JP2005334944A - Continuous casting method for heat-resisting low-alloy steel, and continuously cast slab - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a continuous casting method for heat-resisting low-alloy steel capable of preventing the cracks in the vicinity of the surface skin and the cracks in the central part of a slab by optimizing the cooling conditions at the last stage of solidification for the slab, and to provide the slab. <P>SOLUTION: (1) In the continuous casting method, when heat-resisting low-alloy steel containing 0.03 to 0.08% C, 1.5 to 2.5% Cr, 0.1 to 0.3% Mo and 1.0 to 2.0% W is continuously cast into a round cast slab, the steel is subjected to secondary cooling in the part directly below a die, and is thereafter subjected to forced water cooling at the last stage of solidification, the specific water content in the secondary cooling is controlled to ≥0.30 l/kg, and the specific water content in the forced water cooling at the last stage of the solidification in the meanwhile from a position at which the solid phase ratio in the central part of the cast slab reaches 0.1 to 0.8 to the one at which it reaches ≥0.99 is controlled to ≤0.35 l/kg. (2) The round cast slab of heat resisting low alloy steel is produced by the continuous casting method described in the above (1), and the expansion ratio at the circumferential length in the cross-section of the round cast slab at ordinary temperature is <2.0% to the nominal circumferential length. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a continuous casting method for producing a continuous cast slab free from cracks in the vicinity of the surface of a heat-resistant low alloy steel (hereinafter also referred to as “subcutaneous crack”) and a center part crack, and a slab cast by the method. .

As a technique for improving the internal quality of a round billet slab, Patent Document 1 discloses a method of forced cooling at the end of solidification (end of solidification cooling method). In this method, in the continuous casting of a steel bloom or billet, the solid phase rate at the center of the slab becomes 0.99 or more from a position 0.1 to 2.0 m before the foremost direction of casting of the residual molten metal pool. Until the solidification end forced cooling zone, the slab surface is water cooled at 100 to 300 liters / (min · m ² ). According to the method disclosed here, although it is possible to prevent the center part crack of the slab, when the subcutaneous crack of the slab occurs, and when such a billet is manufactured by the Mannesmann tube method, Not only was the surface of the pipe defective, but there were also operational problems when drilling with a piercer, making pipe production impossible.

  In Patent Document 2, when producing a steel slab having a cross-sectional area of a predetermined value or less by continuous casting, from the position where the solid phase ratio of the slab central portion becomes 0.1 to 0.8 or more until 0.99 or more. In other words, a method for producing a heat-resistant low-alloy steel pipe is disclosed in which the end-solidification period is forcibly cooled at a water density in a predetermined range. However, there is no description about secondary cooling directly under the mold, and this method could not completely prevent the epidermis cracking of the slab.

  In Patent Document 3, the cooling of the surface of the slab by water cooling is started when the solid phase rate at the center of the slab reaches 0.1 to 0.3, and the solid phase rate at the center of the slab is 0. 0. A slab cooling method is disclosed in which the generation of center porosity is reduced by continuing water cooling until it reaches 8 or more. Patent Document 4 discloses a slab center solid phase ratio of 0.2. A method for improving the inner quality of a slab is disclosed in which surface cooling of the slab by water cooling is started at a time of ˜0.8 and water cooling is continued until the slab is completely solidified. However, Patent Documents 3 and 4 only describe the minimum cooling rate for preventing bulging, and did not clearly disclose the cooling rate in secondary cooling.

  However, in the case of round slabs, since the static pressure of the molten steel acts equally on each direction in the circumferential direction of the slab, as described in Patent Document 5, bulging hardly occurs, therefore Under the cooling conditions with the minimum amount of secondary cooling water so as not to cause operational troubles, it was not possible to prevent subepidermal cracking of the slab.

  In the above-mentioned Patent Document 5, a ratio value obtained by dividing the amount of recuperation from immediately below the mold to the first spray cooling by the thickness of the solidified shell immediately below the mold is predetermined based on a preset position of the mold spray. A cooling method is disclosed in which one or more of the cooling water amount and the casting speed are controlled so as to be less than the value. According to the method disclosed herein, the amount of recuperation on the surface of the slab during secondary cooling is reduced, and subsurface cracking of high carbon steel having a C content of 0.3 to 1.0% is prevented. Can do.

  However, although the method disclosed in Patent Document 5 has a great effect on prevention of subepidermal cracking of high carbon steel, the skin of heat-resistant low alloy steel having a C content of 0.03 to 0.08%. It has been difficult to produce a slab for pipe making that has a low prevention effect against under cracks.

JP 2001-62550 A (Claims and paragraphs [0015] to [0027])

JP 2003-64449 A (claims and paragraphs [0025] to [0034]) JP-A-7-1096 (Claims and paragraphs [0009] to [0015]) JP-A-8-332556 (Claims and paragraphs [0007] and [0008]) JP 2000-317598 A (claims and paragraphs [0009] and [0011])

  The present invention has been made to solve the above-mentioned problems at the same time, and the problem is to appropriately control the cooling conditions at the end of solidification of the slab, thereby reducing the subepidermal and central cracks of the slab. It is an object of the present invention to provide a continuous casting method and continuous cast slab of heat-resistant low alloy steel that can be prevented together.

  In order to solve the above-mentioned problems, the present inventors have investigated the subepidermal cracks in a heat-resistant low alloy steel slab, investigated the cause of the occurrence, and found the following findings (a) to (e) To complete the present invention.

  (A) In the subepidermal crack, there are a case where the segregation component phosphorus (P) is concentrated in the cracked portion and a case where P is not concentrated.

  (B) Subepidermal cracking with P concentration is a phenomenon peculiar to heat-resistant low alloy steels containing a large amount of ferrite-forming elements such as Cr, Mo and W at a low carbon content. The temperature range in which ferrite exists is wide, and it is an internal crack caused by bulging due to expansion of the circumferential length of the slab cross section.

  (C) Subcutaneous cracking of (b) above is caused by setting the specific water amount of secondary cooling to 0.30 (liter / kg-steel) or more and the thermal expansion coefficient of the peripheral length of the slab to less than 2%. , Can prevent its occurrence.

(D) subepidermal cracking without the enrichment of P is cast slab surface temperature in the coagulation end cooling zone below the A ₃ transformation temperature, tensile stress acts on the epidermis lower portion transformation expansion occurs in the billet surface It depends. This, while A ₃ transformation temperature by containing a large amount of ferrite-forming elements in a low carbon content is increased, heat-resistant low alloy called billet surface temperature by solidification end cooling method is easily below the A ₃ transformation temperature This phenomenon is unique to steel.

Subepidermal cracking (e) above (d) is the ratio water coagulation end cooling as 0.35 (liter / kg-steel), as the slab surface temperature does not fall below the A ₃ transformation temperature 100 ° C. or higher This can be prevented.

  The present invention has been completed based on the above findings, and the gist of the present invention resides in the continuous casting method shown in the following (1) and the continuous cast slab shown in (2).

  (1) By mass%, C: 0.03-0.08%, Cr: 1.5-2.5%, Mo: 0.1-0.3% and W: 1.0-2.0% When continuously casting a heat-resistant low alloy steel containing slabs into a round slab, the secondary cooling is performed immediately below the mold, and then forced water cooling is performed at the end of solidification. Forced water cooling at the end of solidification from 0.30 (liter / kg-steel) or more until the solid phase ratio at the center of the slab becomes 0.1 to 0.8 or more. The continuous casting method of round slabs with a specific water amount of 0.35 (liter / kg-steel) or less.

  (2) A round slab cast by the continuous casting method according to (1), wherein the expansion rate of the circumferential length of the cross section of the round slab at room temperature is less than 2.0% with respect to the nominal circumferential length A heat-resistant low alloy steel round slab.

  In the present invention, “round cast piece” refers to a cast piece having a circular cross-sectional shape.

  “Heat resistant low alloy steel” refers to an alloy steel manufactured to withstand high temperatures or high temperatures and pressures by containing a low concentration of a third element in carbon steel.

  “End solidification phase” refers to a range in which the solid phase ratio at the center of the slab is 0.1 or more and 0.99 or more.

  “Solid fraction at the center of the slab” refers to the fraction of the solid phase in the coexisting phase of the solid phase and the liquid phase at the center of the slab, and the temperature distribution obtained by actual measurement or heat transfer calculation during solidification And based on the equilibrium diagram.

  And "nominal circumference" means the circumference of the cross section of the slab calculated based on the target diameter of the slab at room temperature.

  According to the method of the present invention, the specific water amount in the secondary cooling just below the mold is set to 0.30 (liter / kg-steel) or more, and the specific water amount in the end-of-solidification cooling is set to 0.35 (liter / kg-steel) or less. By doing so, it is possible to manufacture a round cast slab of heat-resistant low alloy steel that prevents both subepidermal cracking and center part cracking of the slab. In addition, the heat-resistant low alloy steel round slab of the present invention is optimal for the production of heat-resistant low alloy steel pipes by the Mannesmann tube method or the like.

  As described above, in the present invention, when continuously casting a heat-resistant low alloy steel containing C, Cr, Mo, and W into a round cast piece, the secondary cooling is performed immediately below the mold, and then forced water cooling is performed at the end of solidification. A continuous casting method for adjusting the specific water amount of secondary cooling directly under the mold and the specific water amount of forced water cooling at the end of solidification, and a heat-resistant low alloy cast by the continuous casting method. It is a steel slab.

  The inventors of the present invention investigated the subepidermal cracking of the heat-resistant low alloy slab, and based on the results, investigated the mechanism of subepidermal cracking and completed the present invention.

  FIG. 1 is a longitudinal sectional view schematically showing an example of a continuous casting apparatus for carrying out the continuous casting method of the present invention. Molten steel 3 in the tundish 1 is poured into a continuous casting mold 2, cooled in the mold 2, and forms a solidified shell 4 from a portion in contact with the mold 2. A round cast slab 8 that forms a solidified shell 4 and includes an unsolidified portion 9 inside is pulled out of the mold 2 by a pinch roll 6, cooled in a secondary cooling zone 5, and further forced water cooled in a final solidification cooling zone 7. To complete the coagulation. In addition, as forced water cooling in said end-of-solidification cooling zone 7, cooling by air mist is preferable.

  The reason why the range defined by the present invention is limited as described above and the preferable range will be described below.

(A) The specific water amount in secondary cooling, the specific water amount in forced water cooling at the end of solidification, and the expansion rate of the circumferential length of the cross-section of the round slab are cast using the continuous casting apparatus shown in FIG. The occurrence of subepidermal cracking was investigated. In addition, as a continuous casting apparatus, a curved continuous casting apparatus having a curvature radius of 10.5 m is used, and a molten steel of a heat-resistant low alloy steel of the test steel 1 having the component composition shown in Table 1 is used. Cast 191 mmφ round cast slabs. The casting speed was 2.7 m / min, and the superheat degree of the molten steel in the tundish was adjusted to 40 to 50 ° C. Further, in some tests, cooling at the end of solidification was performed in a range from the position where the solid phase ratio at the center of the slab becomes 0.1 to 0.8 to 0.99 or more.

  When investigating the slab where subsurface cracks occurred, there were two types of subsurface cracks: subsurface cracks where P concentration was observed in the cracks and subepidermal cracks where P concentration was not observed. It became clear that it existed. The presence or absence of subepidermal cracks was determined by observing the macrostructure of the slab transverse section and the longitudinal section, and the presence or absence of P element concentration was determined by the EPMA color mapping method.

  Here, the EPMA color mapping method is an analysis method in which a characteristic X-ray unique to each element is emitted by irradiating a sample with an electron beam, and the concentration distribution of each component is detected based on the intensity.

(A) Subcutaneous crack with concentration of element P As a result of further investigation to investigate the cause of the above-mentioned two types of subcutaneous crack, It was found that the circumferential length of the cross section increased (expanded) and bulging occurred.

FIG. 2 is a diagram showing the relationship between the perimeter of the slab cross section and the subepidermal crack. In the figure, the incidence of subepidermal cracking is a value expressed as a percentage by dividing the number of slabs in which cracks having a length (about 0.5 mm) or more that can be visually confirmed are divided by the number of all target slabs.

  From the results shown in FIG. 2, it has been clarified that the subepidermal crack is generated in the slab whose peripheral length of the slab cross section is 612 mm or more. Therefore, based on this result, it was presumed that the subepidermal crack accompanied by the enrichment of the element P was caused by an internal crack caused by bulging of the slab.

FIG. 3 is a diagram further arranged as a relationship between the expansion coefficient of the peripheral length of the slab cross section and the subepidermal crack based on the result of FIG. In FIG. 3, the value calculated by the following formula was used for the expansion coefficient of the circumference.

Expansion rate of perimeter of slab cross section = {(actual value of perimeter of slab cross section−nominal perimeter of slab cross section) / nominal perimeter of slab cross section} × 100 (%)
From the results shown in FIG. 3, it was found that subepidermal cracks occurred when the expansion rate of the circumferential length of the slab cross section was 2% or more, and no subepidermal cracking occurred when the expansion rate of the circumferential length was less than 2%. .

  The result of the figure is the above-mentioned knowledge, that is, in the round slab, the static pressure of the molten steel acts equally in each direction in the circumferential direction of the slab cross section, and thus has a rectangular or polygonal cross section. Unlike slabs, bulging that affects internal cracks (subcutaneous cracks) does not occur. In other words, a circular slab cross-section is itself a mechanically balanced final shape due to bulging. It is different from the findings of.

  The inventors of the present invention have found that the phenomenon of bulging in a round slab having a circular cross-sectional shape is that the heat-resistant low alloy steel of the present invention has a low C content and is a ferrite-forming element such as Cr, Mo. Since a large amount of W and W are contained, it is presumed that this is due to the fact that the temperature range in which δ ferrite is generated is wide in the initial casting and that the high temperature strength of the δ ferrite is low. Therefore, in order to clarify this point, a tensile test at a high temperature of steel was further conducted to investigate the effects of temperature and iron phase on the tensile strength.

  FIG. 4 is a diagram showing the influence of temperature on the tensile strength of steel. In addition, the result of the figure puts together the result of having measured the tensile strength in various test temperature using the tensile test piece of diameter 10mm and the distance between gauge points of 100mm.

  According to the results of FIG. 4, it is clear that the high-temperature tensile strength of the steel is lowered by the presence of the δ ferrite phase. Therefore, the occurrence of subepidermal cracking is caused by the internal cracking of the slab due to the decrease in the strength of the solidified shell due to the formation of δ ferrite and the occurrence of bulging. This phenomenon is unique to heat-resistant low alloy steels. It was confirmed that this was a phenomenon.

  Therefore, investigations were made on the conditions of secondary cooling to make the expansion rate of the peripheral length of the slab cross section less than 2%. FIG. 5 is a diagram showing the relationship between the specific water amount in the secondary cooling zone of continuous casting and the circumferential length of the slab cross section. From the results shown in the figure, it was found that the expansion rate of the circumferential length of the slab cross section can be made less than 2% by setting the specific water amount in the secondary cooling zone to 0.30 (liter / kg-steel) or more.

  When forced cooling is performed at the end of solidification, the final solidification position must be within the range of the end-solidification cooling zone. As the specific water amount of the secondary cooling is increased, the casting speed needs to be increased. Therefore, in consideration of the stability of the initial solidification and the cycle time of cutting by the slab cutting torch, the specific water amount of the secondary cooling is 0. It is preferable to suppress to 60 (liter / kg-steel) or less.

(B) Subcutaneous cracking without concentration of element P It was found that subepidermal cracking in which no concentration of element P was observed occurred only when the end-solidification cooling method was applied. Therefore, the relationship between cooling conditions and subepidermal cracking at the end of solidification was investigated.

  FIG. 6 is a diagram showing the relationship between the specific water amount in the continuous solidification end cooling zone of continuous casting and the subepidermal crack length of the slab. In order to clarify the distinction from subepidermal cracking caused by bulging, the specific water amount in the secondary cooling zone is within a range that does not cause subepidermal cracking caused by bulging. The test was conducted at 0.30 (liter / kg-steel) or more. In addition, the subepidermal crack length in the figure is indicated by the radial subdermal crack length in the cross section of the slab. According to the result of FIG. 6, the epidermal crack length becomes long when the specific water amount in the final solidification cooling zone is higher than 0.35 (liter / kg-steel).

  Therefore, the results of FIG. 6 were further arranged in relation to the slab surface temperature.

FIG. 7 is a diagram showing the relationship between the specific water amount and the surface temperature of the slab in the final solidification cooling zone of continuous casting. From the results of the drawing, when the ratio amount of water in the coagulation end cooling zone is large, substantially below the A ₃ transformation temperature the surface temperature of the slab in the coagulation end cooling zone, resulting in ferrite from austenite phase in the slab surface It was inferred that cracking occurred due to the transformation expansion into the phase and the tensile stress acting on the epidermis. Incidentally, A ₃ transformation temperature of the heat-resistant low-alloy steel was determined by differential thermal analysis.

The above phenomenon has a low C content, the result containing a large amount of ferrite-forming element, by A ₃ transformation temperature of the steel rises, slab surface temperature by forced cooling in the solidification end is readily A ₃ transformation temperature This is a phenomenon peculiar to heat-resistant low-alloy steels that have reached a temperature below.

From the result of FIG. 7, in order to prevent the slab surface temperature when the subepidermal crack length starts to increase rapidly, that is, the slab surface temperature does not fall below (A ₃ transformation temperature−100 ° C.) It became clear that the specific water amount in the cooling zone needs to be 0.35 (liter / kg-steel) or less.

  In addition, from the viewpoint of ensuring the thermal reduction amount at the center of the slab by cooling at the end of solidification, the specific water amount in the cooling zone at the end of solidification is preferably 0.12 (liter / kg-steel) or more.

  Based on the results of (a) and (b) above, in the first invention, the specific water amount of the secondary cooling water is 0.30 (liter / kg-steel) or more, and the specific water amount of forced water cooling at the end of solidification Was set to 0.35 (liter / kg-steel) or less. Moreover, in the 2nd invention, the expansion coefficient of the circumferential length of the cross section of a round slab was made into less than 2.0% with respect to a nominal circumferential length.

(B) Range of solid phase ratio for cooling at the end of solidification (a) Solid fraction of the slab center when starting the cooling at the end of solidification Cooling of the slab surface in the cooling at the end of solidification depends on the solid phase at the center of the slab It is necessary to start from a position where the rate is between 0.10 and 0.80. This is because if the solid phase ratio in the center is less than 0.10, the cooling start time is too early, so that at the time when the shrinkage at the center of the slab increases, a sufficient shrinkage at the surface cannot be secured, This is because sufficient compressive stress cannot be obtained in the central portion, and the effect of reducing the central portion defects cannot be obtained.

  On the contrary, when cooling is started from a position where the solid phase ratio of the slab center part exceeds 0.80, the cooling position is too close to the final solidification position, and the defect reduction effect in the slab center part is exhibited. This is because there is not enough time margin.

(B) Solid phase ratio at the center of the slab at the end of solidification end cooling Cooling of the slab surface at the end of solidification cooling is performed at a position where the solid ratio at the center of the slab is 0.99 or more (preferably, casting It is necessary to cool to a position where the transformation from δ iron to γ iron is completed at the center of the piece. This is because when the solid phase ratio in the central part is less than 0.99, the cooling is completed before complete solidification, and tensile stress is generated in the central part due to thermal expansion due to reheating of the slab surface. This is because the defect is enlarged.

(C) Component composition of heat-resistant low alloy steel as a target The reason for limitation and the preferred range of the component composition of heat-resistant low alloy steel as a target in the present invention will be described. The composition is expressed by mass%.

(A) Heat resistance containing C: 0.03-0.08%, Cr: 1.5-2.5%, Mo: 0.1-0.3% and W: 1.0-2.0% Low alloy steel for:
C: 0.03-0.08%:
C forms carbides with elements such as Nb and V, and has the effect of increasing the high temperature strength. If the C content is less than 0.03%, carbides are not sufficiently precipitated, and the effect of hardenability cannot be sufficiently exhibited. On the other hand, when the content rate exceeds 0.08%, the susceptibility to vertical cracks during continuous casting increases and weldability decreases. Therefore, the range of the C content is set to 0.03 to 0.08%.

Cr: 1.5-2.5%:
Cr is an element having an effect of improving corrosion resistance at high temperatures. In order to acquire the effect, it is necessary to contain 1.5% or more of Cr. On the other hand, when the content rate exceeds 2.5%, the weldability decreases. Therefore, the range of the Cr content is set to 1.5 to 2.5%.

Mo: 0.1 to 0.3%:
Mo has the effect of improving the creep strength. In order to improve the creep strength, the content must be 0.1% or more. On the other hand, even if 0.3% or more is contained, the effect is saturated, and since Mo is an expensive element, economic efficiency is impaired. Therefore, in the present invention, the Mo content is in the range of 0.1 to 0.3%.

W: 1.0-2.0%:
W is an element having an effect of improving the creep strength. In order to improve the creep strength, it is necessary to contain 1.0% or more. However, even if it contains 2.0% or more, the effect is saturated, and W is an expensive element because it is an expensive element. Therefore, the appropriate range of W content is set to 1.0 to 2.0%.

    (B) In the heat-resistant low alloy steel of (a), instead of a part of Fe, Si, Mn and Al are contained within the following content ranges, and further, the P and S content rates are limited. May be.

Si: 0.1 to 0.5%:
Si is an element having a deoxidizing action of molten steel. Si may or may not be contained, but when a deoxidizing effect is required, it is preferable to contain 0.1% or more. However, if the content exceeds 0.5%, the toughness of the heat-resistant low alloy steel decreases, which is not preferable. Therefore, the range of the preferable content rate of Si is 0.1 to 0.5%.

Mn: 0.1 to 1.0%:
Mn is an element having an effect of improving the strength of steel. It may or may not be contained, but when improvement in strength is required, it is preferable to contain 0.1% or more. However, if the Mn content exceeds 1.0%, the mold powder may be altered during casting, which is not preferable. Therefore, it is preferable to contain Mn in the range of 0.1 to 1.0%.

Al: 0.003 to 0.02%:
Al is an element having a strong deoxidizing action of molten steel. It is contained according to the purpose of deoxidation. When the deoxidation effect by Al is required, it is preferable to contain 0.003% or more. However, if the content exceeds 0.02%, the amount of nonmetallic inclusions in the steel increases and the creep strength of the steel decreases, which is not preferable. Therefore, the preferable range of Al content is 0.003 to 0.02%.

P: 0.02% or less:
P is an impurity element inevitably contained in the steel, and the content is preferably as low as possible. In particular, if P is contained in a large amount exceeding 0.02%, the welding characteristics deteriorate, which is not preferable. Therefore, the P content is preferably 0.02% or less.

S: 0.006% or less:
S is an impurity element inevitably contained in the steel, and its content is preferably as low as possible. An increase in the S content is not preferable because hot workability at the time of pipe making is lowered. In particular, in order to prevent the occurrence of inner surface flaws in the pipe, the content is preferably 0.006% or less.

    (C) In the heat-resistant low alloy steel as described in (a) or (b), in place of a part of Fe, at least one of Ti, V, Nb, B and Ca within the following content range It may be a steel containing the balance of Fe and impurities.

Ti: 0.01-0.02%:
Ti is an element having an action of fixing N in steel and suppressing precipitation of AlN which promotes surface cracking of the slab. When the effect of suppressing precipitation of AlN is required, it is preferable to contain 0.01% or more. On the other hand, if the content exceeds 0.02%, the toughness of the steel decreases, which is not preferable. Therefore, the preferable range of Ti content is 0.01 to 0.02%.

V: 0.1 to 0.5%:
V is an element which has the effect | action which forms a carbide | carbonized_material in steel and raises high temperature strength. When the effect is required, it is preferable to contain 0.1% or more. However, if the content exceeds 0.5%, the toughness of the steel is lowered, which is not preferable. Therefore, when it contains V, it is preferable to make the content rate into the range of 0.1 to 0.5%.

Nb: 0.01 to 0.3%:
Nb is an element which has the effect | action which forms a carbide | carbonized_material in steel and raises high temperature strength. When the effect is required, it is preferable to contain 0.01% or more. On the other hand, if the content exceeds 0.3%, the toughness of the steel decreases, which is not preferable. Therefore, when Nb is contained, the content is preferably in the range of 0.01 to 0.3%.

B: 0.002 to 0.01%:
B is an element having an effect of improving the hardenability of steel. When improvement of hardenability is required, it is preferable to contain 0.002% or more. However, if B is contained in excess of 0.01%, the toughness of the steel decreases, which is not preferable. Therefore, when it contains B, it is preferable to make the content rate into 0.002 to 0.01% of range.

Ca: 0.005% or less:
Ca has the effect of preventing nozzle clogging during continuous casting. Ca may or may not be contained, but it is preferably contained when an effect of preventing nozzle clogging is required. However, when the content exceeds 0.005%, Ca inclusions increase and the creep strength of the steel decreases, which is not preferable. Therefore, when Ca is contained, the content is preferably in the range of 0.005% or less.

    (D) In the heat-resistant low alloy steel described in (a) to (c), when the following Cu, Ni, or N is contained as an impurity, the content is preferably within the following range.

Cu: 0.1% or less:
Cu greatly changes the properties of the scale on the surface of the slab. As the Cu content increases, the cooling characteristics of the slab at the time of secondary cooling and cooling at the end of solidification change remarkably, so the content is preferably 0.1% or less.

Ni: 0.2% or less:
Ni also greatly changes the scale properties of the slab surface. As the Ni content increases, the cooling characteristics of the slab at the time of secondary cooling and end-of-solidification cooling change remarkably, so the content is preferably 0.2% or less.

N: 0.01% or less:
Increasing the N content decreases the hardenability of the steel. When high hardenability is required, the N content is preferably 0.01% or less.

  In order to confirm the effect of the continuous casting method of the present invention and the performance of the continuous cast slab, the following casting test was performed and the results were evaluated.

(Test method)
Using a curved continuous casting machine having a radius of curvature of 10.5 m shown in FIG. 1, using heat-resistant low alloy molten steels of test steels 1 and 2 having the steel composition shown in Table 1 above, A round slab having a diameter of 191 mmφ in cross-sectional shape was cast. As the casting method, a method similar to the method described above with reference to FIG. 1 was adopted, and the casting test was carried out by variously changing the specific water amount in the secondary cooling zone directly below the mold and the specific water amount in the final solidification cooling zone.

  The casting speed is 2.7 m / min, the superheat degree of the molten steel in the tundish is adjusted to 40 to 50 ° C., and the cooling in the final solidification cooling zone has a solid phase ratio of 0.1 to 0 at the center of the slab. . Forced water cooling (air mist cooling) of the slab surface from the position where it becomes .8 to the position where the solid fraction of the central part is 0.99 or more by a 12-stage ring spray type cooling device with a casting direction length of 5 m. It went by.

  Mannesmann pipes were further manufactured for the slabs in which no subepidermal cracking occurred, and the quality of the steel pipes was investigated.

(Test results)
Table 2 summarizes the test conditions and test results.

  In the table, subepidermal cracking was evaluated according to the following criteria. That is, the case where the radial crack in the subepidermal portion of the slab cross section could not be visually confirmed was evaluated as good (◯), and the case where the radial crack was confirmed was evaluated as poor (x).

In addition, for the center crack of the slab, after polishing the cross-section of the slab, conduct a penetration flaw test,
The exuded diameter was evaluated. That is, the seepage diameter at the center of the slab cross section is 5
The case of less than mm was evaluated as good (◯), and the case where the oozing diameter at the center was 5 mm or more was evaluated as defective (x).

  Furthermore, pipe making was carried out only for a slab in which no subepidermal cracking occurred, that is, a slab evaluated as good (O) in Table 2 regardless of the presence or absence of P concentration.

  And the evaluation of the generation | occurrence | production state of the flaw of the pipe outer surface after pipe making and the pipe inner surface followed the following reference | standard. In other words, the outer surface and the inner surface of the tube are subjected to an ultrasonic flaw detection test and visual inspection for the presence or absence of wrinkles. If no wrinkles are confirmed, it is evaluated as good (○), and wrinkles are confirmed regardless of the length and thickness. The case was evaluated as bad (x).

  Test numbers 8, 9 and 12 using the test steel 1 and test numbers 14 to 16 using the test steel 2 are the specific water amount and the end-of-solidification cooling in the secondary cooling zone defined in the first invention of the present invention. This is an example of the present invention that satisfies all the ranges of the specific water amount in the belt. In these test examples, the obtained round slabs satisfied the range defined by the second invention in terms of the expansion coefficient of the circumferential length of the slab cross section.

  In any of the above-described casting tests of the present invention examples, a slab having good properties was obtained without occurrence of subepidermal cracking and central cracking. In addition, the steel pipes obtained by manufacturing these cast slabs with Mannesmann's pipe did not generate any outer surface defects or inner surface defects, and the overall evaluation was good.

  On the other hand, in the casting tests of test numbers 1 to 7, 10, 11 and 13, at least one of the specific water amount in the secondary cooling zone and the specific water amount in the end solidification cooling zone falls within the range defined in the first invention. This is a test of a comparative example that has been removed.

  In Test Nos. 1, 2, 4 and 5, since the specific water amount in the secondary cooling was too small, the expansion rate of the peripheral length of the slab cross section was large, and subepidermal cracking accompanied with P concentration occurred. It was impossible to make a pipe using this slab.

  In Test Nos. 3 and 6, since the specific water amount in the secondary cooling was too small, and the specific water amount in the end-of-solidification cooling was too large, the subcutaneous cracking accompanied with the P concentration and the P concentration in the slab Both subepidermal cracks occurred and tube production using these slabs was impossible. In Test No. 10, the specific water amount of the secondary cooling was within the range of the present invention, but because of the excessive specific water amount in the cooling at the end of solidification, subepidermal cracking without P concentration occurred in the slab. Pipe making using this slab was impossible.

  Test Nos. 7, 11 and 13 were such that the specific water amount of the secondary cooling was within the scope of the present invention, but because the end-solidification cooling was not performed, pipe production was possible, but the pipe inner surface after pipe production As a result, internal flaws occurred and the overall evaluation was poor. In order to prevent flaws on the inner surface of the pipe, the specific water amount in the cooling at the end of solidification is preferably in the range of 0.12 to 0.35 (liter / kg-steel).

  According to the method of the present invention, the specific water amount in the secondary cooling just below the mold is set to 0.30 (liter / kg-steel) or more, and the specific water amount in the end-of-solidification cooling is set to 0.35 (liter / kg-steel) or less. By doing so, it is possible to manufacture a round cast slab of heat-resistant low alloy steel that prevents both subepidermal cracking and center part cracking of the slab. The heat-resistant low alloy steel round slab of the present invention is most suitable for the production of heat-resistant low alloy steel pipes by the Mannesmann pipe manufacturing method or the like. Therefore, the continuous casting method and round slab of the present invention can be widely applied as a technique for preventing internal flaws in the field of manufacturing heat-resistant low alloy steel pipes.

It is a figure which shows typically the example of the continuous casting apparatus for enforcing the continuous casting method of this invention. It is a figure which shows the relationship between the perimeter of a slab cross section, and a subepidermal crack. It is a figure which shows the relationship between the expansion coefficient of the perimeter of a slab cross section, and a subepidermal crack. It is a figure which shows the influence of the temperature which acts on the tensile strength of steel. It is a figure which shows the relationship between the specific water amount in the secondary cooling zone of continuous casting, and the perimeter of a slab cross section. It is a figure which shows the relationship between the specific water quantity in the solidification end stage cooling zone of continuous casting, and the subepidermal crack length of slab. It is a figure which shows the relationship between the specific water amount in the solidification end stage cooling zone of continuous casting, and the surface temperature of slab.

Explanation of symbols

1: Tundish 2: Mold for continuous casting 3: Molten steel 4: Solidified shell 5: Secondary cooling zone 6: Pinch roll 7: End-solidification cooling zone 8: Round slab 9: Unsolidified part

Claims (2)

  In mass%, C: 0.03-0.08%, Cr: 1.5-2.5%, Mo: 0.1-0.3% and W: 1.0-2.0% When continuously casting a heat-resistant low alloy steel into a round slab, a secondary casting method in which secondary cooling is performed immediately below the mold and then forced water cooling is performed at the end of solidification, and the specific water amount of the secondary cooling is 0.30. (Liter / kg-steel) The specific water amount of forced water cooling at the end of solidification from the position where the solid phase rate at the center of the slab becomes 0.1 to 0.8 to 0.99 or more. Is a continuous casting method for round slabs, characterized in that it is 0.35 (liter / kg-steel) or less. It is a round slab cast by the continuous casting method according to claim 1, wherein the expansion ratio of the circumferential length of the cross section of the round cast slab at room temperature is less than 2.0% with respect to the nominal circumferential length. A heat-resistant low alloy steel round slab.

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