JP2012161820A - Manufacturing method of nonmagnetic steel using continuous casting - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for manufacturing high-manganese based nonmagnetic steel with high productivity while suppressing surface defects of a cast slab occurring during continuous casting.SOLUTION: There is provided the manufacturing method of high-manganese based nonmagnetic steel using a continuous casting method which has a composition containing, by mass%, C: 0.45-1.3%, Si: 0.05-0.5%, Mn: 10-19%, P: ≤0.10%, S: ≤0.02%, Al: 0.003-0.1%, N: 0.005-0.30%, and has a magnetic permeability of ≤1.1. The manufacturing method is characterized in that a casting temperature T is controlled to satisfy an expression (1): a≤T≤+50, and a casting rate Vc (m/min) is selected within a range of an expression (2): Vc≥0.02×(T-a), where a denotes a value to be determined by an expression (3) based on the composition of steel where expression (3):a=1,557-[53×(%C)+4.5×(%Mn)+45×(%P)].

Description

  The present invention relates to a method for providing a high-quality, particularly non-magnetic, high-strength steel material with reduced surface defects using continuous casting.

  Materials used for power generation turbines and magnetic levitation railways are required to be both high-strength structural materials and non-magnetic materials that do not disturb the magnetic field caused by a permanent magnet or current. Steel containing high concentrations of carbon and manganese (hereinafter referred to as “high carbon-high manganese-containing steel”) has a strength equal to or higher than that of Cr—Ni-containing austenitic stainless steel, and magnetic permeability. Is the most suitable material for non-magnetic structural materials.

  A high carbon-high manganese content steel completes solidification in an austenite single phase during casting, and thus a columnar grain structure is likely to develop. The formed austenite columnar grains are coarse, and the grain boundaries serve as crack initiation points and propagation paths. Furthermore, being an austenite single phase often significantly reduces workability in hot working (such as hot rolling or forging). Therefore, high carbon-high manganese content steel is recognized as a material having high cracking susceptibility during continuous casting or hot rolling.

  In particular, when a high carbon-high manganese content steel having a C content of 0.45 mass% or more and a Mn content of 10 mass% or more is produced using continuous casting, the contents of C and Mn are high. Therefore, various surface defects of the slab are likely to occur. The main slab surface defects are 1) vertical cracks, 2) burrs, and 3) microcracks.

  As a means for suppressing the occurrence of cracks during continuous casting, Patent Document 1 discloses a steel manufacturing method containing C: 0.9 to 1.20% and Mn: 11.0 to 14.0% by weight. The P content is 0.030% or less, the secondary cooling water ratio during continuous casting is adjusted to a range of 0.7 to 1.1 L / kg, and the heating rate of the slab during the heating process is adjusted to 30 to 35. The manufacturing method adjusted to the range of ° C./h is disclosed.

JP 59-13556 A

"Steel Solidification" Appendix Data Collection on Solidification Phenomena of Steel (issued in 1977) “Recent High Manganese Non-Magnetic Steel”, Shigeki Sawa, Journal of the Japan Institute of Metals, Vol. 18, No. 8 (1979), pp. 573-581) “Effects of components and production conditions on physical properties of high-Mn nonmagnetic steel”, Chiaki Ouchi et al., Iron and Steel, Vol. 69, No. 6 (1983), pp. 11-28. 694-702)

  In the method disclosed in Patent Document 1, it is premised on a soaking-pre-rolling step in which the surface of a continuously cast slab is cleaned after continuous casting, and the main object is to prevent the surface cracks from being promoted. It is said. However, according to such a conventional manufacturing method, surface defects such as flaws and cracks at the slab stage of the as-cast continuous casting portion are allowed to some extent, and an increase in cost due to slab maintenance is inevitable. Furthermore, casting may become unstable due to the occurrence of the above-described slab surface defect, and in such a case, an emergency stop of casting is forced, resulting in a significant reduction in productivity and a significant cost loss. .

  It is an object of the present invention to provide a method for producing a nonmagnetic steel containing high concentrations of carbon and manganese with high productivity while suppressing slab surface defects that occur during continuous casting such as surface cracks and surface defects. To do.

  As a result of investigating the cause of surface defects in the slab by paying attention to the properties under high temperature inherent in the high carbon-high manganese content steel, the present inventor effectively generated the surface defects by implementing the following means. The knowledge that it was able to suppress it was acquired.

(1) Appropriately grasp the relationship between the liquidus temperature and the composition and the relationship between the solidus temperature and the composition of high carbon-high manganese content steel, and set the operating conditions for continuous casting based on the relationship. .
(2) In addition to the above means, the reactivity with molten steel is suppressed, and it is suitable for continuous casting of high carbon-high manganese content steel capable of maintaining the lubricity in the mold and the slow cooling effect of the solidified shell. Use mold flux.

The present invention completed based on the above findings is as follows.
(1) By mass%, C: 0.45% to 1.3%, Si: 0.05% to 0.5%, Mn: 10% to 19%, P: 0.10% or less, It has a chemical composition containing S: 0.02% or less, Al: 0.003% or more and 0.1% or less, N: 0.005% or more and 0.30% or less, and the magnetic permeability is 1.1 or less. A method for producing a high manganese nonmagnetic steel by a continuous casting method, wherein the casting temperature T is controlled so as to satisfy the formula (i), and the casting speed Vc (m / min) is within the range of the following formula (ii). Non-magnetic steel manufacturing method characterized in that:
a ≦ T ≦ a + 50 (i)
Vc ≧ 0.02 × (T−a) (ii)
Here, a is a value determined by the following formula (iii) from the composition of steel, and (% C), (% Mn) and (% P) in the following formula (iii) are respectively steel. Content of C, Mn and P in the chemical composition (unit: mass%).
a = 1557− {53 × (% C) + 4.5 × (% Mn) + 45 × (% P)}
... (iii)

(2) The non-magnetic steel continuous casting method according to the above (1), wherein continuous casting is performed using a mold flux containing MnO, CaO, and SiO ₂ as main oxides, which is contained in the mold flux. The MnO content (unit: mass%) satisfies the Mn concentration (unit: mass%) of the continuously cast molten steel and the following formula (iv), and the mass ratio of CaO contained in the mold flux to SiO ₂ CaO / SiO ₂ is 0.9 to 2.0, production method.
0.40 × (Mn concentration in steel) <(MnO content in mold flux) <(Mn concentration in steel) (iv)

(3) The method for producing nonmagnetic steel according to (1) or (2) above, wherein the chemical composition of continuously cast molten steel satisfies the following formula (v).
(% Mn) -19.6 × (% C) + 22.7 × (% C) ² + 72.7 × (% P) <22.4 (v)

  Here, (% C), (% Mn) and (% P) in the above formula (v) are the contents (unit: mass%) of C, Mn and P in the chemical composition of steel, respectively.

  The present invention provides a means for preventing the occurrence of slab surface defects when producing non-magnetic steel containing high concentrations of carbon and manganese through continuous casting. Moreover, the continuous casting edge obtained by carrying out the present invention has low cracking susceptibility during subsequent hot rolling. Therefore, according to the present invention, a nonmagnetic steel material containing high-quality carbon and manganese with good quality can be produced. The obtained steel material has sufficient hot workability even during hot rolling, and can be used for applications requiring non-magnetic performance unique to the material.

It is a graph which shows contrast with a liquidus temperature experimental value and a liquidus temperature estimated value. It is a graph which shows the relationship between casting speed and casting temperature. It is a graph which shows the relationship between basicity and MnO density | concentration (unit: mass%) in mold flux. It is a graph which shows the allowable P density | concentration calculated | required from Formula (H). It is a graph which shows the allowable P density | concentration calculated | required from Formula (H ').

The manufacturing method using continuous casting of the nonmagnetic steel according to the present invention will be described in detail below.
1. Nonmagnetic steel according to the present invention The nonmagnetic steel to be manufactured in the manufacturing method according to the present invention is a high manganese-containing steel whose magnetic properties are not ferromagnetic, the magnetic permeability is 1.1 or less, The phase in is an austenite single phase. Hereinafter, a nonmagnetic steel having a high manganese content and having the above magnetic and structural characteristics is also referred to as “high manganese nonmagnetic steel”.

  The chemical composition of the high manganese nonmagnetic steel according to the present invention is based on the Fe-Mn-ternary system, and as components, mass%, C: 0.45% to 1.3%, Si: 0.00. 05% to 0.5%, Mn: 10% to 19%, P: 0.10% or less, S: 0.02% or less, Al: 0.003% to 0.1%, N: 0 Based on a chemical composition containing 0.005% or more and 0.30% or less. In order to improve the steel characteristics, other elements such as V, Cr, Ni, and Cu may be contained as optional components. The balance is Fe and impurities.

In order to satisfy the magnetic permeability of 1.1 or less, the high manganese nonmagnetic steel according to the present invention needs to have a composition such that the austenite phase has a stable single phase structure. High manganese non-magnetic steel based on Fe-Mn-C ternary system is a stable single phase by austenite at room temperature by using a composition containing both C and Mn, which are austenite stabilizing elements, at a high concentration. Become an organization. Specifically, in high manganese nonmagnetic steel, one of the conditions for obtaining nonmagnetism is that the C content and the Mn content generally satisfy the following relational expression (I). The following relational expression (I) can be derived from a compositional relationship diagram of C and Mn of a general nonmagnetic steel (for example, see FIG. 1 of Non-Patent Document 2 and FIG. 1 of Non-Patent Document 3).
15 × (% C) + (% Mn)> 23 (I)

  Here, (% C) and (% Mn) in the above formula (I) are the contents (unit: mass%) of C and Mn in the chemical composition of steel, respectively.

Next, the composition of the high manganese nonmagnetic steel according to the present invention will be described in detail. Hereinafter, “%” indicating the steel component content means mass%.
C: 0.45% to 1.3% C is an element necessary for stabilizing the austenite phase and ensuring material strength. When the C content is in the range of 0.45% to 1.3%, the steel can be suitably used as a structural material. In particular, in order to stably obtain a low magnetic permeability (1.1 or less) as a nonmagnetic material, the C content needs to be at least 0.45%. However, when the C content exceeds 1.3%, ductility and workability deteriorate. Therefore, the C content is 1.3% or less.

Si: 0.05% or more and 0.5% or less Si is an element necessary for deoxidation, has an effect of solid solution strengthening, and is indispensable for an alloy component. However, since the high manganese nonmagnetic steel according to the present invention is based on the Fe-Mn-C ternary system, the effect is saturated when the Si content exceeds 0.5%. Therefore, the Si content is 0.5% or less.

Mn: 10% or more and 19% or less Mn is an element necessary for stabilizing the austenite phase and ensuring the material strength. In particular, by containing Mn at a high concentration of 10% or more, non-magnetic and high-strength performance, which is characteristic of the austenite phase, can be obtained. However, since the high manganese nonmagnetic steel according to the present invention is based on the Fe-Mn-C ternary system, the workability is greatly impaired when the Mn content exceeds 19%. Moreover, when Mn component is added excessively, cost will become high. Therefore, the Mn content is 19% or less.

P: 0.10% or less P is an impurity element contained in steel and causes a decrease in toughness or hot embrittlement. In the case of the high manganese nonmagnetic steel according to the present invention, if the P content exceeds 0.10%, the weldability is remarkably lowered. Therefore, the P content is 0.10% or less.

S: 0.02% or less Since S is an impurity element contained in the steel and causes a decrease in toughness, it is preferable that S be less. If the S content exceeds 0.02%, the amount of MnS inclusions that become the starting point of corrosion increases, and the corrosion resistance of the steel decreases. Therefore, the S content is 0.02% or less.

Al: 0.003% or more and 0.1% or less Al is an element necessary for deoxidation. It is unavoidable in steel. From the viewpoint of stably obtaining this deoxidation effect, the content is made 0.003% or more. However, since the effect of deoxidation is saturated even if Al is contained excessively, it is not necessary to contain Al in excess of 0.1%. Therefore, the Al content is 0.1% or less.

N: 0.005% or more and 0.30% or less N has an effect of stabilizing the austenite phase and increasing the strength by solid solution or precipitation. Since the affinity for Mn and Cr is large, N can be easily dissolved in steel with a high Mn content. Therefore, if the N content is 0.005% or more, this strength improvement effect can be stably obtained. However, when the N content exceeds 0.30%, the hot workability decreases. Therefore, the N content is 0.30% or less.

The chemical composition of the high manganese nonmagnetic steel according to the present invention can contain the elements exemplified below as optional components in addition to the above basic components.
V: 0.3% or less V is an element useful for improving the strength by precipitation hardening. A small amount may be contained as necessary. However, when the content exceeds 0.3%, the effect is saturated, and rather the workability is significantly lowered. Therefore, when V is contained, the content is made 0.3% or less.

One or more of Cr, Ni and Cu: Total content is 2% or less. Cr, Ni and Cu are elements useful for stabilizing the austenite phase and improving the strength by solid solution strengthening. is there. Therefore, you may contain 1 or more types of these elements as needed. However, when the total content of Cr, Cu and Ni exceeds 2%, the effect is saturated, and rather the workability is significantly lowered. In addition, since these are expensive alloy components, if added excessively, the raw material cost is increased, and the high manganese nonmagnetic steel based on the Fe-Mn-C ternary system originally has the low cost but the non-cost. The characteristic that magnetism is obtained cannot be obtained. Accordingly, when one or more of Cr, Ni and Cu are contained, the total content is made 2% or less.

2. Relationship between Casting Temperature and Casting Speed in Continuous Casting and Composition of High Manganese Nonmagnetic Steel In the method for producing nonmagnetic steel using continuous casting according to the present invention, in the continuous casting process of high manganese nonmagnetic steel, By setting the temperature and casting speed based on the chemical composition of the high manganese nonmagnetic steel, the occurrence of slab surface defects is suppressed, and continuous casting is continued stably for a long time.

  Hereinafter, the relationship between the casting temperature and casting speed in continuous casting and the chemical composition of the high manganese nonmagnetic steel will be described in detail.

(1) Estimation of liquidus temperature and solidus temperature First, in order to control the casting temperature, the composition dependence of the melting point of the high manganese nonmagnetic steel to be cast was examined.
Usually, the casting temperature is managed by the degree of superheat (= casting temperature−liquidus temperature) based on the liquidus temperature. In order to obtain the degree of superheat, a formula for estimating the liquidus temperature from the composition is generally used. As an example of an equation for estimating the liquidus temperature, an equation reported by Kawawa et al. In Non-Patent Document 1 (hereinafter referred to as “Kawawa et al.”) Is shown below.
TL = 1538− {78 × (% C) + 7.6 × (% Si) + 4.9 × (% Mn) + 34.4 × (% P) + 38 × (% S)} (A)

  Here, (% C), (% Si), (% Mn), (% P) and (% S) in the above formula (A) are respectively C, Si, Mn, S in the chemical composition of the steel. And P content (unit: mass%).

  However, the above formula (A) for the liquidus temperature is an estimation formula for ordinary carbon steel, and the high manganese non-magnetic steel according to the present invention has a Mn content that is less than that of ordinary carbon steel. It was found that when the calculation formula (A) is applied to the high manganese nonmagnetic steel according to the present invention, a large error may occur in the calculated value of the liquidus temperature because it is more than an order of magnitude larger.

  For example, the liquidus temperature of 1.0% C-14% Mn steel, which is one of the typical compositions of high carbon-high manganese content steel, is 1390--from the conventional estimation formula (the following formula (A)). 1392 ° C. is calculated.

  On the other hand, it was 1410-1412 degreeC when the actual liquidus temperature was measured about said steel by thermal analysis experiment. That is, a large error of about 20 ° C. occurred between the estimated value and the actually measured value. This is because a general carbon steel has a large value as a contribution coefficient to relatively low concentrations of C and Mn (for example, a temperature drop per element content of 1% is 78 ° C. for C and 4.4 for Mn. 8 to 4.9 ° C.) was set.

  Thus, the conventional estimation formula cannot be applied to the purpose of obtaining the liquidus temperature of high carbon-high manganese steel. Therefore, thermal analysis experiments were performed with various composition systems containing C and Mn to determine the coefficient for the solute, and an equation for estimating the liquidus temperature for high carbon-high manganese steel was determined.

  Here, the details of the thermal analysis experiment carried out in determining the above-mentioned liquidus temperature estimation formula will be described. In an argon gas atmosphere, 70 to 80 g of a steel sample was melted in an alumina crucible having an inner diameter of 20 mm, the furnace temperature was maintained at 1480 ° C. for 15 to 20 minutes, and then the cooling rate was 10 ° C./min using temperature control of the electric furnace. The furnace was cooled. At this time, the sample temperature was measured with a thermocouple in a protective tube immersed in the molten steel sample. Since characteristic points accompanying solidification appear in the cooling curve of the sample, the liquidus temperature and the solidus temperature were evaluated based on these. As the first characteristic point, the maximum temperature or plateau temperature after recuperation when coagulation started (latent heat release started) was defined as the liquidus temperature. Next, the temperature at which internal heat generation obtained by thermal analysis becomes zero was defined as the solidus temperature at which the release of latent heat of solidification was completed. The object of the experiment was a high carbon-high manganese content steel shown in Table 1.

The present inventors determined the following liquid phase temperature estimation formula (B) from the comparison of the obtained experimental results and the composition.
TL = 1527- {53 × (% C) + 4.5 × (% Mn) + 45 × (% P)}
... (B)
Here, (% C), (% Mn) and (% P) in the above formula (B) are the contents (unit: mass%) of C, Mn and P in the chemical composition of steel, respectively.

  Table 2 shows the result of comparing the calculated value of the liquidus temperature derived based on the estimation formulas (A) and (B) with the actually measured value. The error in the newly calculated estimation formula (B) is about 2 ° C. or less in any steel type, and it is possible to estimate with much higher accuracy than the conventional estimation formula (A).

  FIG. 1 shows the relationship (O) between the measured value of the liquidus temperature TL and the estimated value by the estimation formula (B), and the relationship (Δ) between the measured value and the conventional estimation formula (A) in a comparable manner. It is a graph. As shown in FIG. 1, according to the estimation formula (B) according to the present invention, the measured value of the liquidus temperature TL can be reproduced almost accurately. On the other hand, according to the conventional estimation formula (A), the liquidus temperature is estimated to be about 10 to 30 ° C. on the low temperature side compared to the actual measurement value.

Here, the above-described estimation formula (B) is an expression considering only C, Mn, and P, which are components that have a large influence on the change of the liquidus temperature. Considering the contents of Si and S, which do not have as much influence as these elements, but the influence is not so small that they can be completely ignored, the contribution coefficient of the existing estimation formula (specifically only for the contents of Si and S) Specifically, when the contribution coefficient in the equation of Kawawa et al. Is employed, the liquidus temperature is expressed by the following equation (C).
TL = 1529- {53 × (% C) + 6 × (% Si) + 4.5 × (% Mn) + 45 × (% P) + 38 × (% S)} (C)

  Here, (% C), (% Si), (% Mn), (% P) and (% S) in the above formula (C) are respectively C, Si, Mn, S in the chemical composition of the steel. And P content (unit: mass%).

  In addition, in the general melting composition range of high carbon-high manganese content steel, the influence of components (Si, S, etc.) other than C, Mn and P is slight. Specifically, in the high manganese nonmagnetic steel according to the present invention, the fluctuation ranges of the Si and S contents are 0.45% for Si and 0.02% for S, and the above formula (C) Based on this, the temperature fluctuation in the estimated value of the liquidus temperature caused by the fluctuation of the Si and S contents is less than 4 ° C. at the maximum. Therefore, if the amount of temperature drop due to Si and S components in the formula (C) is regarded as 2 ° C., the above formula (B) can be obtained. Even considering the variation range of the Si and S component compositions, the estimation error is less than ± 2 ° C., and the above formula (B) is sufficient for practical use.

Similar to the liquidus temperature TL, the following estimation formula (D) of the solidus temperature TS was obtained from the comparison between the measurement result of the solidus temperature and the composition.
TS = 1333 − {− 108 × (% C) + 125 × (% C) ² + 5.5 × (% Mn) + 400 × (% P)} (D)
Here, (% C), (% Mn) and (% P) in the above formula (D) are the contents (unit: mass%) of C, Mn and P in the chemical composition of steel, respectively.

(2) Casting temperature When performing continuous casting, it is generally necessary to control the degree of superheat of molten steel within an appropriate range based on the liquidus temperature. The appropriate range for casting alloy steel including high carbon-high manganese content steel is empirically 30 to 80 ° C. as the degree of superheat based on the liquidus temperature. If the molten steel superheat degree is lower than the lower limit of 30 ° C, the so-called molten surface skinning occurs in which the molten steel temperature near the molten metal surface, which is particularly easy to cool in the mold, becomes below the liquidus and the molten metal surface partially solidifies. Casting may become unstable. On the other hand, if the degree of superheating of the molten steel is higher than the upper limit of 80 ° C., the solidified shell formed in the mold is easily remelted and easily broken, and there is a risk of occurrence of fogging and breakout in the mold. Assuming that equation (A) is used as an equation for estimating the liquidus temperature of steel containing high carbon and high manganese, the superheat degree of the molten steel greatly deviates from the proper temperature range to the low temperature side (about 30 ° C) and is stable. There is a high possibility that continuous casting is not possible.

Therefore, the appropriate range of the casting temperature T of the high manganese nonmagnetic steel according to the present invention (the molten steel temperature in the molten steel container immediately before the hot water supply to the mold, unit: ° C) is based on the above estimation formula (B). It is set as equation (1).
a ≦ T ≦ a + 50 (1)

Here, a is directly obtained by the equation (3) in which the liquidus temperature estimation formula (B) of the high manganese nonmagnetic steel according to the present invention is added with the appropriate lower limit value of 30 ° C. of the molten steel superheat degree. The lower limit value (unit: ° C) of the proper casting temperature.
a = 1557− {53 × (% C) + 4.5 × (% Mn) + 45 × (% P)}
... (3)

  Here, (% C), (% Mn) and (% P) in the above formula (3) are the contents (unit: mass%) of C, Mn and P in the chemical composition of steel, respectively.

  If the casting temperature T is lower than a, the casting may be performed at a low temperature, which may cause troubles such as coating of the molten metal surface in the continuous casting mold. .

  If the casting temperature T exceeds a + 50, the initial solidified shell formed in the continuous casting mold due to high temperature casting may be broken by the hot pouring flow, which becomes the starting point of the slab surface crack. When the solidified shell is re-dissolved in the mold, a double skin is formed, and the trace becomes a surface defect called slab fogging habit.

(3) Casting speed The casting speed Vc should be appropriately set according to the steel type and casting temperature in order to realize stable continuous casting.

  In the process of growth of the solidified shell formed in the mold, a stress is generated that causes the solidified shell to lift away from the mold due to a thermal contraction difference that occurs with a temperature gradient in the thickness direction of the shell. At this time, if the casting speed is appropriate, the solidified shell is pressed against the mold surface by the molten steel static pressure, so that the solidified shell does not deform. However, when the casting speed is excessively low, there is a high possibility that the solidified shell will be deformed to cause fogging. Therefore, in order to perform stable casting, a lower limit is set to the casting speed from the viewpoint of suppressing deformation of the solidified shell so as to prevent fogging.

As a result of the examination of the lower limit by the present inventors, it has become clear that the factors that determine whether or not the solidified shell is deformed are greatly influenced by the casting temperature and the casting speed. In particular, as shown in FIG. 2, the difference between the lower limit value a of the appropriate casting temperature determined by the above formula (3) and the casting temperature T, that is, Ta is simply the lower limit value of the casting speed. Directly proportional. As a result of performing a casting test on a plurality of steels and obtaining a proportionality coefficient, it has become clear that the following formula (2) is established with respect to the casting speed Vc.
Vc ≧ 0.02 × (T−a) (2)

  That is, by setting the casting temperature T and the casting speed Vc so as to satisfy the above formulas (1) and (2), it is possible to suppress the occurrence of slab surface defects and perform stable continuous casting. .

Even if the above formulas (1) and (2) are satisfied, surface defects, specifically fogging, may occur in the non-stationary stage at the beginning of casting.
Therefore, as a result of further investigations by the present inventors, the relationship between the casting temperature T and the casting speed Vc, which can stably suppress the entire casting including the unsteady stage fogging early in casting, is as follows. It is represented by the following formulas (1 ′), (2 ′) and (2 ″).
a ≦ T ≦ a + 30 (1 ′)
Vc ≧ 0.025 × (T−a) (2 ′)
Vc ≧ 0.5 (2 ″)

  In the above formula (2 ″), the lower limit value of the casting speed is determined based on the fact that the range of the casting temperature at which casting can be performed stably decreases as the casting speed decreases. In the temperature control capability, when trying to control the casting temperature to a target temperature, an error of ± 10 ° C. inevitably occurs with respect to the target temperature. Further, the above formula (2 ′) means that when the casting speed decreases, the temperature range of Ta allowed at the casting speed (that is, the casting temperature range in which casting can be stably performed) decreases. In particular, when the casting speed is less than 0.5 m / min, the allowable temperature range of Ta is less than 20 ° C. Therefore, as shown in the above formula (2 ″) The lower limit of casting speed is 0.5m Cast as min is decided to ensure the stable performed.

  On the other hand, if the casting speed is excessively high, the cooling in the mold becomes insufficient, and the solidified shell is not formed soundly in the mold, and is secondarily cooled out of the slab or mold in the mold. The risk of breakout of the solidified shell is increased.

(4) Mold Flux Next, a mold flux suitable for continuous casting of the high manganese nonmagnetic steel according to the present invention was examined. In continuous casting of steel, mold flux is supplied onto the molten steel surface in the mold to form a molten layer by supplying heat from the molten steel, keeping the molten surface warm and flowing between the mold and the solidified shell. Acts as a thermal resistance and lubricant.

The mold flux is mainly composed of a mixture of oxides such as CaO and SiO ₂ and powder or granular carbon. The molten steel of high carbon-high manganese content steel supplied to the continuous casting mold has a high Mn content, and Mn is easily oxidized, so that it easily reacts with an oxide component in a general mold flux. When Mn in the molten steel reacts with the molten mold flux, for example, the MnO concentration in the molten mold flux is increased by the reaction formula (E), and the molten mold flux composition changes from the initial composition. When the molten mold flux composition changes, the viscosity of the molten layer changes, the amount of molten mold flux flowing between the mold and the solidified shell and the heat transfer characteristics change during operation, and the solidification behavior in the mold is unstable. It becomes a factor that hinders operation.
2 [Mn] + (SiO ₂ ) = 2 (MnO) + [Si] (E)
[Mn]: Solute Mn in molten steel
[Si]: Solute Si in molten steel
(MnO): MnO in molten mold flux
_{(SiO 2): SiO} 2 in the molten mold flux

  Therefore, by a crucible reaction experiment, it was decided to examine a mold flux that hardly causes the reaction of the above formula (E) even when contacted with Mn-containing molten steel, and hardly changes in composition.

Details of the crucible reaction experiment will be described. In an argon gas atmosphere, about 3 kg of a molten steel sample containing 1% C and 13% Mn was melted in a magnesia crucible and maintained at 120 ° C. with a degree of superheat (difference between liquidus temperature and casting temperature). Various kinds of fluxes preliminarily adjusted with SiO ₂ and CaO as main components were supplied onto the molten steel in the crucible. The SiO ₂ content of the flux used is 25 to 50% by mass, and the basicity expressed by CaO / SiO ₂ (mass ratio) is 0.5 to 2.0. Further, in order to adjust the viscosity and the freezing point, 3 mass% or less of Al ₂ O ₃ , 12 mass% or less of Na ₂ O, and 12 mass% or less of F are added to the flux supplied into the crucible. ing.

  When 30 minutes had passed since the mold flux was supplied into the crucible, samples were taken from the molten steel and the molten mold flux, and the respective compositions were analyzed and evaluated. As a result, Mn was detected from the molten mold flux, and the reaction of the above formula (E) occurred, that is, the transition of Mn in the molten steel to the molten mold flux and the transition of Si in the molten mold flux to the molten steel. confirmed.

  FIG. 3 shows the relationship between the basicity and the MnO concentration in the molten mold flux obtained as a result of the above crucible reaction test. As shown in FIG. 3, when the basicity in the molten mold flux is low (less than 0.9), MnO concentration in the molten mold flux tends to easily transfer Mn from the molten steel to the molten mold flux as the basicity decreases. The tendency to increase was confirmed. On the other hand, when a mold flux having a basicity of 0.9 or more was used, the MnO concentration in the molten mold flux supplied on the molten steel containing 13% by mass of Mn was saturated at about 10% by mass. That is, it became clear that the transition reaction represented by the above formula (E) can be limited by increasing the basicity in the molten mold flux to some extent.

  Based on the above findings, in order to confirm the stability of the mold flux, various Mn contents (including steels with a high concentration higher than the target composition, including steel composition with a maximum Mn content of 26%) A quantity of molten steel was subjected to a mold flux evaluation continuous casting test to evaluate the stability of the mold flux and the surface quality of the slab. As a result, in continuous casting experiments using molten steels with Mn contents of 13%, 19%, 24%, and 26%, stable casting is possible by using the mold flux of MF1 in Table 3 to be described later. Surface defects could also be suppressed. In addition, the Mn-containing steel having a higher concentration than the above-described composition range was used in the mold flux evaluation continuous casting test, confirming that the mold flux can be applied to a wider range of compositions, and the molten mold flux characteristics during casting are This is to confirm that it is stable.

From the above crucible experiment and flux evaluation continuous casting test, it was found that the mold flux preferably has the following compositional characteristics.
(Feature 1) The mass ratio CaO / SiO ₂ between CaO and SiO ₂ is adjusted to a range of 0.9 to 2.0.

(Feature 2) The MnO content in the mold flux is adjusted to the range of the following formula (F).
0.40 × (Mn concentration in steel, unit: mass%) <(MnO content in mold flux, unit: mass%) <(Mn concentration in steel, unit: mass%) (F)
Hereinafter, the above features will be described.

As shown in the experiment using the crucible described above, when the basicity CaO / SiO ₂ is controlled to be 0.9 or more, Mn migration from steel to molten mold flux can be suppressed. If the basicity CaO / SiO ₂ is less than 0.9 decreases the function of suppressing the migration of the molten mold flux in steel Mn. Further, when the basicity CaO / SiO ₂ is excessively high, the solidification temperature rises and the inflow characteristics become unstable because of excessive crystallization. Therefore, the upper limit of the appropriate CaO / SiO ₂ range is set to 2.0.

In the mold flux, CaO, SiO ₂ and MnO are “main components”, and F, Na ₂ O, Al ₂ O _{3 and the} like are added as subcomponents. Here, the “main component” means that the total of the three components is 70% by mass or more. The specific content of each component contained in the mold flux is appropriately set according to the viscosity and solidification temperature characteristics required for the mold flux.

The above formula (F) in the feature ₂ is a range in which the composition does not change so as to significantly affect the physical properties of the molten mold flux, assuming that CaO / SiO ₂ is in the range shown in the feature 1. Is shown. As clarified by the above-described continuous casting experiment for mold flux evaluation, the minimum MnO content in the molten mold flux with respect to the Mn concentration in the steel is about 40% (MnO concentration in the mold flux with respect to the molten steel having a Mn concentration of 26% by mass is 10%. Mass) until equilibrium is reached with the transition of Mn from steel to molten mold flux. Based on the above examination, the lower limit of MnO / Mn is set to 0.40.

  In particular, in order to suppress an increase in the MnO content in the molten mold flux, it is desirable that MnO / Mn is 0.53 (MnO concentration in the mold flux with respect to molten steel having a Mn concentration of 19% by mass is 10% by mass) or more.

On the other hand, when the MnO content is excessively high, there is a concern that inflow characteristics become unstable, so the upper limit of MnO / Mn is 1.0.
Table 3 shows examples of mold fluxes suitable for continuous casting of nonmagnetic steel containing high manganese.

(5) Characteristics of composition with good hot workability As described above, the relational expression between the solidus temperature TS and the composition of the high carbon-high manganese content steel obtained from the thermal analysis experiment is the following formula (D). .
TS = 1333 − {− 108 × (% C) + 125 × (% C) ² + 5.5 × (% Mn) + 400 × (% P)} (D)

In order to achieve good hot workability, it is preferable that the solidus temperature is higher than 1200 ° C. which is the upper limit of normal hot working temperature such as bending correction, forging or rolling during continuous casting. . When 1210 ° C. is selected as the lower limit of the hot embrittlement temperature, the following formula (G) representing the composition condition is determined so that the right side of the formula (D) is greater than 1210 ° C.
1333 − {− 108 × (% C) + 125 × (% C) ² + 5.5 × (% Mn) + 400 × (% P)}> 1210 (G)

  Here, (% C), (% Mn) and (% P) in the above formula (G) are the contents (unit: mass%) of C, Mn and P in the chemical composition of steel, respectively.

If (% Mn) is arranged from the formula (G), the formula (H) representing the relationship between the Mn content, the C content and the P content in the steel composition is obtained.
(% Mn) -19.6 × (% C) + 22.7 × (% C) ² + 72.7 × (% P) <22.4 (H)
Here, (% C), (% Mn) and (% P) in the above formula (H) are the contents (unit: mass%) of C, Mn and P in the chemical composition of steel, respectively.

As a more desirable composition condition for realizing good hot workability, the composition satisfies a solidus temperature, that is, a formula (G ′) in which the right side of the formula (D) is higher than 1225 ° C. The composition condition is represented by the following formula (H ′).
1333 − {− 108 × (% C) + 125 × (% C) ² + 5.5 × (% Mn) + 400 × (% P)}> 1225 (G ′)
(% Mn) -19.6 × (% C) + 22.7 × (% C) ² + 72.7 × (% P) <19.6 (H ′)

  Here, (% C), (% Mn) and (% P) in the above formulas (G ′) and (H ′) are the contents of C, Mn and P in the chemical composition of steel (unit: Mass%).

FIG. 4 shows the relationship between the Mn content, the C content, and the allowable P content in various steel compositions determined by the above formula (H) determined with the solidus temperature of 1210 ° C. as the limit.
Further, FIG. 5 shows the relationship between the Mn content, the C content, and the allowable P content in the steel composition calculated by the formula (H ′) and determined with the solidus temperature of 1225 ° C. as the limit.

Example 1
The test by the continuous casting machine was done using 2.5 ton molten steel. Molten steel having a predetermined composition was melted in a melting furnace and poured into a tundish through a ladle. Adjusting the molten steel temperature of the tundish to 1430-1490 ° C., supplying hot water to the internal water-cooled copper plate mold that vibrates up and down from the immersion nozzle, and continuously casting under the conditions of Table 4 using the mold flux of the symbol MF1 composition of Table 3 In the lower part of the mold, secondary cooling by water spray was performed to obtain a slab slab having a thickness of 100 mm × width of 600 mm × length of 5000 mm. In the secondary cooling, the amount of water per 1 kg of slab weight (specific water amount) was set to 0.5 to 1.0 L. After cooling the slab to room temperature, the slab was examined for the presence or absence of surface defects on the slab, and a part was used as a base material sample for a hot rolling test. The results are as shown in Table 4.

No. of the present invention example In No. 1, an extremely slight fogging occurred within 60 seconds from the beginning of casting, but it was almost sound. No. of the present invention example In Nos. 2 to 4, there was no fogging defect on the surface of the slab, and there were no cracks during the hot rolling test.
On the other hand, no. In all of Nos. 5 to 6, fogging defects on the slab surface occurred. Comparative Example No. In No. 7, the molten metal surface in the mold caused by the low temperature molten steel was peeled off.

(Example 2)
Based on the knowledge obtained in the above tests, a full-scale test was conducted.

  A Mn alloy was added to the mother molten steel refined in the converter, and a high carbon-high manganese content steel having a predetermined composition was melted. The obtained molten steel was poured into a tundish from a ladle, poured into a mold from an immersion nozzle provided at the bottom of the tundish, and continuous casting was performed. The main casting conditions are as follows.

(Non-magnetic steel casting conditions 1)
-Molten steel composition: 0.94% C, 0.35% Si, 13.9% Mn, 0.01% P, 0.008% S in mass%. Note that a for this composition is 1444 ° C.

-Molten steel amount: 80 tons
-Molten steel temperature of tundish: 1450-1477 ° C
・ Cross section size: 300mm × 400mm
・ Continuous casting method: vertical bending mold, bending part radius 15m
・ Casting speed: 0.7m / min
Mold flux: Symbol MF2 composition in Table 3 Secondary cooling specific water amount: 0.3 L / kg
When continuous casting was performed under such conditions, casting was stable. The obtained slab was sound over its entire length, and no fogging occurred.

(Non-magnetic steel casting conditions 2)
-Molten steel composition: 1.0% C, 0.3% Si, 14.1% Mn, 0.025% P, 0.005% S in mass%. The a for this composition is 1439 ° C.

-Molten steel amount: 160 ton (80 ton continuous casting)
-Molten steel temperature of tundish: 1445-1470 ° C
・ Cross section size: 330mm × 450mm
-Continuous casting method: curved type, bending part radius 16m
・ Casting speed: 0.7m / min
Mold flux: Symbol MF2 composition in Table 3 Secondary cooling specific water volume: 0.25 L / kg

When continuous casting was performed under such conditions, casting was stable. The obtained slab was sound over its entire length, and no fogging occurred.
As described above, by satisfying the conditions of the present invention, stable production is possible by any type of continuous casting method of vertical bending type and curved type, and a sound slab to be a hot rolling base material is obtained. It was.

  The slab obtained by continuous casting under each of casting conditions 1 and 2 was reheated to 1100 ° C. and hot-rolled to obtain a bar steel. During this hot rolling, no cracks, wrinkles, etc. occurred, and a high-quality nonmagnetic steel material was obtained.

  About the evaluation result obtained by the above Example 1 (Example on a laboratory scale) and Example 2 (Example on an actual machine scale), casting speed V and Ta (casting temperature T and the above formula (3) FIG. 2 shows a graph organized by the relationship with the difference between the a value calculated in (1). As shown in FIG. 2, fogging occurs when the casting speed V is too low or when Ta is too high.

Claims (3)

C: 0.45% to 1.3%, Si: 0.05% to 0.5%, Mn: 10% to 19%, P: 0.10% or less, S: 0% by mass High manganese having a chemical composition containing 0.02% or less, Al: 0.003% or more and 0.1% or less, N: 0.005% or more and 0.30% or less, and a magnetic permeability of 1.1 or less A non-magnetic steel produced by a continuous casting method,
While controlling so that casting temperature T satisfy | fills Formula (1),
A method for producing nonmagnetic steel, wherein the casting speed Vc (m / min) is selected within the range of the following formula (2):
a ≦ T ≦ a + 50 (1)
Vc ≧ 0.02 × (T−a) (2)
Here, a is a value determined by the following formula (3) from the composition of steel, and (% C), (% Mn) and (% P) in the following formula (3) are respectively steel. Content of C, Mn and P in the chemical composition (unit: mass%).
a = 1557− {53 × (% C) + 4.5 × (% Mn) + 45 × (% P)}
... (3) MnO, CaO, a continuous casting method of a non-magnetic steel according to claim 1 for continuous casting using a mold flux containing SiO ₂ as a major oxides, containing MnO contained in the mold flux the amount (unit: mass%) Mn concentration of the molten steel to be continuously cast are: and (unit weight%) and the following formula (4) full, the mass ratio CaO / SiO ₂ with respect to SiO ₂ of CaO contained in the mold flux The manufacturing method whose is 0.9 or more and 2.0 or less.
0.40 × (Mn concentration in steel) <(MnO content in mold flux) <(Mn concentration in steel) (4) The manufacturing method of the nonmagnetic steel of Claim 1 or 2 with which the chemical composition of the molten steel continuously cast satisfy | fills following formula (5).
(% Mn) -19.6 × (% C) + 22.7 × (% C) ² + 72.7 × (% P) <22.4 (5)
Here, (% C), (% Mn) and (% P) in the above formula (5) are the contents (unit: mass%) of C, Mn and P in the chemical composition of steel, respectively.

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