JPH1133688A - Method for continuously casting steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent the surface cracking of traverse cracking, traverse crazing, etc., while securing sufficiently high correcting temp. without adding Ti. SOLUTION: A steel is cast so as to contain 0.003-0.06 wt.% Al and/or 0.005-0.1 wt.% Nb with a curving type or a vertical and bending type continuous caster. In such a case, a cast slab is cooled at 4-15 deg.C/sec cooling speed of the surface layer just after coming out from a mold and the cooling is stopped in the temp. range of < Ar3 point to the temp. at non-completion of γ phase transformation on the surface layer temp. and then, the surface temp. is returned back to 950-1200 deg.C and the correction is executed with the following inequality. 17-0.002×D>ε. In this inequality, D is the thickness of the cast slab (mm) and εis correcting strain (%).

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of continuously casting steel for preventing surface cracks of a steel slab containing Nb or the like.

[0002]

2. Description of the Related Art Nb, V, N
The production of low alloy steels containing various alloying elements such as i, Cu, etc. is increasing. However, during the continuous casting accompanied by the addition of these alloying elements, surface cracks called lateral cracks and lateral cracks on the slab surface (hereinafter referred to as "surface cracks").
May occur, which is a manufacturing problem. These surface cracks are formed along the austenite (hereinafter referred to as “γ”) grain boundaries weakened by precipitation of AlN or NbC or the like near the surface or pro-eutectoid ferrite (hereinafter referred to as “ferrite”) formed at the γ grain boundaries. Along with “α”),
It is known that this occurs when straightening stress for slab straightening is applied. Therefore, severe surface cracking occurs when the temperature is corrected at 600 to 850 ° C., which is a temperature range where γ is transformed into α. Therefore, a method is usually employed in which a temperature range in which the hot ductility of the surface is reduced (hereinafter, referred to as “brittle temperature range”) is avoided on a low temperature side or a high temperature side to perform correction and suppress surface cracking. . However, it is impossible to prevent surface cracking only by controlling the surface temperature at the time of slab correction, and various methods have been proposed.

For example, immediately after leaving the mold, the slab surface temperature is changed from γ to α once by forcibly cooling the slab to 650 to 700 ° C. A method of adopting a cooling pattern to be avoided on the side has been proposed (Japanese Patent Publication No. 58-3790). However, in steel types having high crack susceptibility due to the high content of alloying elements such as Ni, it is not possible to change the cooling pattern to avoid the brittle temperature range at the time of slab correction to a lower temperature side.

[0004] Also, before charging the heating furnace, the surface layer of the slab is 350
There is also disclosed a method of cooling and maintaining the temperature in a temperature range of -500 ° C for 1 minute or more (JP-A-5-329505). This method is a method in which the slab surface temperature is once lowered, thereby transforming most or all of the slab from γ to α, thereby reducing the γ grain size and systematically reducing cracking sensitivity. . However, once most of the slabs have dropped to 700 ° C. or less, it is not thermally possible to avoid the embrittlement temperature range on the high temperature side even if the slabs are subsequently double heated.

Further, since surface cracks occur at the γ grain boundaries, many proposals have been made to focus on the γ grain size and to reduce the γ grain size. In order to suppress the growth of γ grains, the applicant has set the cooling rate from the temperature at which γ becomes a single phase during solidification by 1
0 ° C./sec or higher (Japanese Patent Application Laid-Open No. 63-63559) and a method of setting the relational expression of the mold length, drawing out the slab early, and immediately performing secondary cooling (Japanese Patent Application Laid-Open No. 61-195)
742). However, the position during continuous casting where the slab surface reaches the γ single-phase temperature is usually in the mold immediately after solidification, it is difficult to control the cooling rate, and the mold length is extremely shorter than usual. Doing so can easily lead to operational problems, which is an obstacle to practical application.

On the other hand, AlN or N
It is known that bC is precipitated, and stress concentration accompanying precipitation of AlN or the like promotes cracking. On the other hand, in order to suppress AlN precipitation concentrated on the γ grain boundaries, Ti is often added to precipitate TiN in the grain boundaries and in the grains at a high temperature range, and a high effect is obtained. Further T
A method of controlling the precipitation of AlN by controlling the cooling conditions in addition to the addition of i has also been proposed (Japanese Patent Publication No. 55-1979).
No. 7106). However, due to material property requirements, T
There are many steel types to which i cannot be added, and there is a problem in that precipitation control of AlN or the like depending only on cooling conditions lacks stability.

Therefore, although many methods have been proposed for preventing the surface cracks of cast slabs, there are problems such as (a) it is not possible to secure the straightening temperature, and (b) it cannot be applied to steel that cannot contain Ti. Yes, surface cracks are still occurring frequently.

[0008]

SUMMARY OF THE INVENTION The object of the present invention is to solve the problem of Ti
It is an object of the present invention to provide a continuous casting method of steel that prevents surface cracking while adding a sufficiently high straightening temperature without adding.

[0009]

Means for Solving the Problems In order to solve the above-mentioned problems, the present inventors focused on a cooling pattern of a slab surface immediately below a continuous casting mold and a distortion amount at the time of straightening, and the like. As a result of repeated studies using, the following items could be confirmed.

(A) The main causes of surface cracks are AlN and NbC
Precipitates are concentrated on the γ grain boundaries.However, in order to prevent concentrated precipitation of AlN etc. on the γ grain boundaries, all the γ is cooled to the low temperature region where the transformation is completed as in the past. do not have to.

(B) When the surface is reheated after the cooling is stopped during the transformation of γ to produce pro-eutectoid α, the above precipitates can be dispersed and precipitated in the grain boundaries and in the grains. And a sufficiently high correction temperature can be ensured.

(C) On the premise of the above (a) and (b), in the case of straightening, if a certain relationship is satisfied between the amount of straightening and the thickness of the slab, it is possible to reliably prevent surface cracks. it can.

The present invention has been completed by combining the above items, and the gist of the invention resides in the following continuous casting method.

"By weight, Al: 0.003 to 0.0
6% and Nb: a method for producing a steel slab containing one or two of 0.005 to 0.1% by a curved or vertical bending type continuous casting machine, and drawn from a mold. After cooling the slab under the condition that the cooling rate in the surface layer is 4 to 15 ° C./sec, the cooling is stopped in a temperature range where the surface temperature is less than the Ar ₃ point and the transformation of the austenite phase is not completed, and then the surface layer is cooled. A continuous casting method for steel that is reheated to 950 to 1200 ° C. and that corrects a slab under the following formula to prevent surface cracks.

1.7−0.002 × D> ε Here, D represents slab thickness (mm), and ε represents straightening strain (%). In the above invention, the steel refers to a low alloy steel, more specifically, a low alloy steel having a C content of 0.2% or less.
“Surface layer” refers to a surface portion up to 5 mm inward from the slab surface. In the following description, the cooling rate of the surface layer is 4 to
Cooling under the condition of 15 ° C./sec may be referred to as “rapid cooling”. The quenching temperature refers to the lowest temperature at which the surface layer reaches by quenching. The recuperation temperature refers to the maximum temperature that the surface layer reaches by recuperation. Reheating after quenching the surface layer is called "quenching reheating". The correction distortion amount refers to the distortion amount that the surface, not the surface layer, receives at the time of correction. In the case of multipoint correction, it refers to the total correction distortion amount at each point. For example, in the case of N-point correction, the correction distortion amount ε = ε ₁ + ε ₂ +... + Ε
_N.

Next, the technical contents of the method of the present invention will be described in detail.

For the generation of lateral cracks, Al precipitated in steel
It has been found that N and NbC have a great effect.

FIG. 3 is a drawing showing the solubility products of Al and N and Nb and C in steel. FIG. 3A shows the solubility product of Al and N, and FIG. 3B shows the solubility product of Nb and C. The solubility product increases with increasing temperature, and γ
It can be seen that the solubility product is larger in the middle. For example, C:
0.07%, Al: 0.03%, Nb: 0.03%,
In the case of steel containing N: 35 ppm (hereinafter referred to as “steel S”), when the precipitation onset temperatures of AlN and NbC are determined based on the above solubility product, both are about 1050.
° C and 970 ° C. Steel S is in the γ phase at these temperatures. That is, both AlN and NbC should begin to precipitate in the γ phase in an equilibrium manner. AlN and NbC start to precipitate in the γ phase in most low alloy steels except for the α-type stainless steel, not limited to the steel S.

Next, the precipitation behavior during actual continuous casting will be considered. Simulated temperature history in normal continuous casting (a)
Two cases of slow cooling and (b) rapid cooling / reheating will be considered.

FIG. 2 is a drawing showing the temperature histories of the surface layer of the slab and the precipitation behavior of AlN and NbC accompanying these temperature histories. FIG. 2A shows the case of slow cooling, and FIG. 2B shows the case of rapid cooling and reheating. FIG. 2 (c) shows the time transition of the surface temperature of the slab in the case of slow cooling and rapid cooling and reheating.

In the case of gradual cooling, the surface layer temperature of the slab becomes equal to or lower than the precipitation start temperature, and when the slab is further cooled to some extent, precipitation of AlN and NbC at the γ grain boundary 1 starts. At this time, the precipitates 2 are coarse and distributed in a point sequence along the grain boundaries. At the time of slab rectification, precipitates 3 such as AlN precipitate in the grains by dynamic precipitation due to the correction stress. When coarse AlN or the like 2 precipitates at the grain boundary 1, a non-precipitation zone 4 is generated near the grain boundary. Further, if the Ar content is ₃ points or less, “pro-eutectoid α” 6 precipitates at the grain boundary.
The straightening stress concentrates on a fragile part such as the like, and cracks occur (Y. Maehara et al: Material)
Science Engineering 62 (1
984) 109).

On the other hand, if the alloy is quenched immediately below the mold to the Ar ₃ point or lower, sufficient time for AlN or NbC to precipitate at the γ grain boundary cannot be obtained. In addition, since the solubility product in pro-eutectoid α is significantly smaller than that in γ,
With the formation of Al, 5 such as AlN is precipitated inside the pro-eutectoid α and at the interface. Considering the diffusion rate, the size of the pro-eutectoid α grains generated at this time is considered to be about 10 μm, and the precipitates of AlN and NbC are dispersed in these grains. AlN and NbC at the time of the above-mentioned slow cooling precipitate at the grain boundaries in a point sequence, whereas
Once quenched, the precipitates 7 have a distribution distributed around the grain boundaries.

On the other hand, with respect to the precipitates at the time of recuperation, as in the case of steel S described above, when the precipitation starting temperatures of AlN and NbC in the equilibrium state are 1050 ° C. and 970 ° C.,
In view of the rate of solid solution and diffusion, these precipitates do not completely dissolve unless the temperature greatly exceeds these temperatures at the time of reheating. From previous experience, precipitates remain in most low alloy steels when the reheat temperature is less than 1200 ° C. When quenched and reheated, the distribution of AlN and NbC at the correction point is different from that at the time of slow cooling, and there is no fragile portion such as a non-precipitation zone near the grain boundary, and the susceptibility to surface cracking is improved.

[0024]

Next, the reason why the limited range of the present invention is set as described above will be described. In the following description, “%” representing the content of alloying elements in steel means “% by weight”.

1. The chemical composition of steel The reasons for limiting the contents of Al and Nb are as follows.

The method of the present invention is directed to a steel having high surface cracking susceptibility because it contains one or two of Al and Nb.

Al is added for deoxidation and control of metal structure. If the Al content is less than 0.003%, the deoxidation is insufficient, and the metal structure of the product cannot be made fine,
As a result, it becomes difficult to secure toughness. On the other hand, 0.06%
If it exceeds 300, the precipitation start temperature of AlN becomes high and the precipitate becomes coarse, so that the effect of preventing cracking cannot be obtained. Therefore, in the steel according to the method of the present invention, Al is set to 0.003.
For steel containing up to 0.06%.

Nb is generally added to improve the strength of steel. If the Nb content is less than 0.005%, NbC
The effect of improving the strength due to the precipitation cannot be obtained. On the other hand, 0.1
%, The effect of improving strength saturates and the toughness of the heat affected zone decreases, so the upper limit is made 0.1%.

Also, steel containing C in the range of 0.04% to 0.2% is highly sensitive to such surface cracks. When the C content is in the above range, it is possible to control the precipitation of NbC by changing the above-mentioned cooling conditions. Therefore, the method of the present invention is desirably used for steel containing 0.04 to 0.2% of C. .

The surface cracking susceptibility is closely related to the “carbon equivalent of peritectic reaction” (hereinafter referred to as “Cp”) in the following formula. It is known to grow. The method of the present invention can suppress surface cracking even when Cp is in the range of 0.1 to 0.18. Therefore, Cp is 0.1 to 0.18
Is particularly effective for steels in the range of

[0031] 2. Cooling Next, the rapid cooling of the slab immediately after leaving the mold, that is, the cooling rate of the secondary cooling will be described.

When the cooling rate of the surface layer is low, coarse AlN or NbC precipitates in a dotted line in the γ region as described above, and AlN or Nb
The effect of dispersing and depositing C cannot be expected. For this reason,
The lower limit of the cooling rate is 4 ° C./sec. In order to enhance the effect of quenching, the amount of water in the secondary cooling spray and mist will be increased, but if the amount of water is excessively increased, the solidification shell is broken by water pressure, and water is blown up from the gap between the mold and the slab, It leads to big troubles such as. Therefore, the cooling rate of the surface layer cannot be made higher than 15 ° C./sec. On the other hand, if the cooling rate exceeds 15 ° C./second, the temperature of the slab surface tends to be uneven, and there is a danger of cracking due to thermal stress. From the viewpoint of stable operation, it is more desirable that the cooling rate to the Ar ₃ point be less than 10 ° C./sec.

The temperature at which the cooling is stopped, that is, the quenching temperature, is within a temperature range of less than the Ar ₃ point and in which all the γ phases do not end the transformation. Unless quenched to less than the Ar ₃ point, pro-eutectoid α does not form at the grain boundaries, and A
Precipitation of 1N, NbC, etc. cannot be prevented. On the other hand, when the γ phase is rapidly cooled until the transformation is completed, the effect of making the γ particle size finer increases, but the temperature of the surface layer of the slab at the correction point cannot be sufficiently high. In the vicinity of the transformation end temperature, pearlite precipitates and precipitates such as cementite occur, which adversely affects the structure of the slab after continuous casting. Therefore, the transformation must not be completed during rapid cooling after passing through the mold, and it is necessary to stop the rapid cooling during the transformation to recover the heat. The Ar ₃ point is 700 to 850 ° C. in the case of the above C content and the like.

The temperature at which the transformation of the γ phase is completed is 400
４５０450 ° C. Note that both the Ar ₃ point and the temperature at which the γ phase completes transformation are dependent on the cooling rate, and the higher the cooling rate, the lower the temperature. However, in the case of the low alloy steel targeted by the method of the present invention, when the cooling rate is in the range of 4 to 15 ° C./sec, it can be regarded as substantially constant. If the temperature fluctuates greatly, the Ar ₃ point is a temperature at a cooling rate of 15 ° C., and the temperature at which the γ phase completes transformation is 4 ° C./sec.

In the actual secondary cooling of continuous casting, the cooling effect is varied between the rolls due to the roll contact portion, the heat radiating portion, the spray portion, the accumulated water portion, etc., and the cooling and reheating are repeated. However, the influence of these temperature fluctuations on precipitation is small, and the cooling rate in the present invention may be determined by the average temperature change in the strand.

3. Continuous cooling after reducing the temperature of the slab surface layer below the temperature at which the γ phase completely completes the transformation, remarkably reduces the surface layer temperature at the straightening point and increases the strength of the slab. There is a problem that the operation cannot be performed. Even if the amount of cooling water is reduced, it is inevitable that the surface layer temperature will gradually decrease unless the heat is restored, and the same problem occurs. Furthermore, if the surface temperature of the slab at the time of recuperation is not sufficiently ensured, the surface temperature of the slab at the correction point becomes about 800 ° C. and enters the embrittlement region near the Ar ₃ point, so that a surface crack occurs. Therefore, at the time of reheating, the supply of cooling water is stopped, and the reheating temperature is set to 950 ° C. or higher. In order to stably maintain the correction temperature, it is more preferable that the reheating temperature is 1000 ° C. or higher.

However, when the reheating temperature exceeds 1200 ° C., the precipitate is dissolved, and the temperature of the surface layer of the slab is reduced to Ar.
_The effect of quenching to ₃ points or less disappears. In the recuperation method, it is not necessary to heat the surface, it is economical to reduce or stop the amount of cooling water, and to recuperate by the calorie of the slab, and to obtain a uniform surface temperature.

4. Correction Next, a description will be given of a change in susceptibility to cracking due to the thickness and strain amount of a slab in correction. The knowledge about the straightening was obtained as a result of investigating the state of occurrence of cracks under various continuous casting conditions and slab thickness conditions while keeping conditions such as the composition of the steel and the straightening temperature substantially constant.

FIG. 1 is a diagram showing the range of the amount of correction strain at which surface cracks occur and the thickness of the slab. In this case, the cooling conditions were such that the mold was once cooled to about the Ar ₃ point at a rate of about 6 ° C./sec after leaving the mold, and then reheated. The amount of straightening is determined by the specifications of the continuous casting machine and the thickness of the slab, and is determined by the following equation. In the case of multipoint correction, the sum of the distortion amounts at each correction point is defined as the distortion amount.

[0040]

【number】

Here, D is the slab thickness (mm), and
Rn and Rn + 1 represent the radius of curvature of the curvature before and after the correction.

As a result of this study, it was found that as the thickness of the slab increases, cracking occurs with a smaller amount of strain. The conditional expression for preventing surface cracking is set as follows from the boundary line of the presence or absence of cracking in the figure.

1.7−0.002 × D> ε Since the object of the present invention is to prevent surface cracks that occur when the slab is straightened, it is curved or vertical. Bending type (VB
The target is a method of manufacturing using a continuous casting machine of (type).

[0044]

EXAMPLES Next, the effects of the method of the present invention will be described with reference to examples.

[Embodiment 1] In Embodiment 1, the straightening condition is a fixed condition that satisfies the equation, that is, a slab thickness of 235 m.
m, the correction distortion amount was set to 0.95%, and a test was performed in which the secondary cooling conditions were variously changed. As continuous casting machines, all are operating on the actual production line, a three-point straightening type curved continuous casting machine having a length of 23 m, and a one-point straightening type VB having a length of 12 m.
A mold continuous casting machine and a one-point correction type curved continuous casting machine having a length of 18 m were used. Secondary cooling conditions varied the amount of cooling water, and changed the rapid cooling speed, rapid cooling temperature, reheat temperature, and the like. The surface layer temperature of the slab was measured by a thermocouple biting into the surface layer from immediately below the mold and a radiation thermometer installed in the chamber.

Table 1 shows the chemical composition of the molten steel used in the continuous casting test. The Ar ₃ point of the steel in the above cooling rate range is about 850 ° C., and the temperature at which the γ phase completes 100% transformation is about 475 ° C.

[0047]

[Table 1]

Since it is difficult to determine the occurrence of surface cracks as it is with black scales, a scarf having a thickness of about 1 mm was applied to the surface of the slab and evaluated by visual observation. Three or more surfaces of 1 m ² or more including the width from the center to the edge of the steady portion of the slab were inspected, and the total length of cracks generated was converted to about 1 m ² and evaluated.
In addition, it was previously confirmed that this inspection method was a representative test method by extracting an average condition of a standard slab surface.

Table 2 is a list showing the test results.

[0050]

[Table 2]

As described above, in order to suppress surface cracking, it is important that the surface layer temperature at the correction point is outside the range of the embrittlement temperature range. Test Nos. 1 to 3, which are examples of the present invention,
In Test Nos. 4, 6, and 7, which are comparative examples, the surface temperature at the correction point was almost 880 to 9 so as to avoid the embrittlement temperature range.
The amount of cooling water is controlled so as to be about 20 ° C.

When the specimen was gradually cooled as in Test No. 4 which is a comparative example, cracks having a depth of 5 mm or more occurred on the entire surface of the slab despite the same correction temperature as described above. Therefore, when the cooling rate was as low as 2 ° C./sec or 3 ° C./sec even after cooling to the Ar ₃ point, the heat was recovered and the crack was generated even at the same correction temperature. Test No. 8 in which the surface layer temperature reached only immediately above the Ar ₃ point even after quenching once had cracks similar to Test No. 4, and the quenching effect was not recognized. Reheat temperature is 123
In Test No. 7, which was too high at 0 ° C., severe cracking occurred.
For Test No. 5 in which the recuperation temperature after quenching was low, the correction temperature could not be secured because the recuperation temperature was low, and cracks occurred.

On the other hand, the cooling rate from the mold to the Ar ₃ point was 5.0 ° C./sec or 9.2 ° C./sec. In Test Nos. 1 to 3, no surface cracks occurred.

According to the results, as long as the formula is satisfied at the time of straightening, the temperature of the slab surface is once quenched immediately below the mold to reduce the temperature of the slab surface to less than the Ar ₃ point. It has been found that surface cracks can be prevented by setting the temperature history so that the temperature of the solidified molten steel is restored to 950 ° C. or higher by the latent heat of solidification.

[Example 2] Next, the results of changing the conditions for applying distortion during straightening while setting the cooling conditions in continuous casting within the scope of the method of the present invention will be described.

Table 3 shows secondary cooling conditions and correction conditions in Example 2.

[0057]

[Table 3]

The method for investigating the occurrence of cracks is the same as in the first embodiment. According to the test results shown in Table 3, in Test No. 11 in which the thickness of the slab was 300 mm and the conditions of the formula were not satisfied, the cooling conditions and the straightening conditions were Test Nos.
Despite being equivalent to 3, surface cracks occurred. In addition, the surface layout also occurred in Test No. 12 where the distortion amount was changed by changing the roll layout for Test No. 11 and was outside the range of the formula.

On the other hand, if the secondary cooling conditions and the formula for straightening of the method of the present invention are satisfied even if the thickness and strain amount of the slab are changed, cracks as shown in Test Nos. 9 and 10 are observed. Did not occur.

As a result, even if the secondary cooling conditions were within the range of the method of the present invention, it was clarified that cracking would occur if the slab thickness and strain amount did not satisfy the equations in straightening.

[0061]

According to the method of the present invention, it is possible to manufacture a continuous cast slab in which surface cracks are suppressed by securing a sufficient correction temperature without adding Ti or the like.

[Brief description of the drawings]

BRIEF DESCRIPTION OF DRAWINGS FIG. 1 is a view showing the effects of straightening strain and slab thickness on surface cracking in straightening.

FIG. 2 is a diagram schematically illustrating a precipitation behavior of a precipitate in continuous casting. (A) is for slow cooling, (b) is for rapid cooling,
(C) shows each temperature transition of (a) and (b).

FIG. 3 is a diagram showing the temperature dependence of the solubility product of the constituent elements of the precipitate. (A) shows the case of Al and N, and (b) shows the case of Nb and C.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Austenitic grain boundary 2 ... Precipitates, such as AlN, which precipitated on austenite grain boundary 3 ... Precipitates, such as AlN, which precipitated in a grain 4 ... No precipitation zone 5 ... Precipitates, such as AlN which precipitated in pro-eutectoid ferrite 6 ... proeutectoid ferrite 7 ... precipitates such as AlN deposited during reheating

Claims (1)

[Claims] (1) Al: 0.003 to 0.06 by weight.
% And Nb: a method of producing a steel slab containing one or two of 0.005 to 0.1% by a continuous casting machine of a curved type or a vertical bending type, wherein the casting is drawn from a mold. After cooling the piece under the condition that the cooling rate in the surface layer is 4 to 15 ° C./sec, and stopping the cooling in the temperature range where the surface layer temperature is lower than the Ar ₃ point and the transformation of the austenite phase is not completed, the surface layer is cooled to 950. A continuous casting method of steel for preventing surface cracks, wherein the method is reheated to ~ 1200 ° C and straightened under a condition of the following formula. 1.7−0.002 × D> ε Here, D represents the thickness of the slab (mm), and ε represents the amount of corrective strain (%).