9% Cr containing steel, and the nitrogen content rate and hydrogen content rate in steel. It is a figure which shows the relationship between the surface flaw occurrence rate of the partial rolling billet manufactured from the continuous casting slab of 9% Cr containing steel, and the hydrogen content rate and sulfur content rate in steel.

Claims (2)

The value obtained by dividing Cr equivalent expressed by the following formula (1) by the Ni equivalent expressed by the following formula (2) is 1.5 or more and contains Cr and Ni and is once in the solidification process. It generates a δ phase, a method of casting an alloy steel having a nitrogen content of greater than nitrogen solubility in the δ phase,
The hydrogen content in the molten steel is less than 10 ppm, and the sulfur content in the molten steel is 7 ppm or less ,
A method for casting an alloy steel containing Cr and Ni, comprising producing a slab in which a pinhole extending approximately 10 mm or more substantially perpendicular to the surface is not formed.
Cr equivalent = Cr + 1.37 × Mo + 1.5 × Si + 2 × Nb + 3 × Ti (1)
Ni equivalent = Ni + 22 × C + 0.31 × Mn + 14.2 × N + Cu (2)
Here, the element symbols in the formulas (1) and (2) represent the content (mass%) of each element in the steel. Casting method of alloy steel containing Cr and Ni as defined in claim 1, wherein the casting method is a continuous casting method.

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