JP2917641B2 - Steel continuous casting method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for continuously casting steel without causing internal cracks in a slab.

[0002]

BACKGROUND OF THE INVENTION Continuous casting of steel has been continuously improved since its inception and is nearing completion, but nonetheless still has some unsolved problems. One of them is "internal cracking" that occurs inside the slab.

[0003] In continuous casting, molten metal supplied into a mold is cooled by the mold to form a solidified shell, and then, while being supported by a support roll, is subjected to secondary cooling by water spray or the like to gradually solidify. A method of cutting a slab that has progressed and finally completely solidified to a predetermined length is adopted. During this time, there is an unsolidified part inside the slab,
It is considered that the above-mentioned internal cracks occur due to the occurrence of tensile strain due to bending, straightening, bulging, misalignment, thermal stress, and the like of the slab during continuous casting, causing the solidification interface to tear. In recent years, casting has been accelerated to improve productivity.However, by increasing the casting speed, the frequency of occurrence of internal cracks has increased, and the occurrence of internal cracks is a factor that determines the casting speed. I have.

[0004] In continuous casting for blooms, rounds, billets for wires and pipes, and slabs for thick plates, etc., a technique of unsolidifying and reducing the slab is adopted to improve center segregation and center porosity. It is getting. When the slab is rolled down, tensile strain is generated at the solidification interface. At this time, if the rolling amount, the rolling timing and the like are wrong, internal cracks will be caused. Regarding these, the following techniques are known.

Iron and steel, 73 (1987), S909 "Development of on-line internal crack diagnosis and prevention system" This is to diagnose internal cracks by comparing with limit strain by strain calculation incorporating operation information during casting. , To control the prevention system.

Materials and Processes, Vol. 4 (1991), 297
"Improvement of center segregation by strong reduction" This is to reduce the solidification interfacial strain as much as possible by performing unsolidification reduction using a large-diameter roll, thereby preventing internal cracking during reduction.

Japanese Unexamined Patent Publication No. Hei 3-174962, "Continuous casting of steel" This is a method proposed by the present inventors, but its main feature is to consider the accumulation of strain applied to the slab between ZST and ZDT. Then, the casting conditions and the design conditions of the apparatus are determined so that the sum of the strains does not exceed a certain limit value (limit strain). However, although it is possible to derive this critical strain from the composition of the steel to some extent, knowledge on the relationship between the steel composition and the total strain limit is still insufficient.

That is, it is empirically known that Mn is a component that greatly affects the susceptibility of a steel slab to the occurrence of internal cracks. Does not consider the Mn component in relation to the sum of the strains added, and also in other conventional technologies, comprehensively and quantitatively, taking into account the essential components C, Mn, P, and S of steel. There is no finding that has been examined and made practical for a wide range.

[0009]

Various measures have been proposed to prevent internal cracks in continuous cast slabs, as described above, but none of them is practically sufficient.

It is an object of the present invention to provide C, an essential component of steel,
Considering Mn, P, and S comprehensively in a practically wide range of contents, the total limit value of the applied strain is directly and quantitatively determined from these components, and the strain of ZST to ZDT is determined. An object of the present invention is to provide a continuous casting method in which casting is performed under a condition that the sum does not exceed this value to prevent the occurrence of internal cracks and to cast a high-quality slab at a high speed.

[0011]

Means for Solving the Problems Based on the above-mentioned invention, the present inventors have further experimentally studied a wide range of effects on Mn cracking susceptibility, which have been known only empirically. did. As a result, especially regarding the relationship with S,
Instead of affecting each other independently, they influence each other to form MnS, and the ratio of Mn to S ([Mn] / [S]) and C content are closely related to crack susceptibility. We found that there was relation. It has also been found that the limit value of the total amount of strain exerted on the occurrence of internal cracks can be derived from the contained components by a simple mathematical formula by combining the ratio of Mn and S and the C content with some constants. The present invention has been made based on these findings, and its gist lies in the following continuous casting method.

[0012] In the continuous casting of steel in a component range of C: 0.85% or less, P: 0.03% or less, Mn: 2.5% or less, S: 0.001 to 0.1% by weight, each part of the billet has at least tensile strength. Casting under the condition that the total amount of applied strain does not exceed the value of ε _C (%) defined by the following equation while in the temperature range between the appearance temperature (ZST) and the ductility appearance temperature (ZDT) It is a continuous casting method characterized by the following.

Ε _C = K ₁ × {[Mn] / [S]} − K ₂ [C] + K ₃ (B) where [Mn], [S] and [C] are Mn,
It is the content (% by weight) of S and C, and K ₁ , K ₂ and K ₃ are the following constants.

[Mn] / [S] ≦ 100 and [C] ≦ 0.22: K ₁ = 0.012, K ₂ = 7.5, K ₃ = 2.1 [Mn] / [S]> 100 and [C] ≦ 0.22 When: K ₁ = 0.004, K ₂ = 7.5, K ₃ = 3.0 [Mn] / [S] ≦ 100 and 0.22 <[C] ≦ 0.85: K ₁ = 0.012, K ₂ = 1.4, K ₃ = 0.86 [Mn] / [S]> 100 and 0.22 <[C] ≦ 0.85: K ₁ = 0.004, K ₂ = 1.4, K ₃ = 1.6

[0015]

Each part of the slab during continuous casting first undergoes a tensile strength appearance temperature (ZST) and a ductile appearance temperature (ZDT) from complete liquid state to solid-liquid coexistence state to complete solidification and cooling. It has been known. ZST and ZDT are values determined by the type of steel, and the values are, for example, Arch. Eisenhuttenwesen
54 (1983) .357 by a known method. It is also known that the values of ZST and ZDT correspond well with the solid fraction.
DT almost coincides with the point of the solid phase ratio of 0.99 ("Iron and steel"'87 -S896).

FIG. 1 is a view schematically showing a solidification process of a slab in continuous casting. In the figure, the molten steel injected into the mold M is cooled as it is drawn downward, and the solidified layer (shell) S gradually becomes thicker.
There is a solid-liquid coexistence layer (S + L) between the solidified layer S and the liquid phase (L) at the center. Now, the longitudinal direction of the slab (drawing direction)
If at a certain position P ₁ and the point of the solid fraction 0.8 ₁ point A, tensile strength begins to appear here. That is, the temperature at point A ₁ is ZST. The solid fraction _a point B at the same position P ₁
Assuming a point of 0.99, ductility appears from this and this temperature is ZD
T.

If the point A ₁ is followed in the direction of drawing the slab, ZDT is reached at the point A ₂ and ductility appears.
ZST is the maximum temperature at which stress is applied and strain begins to occur,
ZDT is a limit temperature at which the cracking does not occur due to ductility even if strain occurs at a temperature lower than this. Each part of the slab is subjected to various strains as described above during the continuous casting, due to bending, bulging, straightening, thermal stress, and the like, while passing from ZST to ZDT.

As described in Japanese Patent Application Laid-Open No. 3-17496, the inventors of the present invention do not reduce the strain received while each part of the slab is in the temperature range from ZST to ZDT each time it receives the strain. Is stored as it is, the total amount of the stored strain (Σε, where ZST to ZDT, hereinafter referred to as E)
Exceeds a certain limit (critical strain ε _C ), cracks occur,
It was confirmed by repeating various experiments that cracking did not occur if it was less than that. FIG. 2 illustrates one of the experiments, which describes a method of measuring the critical strain for crack initiation (for details, see the papers of the present inventors, Materials and Processes, Vol.
988), p.1229).

The critical strain for the occurrence of internal cracks is determined by applying various deformations to a small ingot cast under the same conditions as in a continuous cast slab, and examining the magnitude of the strain and the occurrence of cracks. Asked by the thing. That is, a uniaxial tensile deformation was applied to an ingot whose center portion was in an unsolidified state as shown in FIG. 2 to generate strain. The amount of deformation, the number of deformations, and the time interval between deformations were variously changed, and these were combined to give a deformation. Also, set a thermocouple for temperature measurement inside the ingot,
While confirming the temperature, the amount of strain applied between ZST and ZDT in the total amount of strain was determined.

By changing the cooling conditions within a range that can be considered for continuous cast slabs, the cooling rate at the temperature measurement point is changed,
The time during which each point was between ZST and ZDT was varied.

FIG. 3 shows that C: 0.15%, Mn: 0.6%, S:
For steel having a composition of 0.012% and P: 0.02%, the abscissa indicates the cooling conditions and the ordinate indicates the sum of the amount of strain generated, and the result of examining the occurrence of internal cracks is shown. The amount of distortion is shown separately from the total amount of generated distortion (Σε) and the total amount of distortion added between ZST and ZDT (E).

As shown in FIG. 3, when E exceeds the critical strain ε _C obtained by the equation of the present invention, which is about 1.6%, internal cracks occur, and this is the critical strain. It can also be seen that this critical strain has not changed with cooling conditions. On the other hand, the crack initiation limit value determined by the total strain
Greater than about 1.6% above. This is because the strain applied below the ZDT is also counted, and the strain is absorbed as elongation due to the ductility of the solidified shell and does not cause cracking. Further, the limit value of crack generation due to the generated total strain Δε changes depending on the cooling conditions, and increases as the cooling rate increases.

E is a combination of the various added strains described above, but cracks occur when the total strain exceeds the limit value regardless of the type of deformation. The main types of strain applied to a slab in continuous casting are bending strain, straightening strain, bulging strain, misalignment strain, and thermal strain. Of these strains, the sum of the strains applied when the cast slab is between ZST and ZDT is represented by Σε _r , Σε _u , and Σε, respectively.
_b, if Shigumaipushiron _m and Shigumaipushiron _t, the sum of these, i.e. E is as follows.

Σε _r + Σε _u + Σε _b + Σε _m + Σε _t = E If E exceeds a certain limit value ε _C (about 1.6% in FIG. 3 described above), internal cracks occur. Therefore, it is necessary to perform the casting while maintaining at least the condition of E ≦ ε _C until the slab is completely solid. If there is a temperature range where ductility is extremely small even in a temperature range equal to or lower than ZDT depending on the type of steel, it is preferable that the temperature range is expanded to a temperature range where ductility becomes sufficiently large and the total strain during that time is suppressed to a limit strain or less. .

Next, in order to extend the above knowledge to a practically wide range of component composition, a similar test was conducted by the method shown in FIG. The relationship with the susceptibility of internal cracking was investigated. That is, C: 0.85% or less, P: 0.03% or less, Mn:
In the range of 2.5% or less and S: 0.1% or less, different deformations are added to the ingot with various steel compositions, and the relationship between the total strain and the occurrence of cracks at that time is investigated. The critical strain for the occurrence of internal cracking was determined. FIG. 4 shows the results. The vertical axis E (%) is the total amount of strain applied between the ZST and ZDT of the ingot, and the horizontal axis ε _C (%) is the limit strain obtained for each steel composition. In FIG. 4, the oblique line indicates E = ε _C , and the vertical line of ε _C = 13 indicates the maximum possible value of ε _C obtained by the above equation (a), that is, the applicable limit.

Of the components, if P is 0.03% or less, P is irrelevant to internal cracking, and Mn and S do not independently affect each other, but nonmetallic inclusions Mn that serve as starting points for internal cracking.
In relation to S generation, [Mn] / [S] and further [C]
It affects the critical strain ε _C in accordance with the equation (1), and is determined to be defined by the equation (A). The constants K _{1 to} K ₃ are numbers determined for each component as shown in the following. These are obtained from the results of FIG. 4 by performing multiple regression analysis on the assumption of the structure of the above equation (A). It is a thing.

According to the method of the present invention, when the composition of the steel becomes clear before the start of casting, the critical strain determined for each composition component can be immediately calculated by a simple formula.

[0028]

EXAMPLES Continuous casting was carried out under the following conditions while changing the composition of steel within the scope of the present invention.

Continuous casting machine: Vertical bending (V-B) type Bending: 5 points Straightening: 4 points Slab size: 2000 mm width x 250 mm thickness Molten steel superheat degree: 40-50 ° C (in a tundish) Casting speed: 0.5 ~ 2.5m / min Cooling water (specific water) volume: 1.0 ~ 2.5l / kg (steel) In addition, in each roll arrangement, the solid fraction at the center of the slab is 0.3 ~
In the final stage of solidification at 0.8, a maximum reduction of 4 mm was applied to the slab. During casting, the surface temperature and roll pressure of the slab were continuously measured from the initial stage to the final stage, and the strain inside the slab was continuously calculated by stress analysis. In this way, the epsilon _C according to formula to obtain the limit strain of the present invention calculated for each component of the steel, to monitor the operation while constantly compared with the strain amount of the internal slab, the amount of distortion limit epsilon _C value In the case of exceeding, a measure was taken to automatically take measures such as increasing the amount of secondary cooling water, decreasing the casting speed, or decreasing the rolling reduction.

On the other hand, the comparative example was carried out under the same conditions without using the ε _C value according to the formula of the present invention.

These results were obtained from the steel composition [M
n] / [S] and the results are shown in Table 1. As apparent from Table 1, the case according to the method of the invention, the frequency of occurrence of internal cracks is to 0.04 to 0.3 times / charge, Comparative Examples not using the epsilon _C value is 0.4 to 1.3 times / charging, the The effect of the invention method for preventing internal cracks is clear.

[0032]

[Table 1]

[0033]

According to the method of the present invention, the total limit value of the strain applied according to the composition of the steel to be cast is calculated and calculated, and the risk of internal cracking is predicted, and the risk of internal cracking is added to the slab before or during casting. By setting the casting conditions so that the internal strain does not exceed the limit value, it is possible to effectively prevent the occurrence of internal cracks.

[Brief description of the drawings]

FIG. 1 is a schematic cross-sectional view of a continuous cast slab illustrating ZST and ZDT.

FIG. 2 is an explanatory diagram of an experimental method for obtaining an internal crack generation limit strain.

FIG. 3 is an example showing the relationship between the sum of strain applied to an ingot and the occurrence of internal cracks obtained by the method of FIG. 2;

FIG. 4 is a graph showing the sum of strain applied to an ingot and the relationship between critical strain and the occurrence of internal cracks according to the method of the present invention.

Claims (1)

(57) [Claims] In a continuous casting of steel having a composition of C: 0.85% or less, P: 0.03% or less, Mn: 2.5% or less, S: 0.001 to 0.1% by weight, each part of a billet is Casting under the condition that the total amount of applied strain does not exceed the value of ε _c (%) defined by the following equation at least in the temperature range between the tensile strength appearance temperature (ZST) and the ductility appearance temperature (ZDT) A continuous casting method. ε _C = K ₁ × {[Mn] / [S]} − K ₂ [C] + K ₃ (B) where [Mn], [S] and [C] are Mn in molten steel,
It is the content (% by weight) of S and C, and K ₁ , K ₂ and K ₃ are the following constants. [Mn] / [S] ≦ 100 and [C] ≦ 0.22: K ₁ = 0.012, K ₂ = 7.5, K ₃ = 2.1 [Mn] / [S]> 100 and [C] ≦ 0.22: K ₁ = 0.004, K ₂ = 7.5, K ₃ = 3.0 [Mn] / [S] ≦ 100 and 0.22 <[C] ≦ 0.85: K ₁ = 0.012, K ₂ = 1.4, K ₃ = 0.86 [Mn ] / [S]> 100 and 0.22 <[C] ≦ 0.85: K ₁ = 0.004, K ₂ = 1.4, K ₃ = 1.6