JP4949052B2 - Continuous casting method of steel preventing internal cracking - Google Patents

Description

  The present invention relates to a method for continuously casting steel at a maximum casting speed that does not cause internal cracks.

  In continuous casting of steel, it is required to improve the casting speed as much as possible from the viewpoint of improving productivity. However, if the casting speed is increased too much, an internal crack occurs in the slab, resulting in the entire slab being discarded as a defective product. For this reason, the casting speed has been regulated so as not to generate internal cracks, but the sensitivity to internal cracks (ease of internal cracks expressed by the internal crack limit strain εcr) varies depending on the steel type. It was difficult to determine the limit of occurrence of internal cracks in advance.

  Therefore, conventionally, for example, the maximum casting speed Vc is set based on a database in which the relationship between the upper limit values of C, P, and S of the steel type to be cast and the maximum casting speed Vc is accumulated. However, since this method is based on past experience values, depending on the type of steel, even though higher speed casting is possible, there is a production loss or setting that is considerably slower than that. In spite of casting at the maximum casting speed Vc or less, internal cracks may occur.

  In Patent Document 1, focusing on the fact that the internal crack susceptibility depending on the steel type varies depending on the ratio of Mn / S in steel and C, the internal crack limit strain εcr is calculated by the mathematical formula described in the claim, and this A method of setting casting conditions in consideration of values is disclosed. However, only three elements of Mn, S and C in the steel are considered here, and it is impossible to determine the optimum casting conditions for various steel types currently produced by this formula. The mathematical formula used is an empirical formula under specific limiting conditions and is not universal.

Further, according to the experience of the present inventors, although the same steel type is continuously cast, the component values in the steel, especially P, S, etc. fluctuate slightly for each charge. The occurrence of cracking may change. For this reason, as disclosed in Patent Document 1, the method of calculating the internal crack limit strain εcr using the component standard values relating to Mn, S, and C of the steel to be cast cannot completely prevent internal cracking. As described in the examples.
JP-A-5-185183

  Therefore, the object of the present invention is to solve the above-mentioned problems of the prior art, and accurately calculate the maximum casting speed at which there is no possibility of generating internal cracks by calculation in advance, for various steel types including those with no casting experience in the past. It is an object of the present invention to provide a continuous casting method of steel that prevents internal cracking and can improve productivity as much as possible.

The present invention made in order to solve the above-mentioned problems is a component of five or more elements including C, Si, Mn, P, and S as component values of steel after secondary refining in continuous casting of steel. Analyzing the values for each charge, calculating the embrittlement zone width η from the solid fraction obtained by calculating the microsegregation from those values, and further calculating the internal crack limit strain εcr, each position of the slab during continuous casting in determines the maximum casting speed Vc the maximum value does not exceed the calculated internal cracking limit strain εcr of the total strain εt is calculated, characterized in that the continuous casting at a maximum casting speed Vc is there.

In the present invention, the calculation of microsegregation is obtained by performing solute element concentration between dendrite trees during solidification by a numerical calculation method taking into account solute partitioning between δ / γ and solute diffusion in the solid phase. It is preferable to determine the embrittlement zone width η from the solid phase ratio at the center of the dendritic tree at an arbitrary temperature until the completion of solidification, and calculate the internal crack limit strain εcr based on this .

  According to the present invention, the internal crack limit strain εcr is calculated according to the component value of the steel after the secondary refining, and the total casting speed Vc at which the total strain εt during continuous casting does not exceed the calculated internal crack limit strain εcr. Since continuous casting is performed at, continuous casting at a maximum casting speed Vc corresponding to the component value of steel that varies with each charge is possible, and productivity can be improved. In addition, by analyzing the component values of five or more elements including C, Si, Mn, P, and S for each charge and calculating the internal crack limit strain εcr from the relationship between microsegregation and solid fraction, Therefore, it is possible to improve the productivity by obtaining the maximum casting speed Vc with high accuracy in advance, without causing the occurrence of internal cracks, for various steel types including those with no casting experience.

  Embodiments of the present invention will be described below. First, internal cracks during continuous casting of steel, which is a problem in the present invention, will be described. Internal cracking is considered to be a phenomenon in which columnar crystals (dendritic trees) existing at the solid-liquid interface of the slab are opened due to tensile strain at the initial stage of solidification, and the concentrated liquid phase enters there. FIG. 1 is a schematic diagram of this internal crack generating mechanism, with the left side being the center side (high temperature side) of the slab and the right side being the surface side (low temperature side).

  In FIG. 1, ZDT (Zero Ductility Temperature) indicates a temperature line where the solid phase ratio = 1.0, and the surface side of this line is a solid phase region, which is a region where ductility is exhibited as shown. Also, ZST (Zero Strength Temperature) indicates a temperature line that develops strength, and the center side of this line is a zero-strength region due to the presence of a liquid phase. ZST corresponds to a solid phase ratio of 0.7. An intermediate region between these ZDT and ZST, that is, a region having a solid phase ratio of 0.7 to 1.0 is an embrittlement region.

  In the solidification process, columnar crystals (dendritic trees) are generated from the solid phase region to the liquid phase region as shown in FIG. 1, but when a strain in the casting direction is applied to the slab in this state, the above described Opening of dendritic trees occurs in the embrittlement zone between ZDT and ZST. The strain at which this opening begins to occur is the internal crack limit strain εcr. Therefore, the internal crack sensitivity can be evaluated by the internal crack limit strain εcr.

  In Patent Document 1, the internal crack limit strain εcr is obtained as a function of (Mn / S) and (C) of the components in steel based on experiments. On the other hand, in the present invention, the component values of steel after secondary refining are analyzed for 7 elements of C, Si, Mn, P, S, B, and N, and the internal crack limit strain εcr is microscopically determined according to these values. Calculate using a segregation model. Here, the micro-segregation model is a mathematical model for simulating the concentration of solute elements between dendritic trees in the solidification process shown in FIG. 1, and is itself “Iron and Steel” No. 11 in 1987. In the paper entitled "Effects of Alloying Elements on Microsegregation between Dendrites of Carbon Steel". However, the use of this microsegregation model for the calculation of the internal crack limit strain εcr is the first proposal in the present invention and has not been performed before.

  The outline of the micro-segregation model will be described below to the extent necessary for understanding the present invention. For academic details, please refer to the above paper. In this microsegregation model, the shape of the dendrite is approximated by a regular hexagonal cross section as shown in FIG. Then, the diffusion of the solute in the principal axis direction is ignored, and the three-dimensional diffusion of the solute in the portion between the dendrites is approximated by the one-dimensional diffusion in the radial direction of the triangle OPQ shown in FIG. Here, as the solute, five or more elements including C, Si, Mn, P, and S can be used.

  It is assumed that local equilibrium is established at the solid / liquid interface and the δ / γ interface, and solute equilibrium distribution corresponding to the equilibrium distribution coefficient occurs. Qualitatively, during the transformation from δ to γ, Si, P, Mo, etc. are redistributed from the γ phase (between trees) to the δ phase (core), while C, Mn, etc. are reversed from the δ phase to the γ phase. Will be redistributed. Thus, the change of the interfacial concentration that progresses sequentially as the solidification progresses is numerically calculated using a computer. The calculation is performed by dividing the cross section of dendrites into meshes and calculating the mass balance in minute time increments. The interface concentration is calculated in this way because it greatly changes the freezing point. Then, each phase is generated according to the transformation temperature of liquid phase / δ / γ obtained from the phase diagram (1 and 2 below), and the ratio of each phase at each time point is calculated from the interfacial concentration calculated sequentially. And calculate the solute element concentration.

Solid / liquid interface temperature = 1536-78 (% C) -7.6 (% Si) -4.9 (% Mn) -34.4 (% P) -38 (% S) ... 1 set δ / γ interface temperature = 1392 +1122 (% C) -60 (% Si) +12 (% Mn) -140 (% P) -160 (% S) ··· 2 formulas (% C) in these formulas It means a value expressed by mass% of the content of the solute. In addition, when applying a model to an element X other than C, Si, Mn, P, S, from the slope of the solid / liquid interface temperature in the Fe-X binary system, the δ / γ interface temperature with respect to the X concentration, The coefficients of Formula 1 and Formula 2 can be determined approximately.

This micro-segregation model is a model created in order to determine the solid-liquid ratio determined by the solute distribution in the dendrite during the solidification process and the decrease in liquidus temperature due to solute concentration. The solid phase ratio is determined from the ratio of each phase in the solidification process determined by the microsegregation calculation, and ZDT is determined as a temperature where the solid ratio is 1.0, and ZST is determined as a temperature where the solid ratio is 0.7. As is clear from the above formula, these values calculated from the interface temperature and interface temperature vary depending on the values of C, Si, Mn, P, S, B, N, etc. in the steel after secondary refining. However, ZDT and ZST can be accurately obtained by using the micro-segregation model.

Next, the embrittlement zone width η between them is obtained from ZDT and ZST by the following equation.
η = [1.0-{(ZDT-T ₀ ) / (ZST-T ₀ )} ^1.291 ] × d (3 formulas) where T ₀ is the slab surface temperature and d is the solidified shell thickness (mm).

According to the studies by the present inventors, there is a relationship as shown in FIG. 4 between the embrittlement region width η and the above-described internal crack limit strain εcr. Therefore, the internal crack limit strain εcr is calculated by the following four formulas.
εcr ＝ 6.02 × η ^-2.13・ ・ ・ ・ ・ ・ ・ ・ 4 formula

In the continuous casting process, distortion due to bending back deformation of the slab, bulging distortion, distortion due to roll misalignment, and the like are added to the slab, and the sum of these is called the total distortion εt. The total strain εt is calculated at each slab position during continuous casting. As a result of conducting a survey on various steel types, as shown in FIG. 5, it is confirmed that if the internal crack limit strain εcr calculated using the micro-segregation model is equal to or less than the total strain εt, no internal crack occurs. It was.

Therefore, the casting conditions may be set so that the maximum value of the total strain εt during continuous casting does not exceed the calculated internal crack limit strain εcr. If the continuous casting equipment is the same, the total strain εt increases in proportion to the casting speed. Therefore, the maximum casting speed Vc is determined so that the maximum value of the total strain εt does not exceed the calculated internal crack limit strain εcr. Casting becomes possible, and productivity can be maximized.

  Since the relationship between the total strain εt and the casting speed differs depending on the individuality of the continuous casting equipment, it is difficult to express the relationship between the calculated internal crack limit strain εcr and the maximum casting speed Vc by a general formula. However, if it is expressed roughly, ignoring variation, in the continuous casting equipment of the applicant company, the maximum casting speed Vc when the internal crack limit strain εcr is 0.06% is 1.0 mpm, and when 0.2%, 1.4 mpm, At 0.4%, it is 1.8mpm, and at 0.6%, it is around 2.2mpm.

  Each step of the present embodiment described above is summarized in a block diagram as shown in FIG. According to the present invention, for a variety of steel types including those with no casting experience in the past, the maximum casting speed Vc without the possibility of causing internal cracks is accurately determined in advance to improve productivity as much as possible. be able to. Next, examples of the present invention will be described.

  The present invention was applied to 40 kg steel for general planks. This steel grade has a standard composition of 0.13% C-0.10% Si-0.85% Mn-0.02% P-0.01% S, and was conventionally continuously cast at a maximum casting speed Vc = 1.3m / min. is there. When the internal crack limit strain εcr was calculated using the microsegregation model according to the present invention, εcr = 0.3%. Therefore, the speed was increased to Vc = 1.4 m / min, and it was confirmed that continuous casting was possible without causing internal cracks.

  When the component value of the steel after secondary refining with a certain charge was analyzed for the same steel type, it was 0.13% C-0.10% Si-0.85% Mn-0.02% P-0.008% S. When the internal crack limit strain εcr was calculated using the microsegregation model according to the present invention, εcr = 0.26%. Therefore, the speed was increased to Vc = 1.5 m / min, and it was confirmed that continuous casting was possible without causing internal cracks.

  When the component value of the steel after secondary refining with another charge was analyzed for the same steel type, it was 0.13% C-0.10% Si-0.85% Mn-0.015% P-0.008% S. When the internal crack limit strain εcr was calculated using the microsegregation model according to the present invention, εcr = 0.31%. Therefore, the speed was increased to Vc = 1.6 m / min, and it was confirmed that continuous casting was possible without causing internal cracks.

  The present invention was applied to B-containing 80 kg high tension. This steel grade has a standard composition of 0.18% C-0.20% Si-1.50% Mn-0.02% P-0.005% S-0.002% B. Conventionally, B-containing steel was cast at Vc = 1.0 m / min. When the internal crack limit strain εcr was calculated using the microsegregation model according to the present invention, εcr = 0.06%. Therefore, the speed was increased to Vc = 1.1 m / min, and it was confirmed that continuous casting was possible without causing internal cracks.

  The present invention was applied to a high N content low carbon steel. This steel grade has a standard composition of 0.06% C-0.02% Si-0.25% Mn-0.020% P-0.015% S-0.0120% N. Conventionally, the steel was cast at the same composition as Vc = 2.2mpm, which is the same as the steel type not containing N. However, about 3% of internal cracks occurred, resulting in production loss. When the internal crack limit strain εcr was calculated using the microsegregation model according to the present invention, εcr = 0.5%. Therefore, the speed was reduced to Vc = 2.0 mpm, and internal cracks could be eliminated.

  As described above, according to the present invention, the maximum casting speed Vc can be increased to the maximum without causing internal cracks in accordance with the component values of the steel after secondary refining, and productivity can be increased. It is possible to improve. The present invention can also be applied to steel types that have not been cast in the past, and an optimum maximum casting speed Vc can be set in advance.

It is a schematic diagram of an internal crack generating mechanism. It is a figure which shows the shape of the dendrite in a microsegregation model. It is a figure which shows the three-dimensional diffusion of the solute in a microsegregation model. It is a graph which shows the relationship between the embrittlement zone width | variety (eta) and the internal crack limit strain (epsilon) cr. It is a graph which shows the relationship between internal crack limit distortion | strain (epsilon) cr, total distortion (epsilon) t, and internal crack generation | occurrence | production. It is a block diagram which shows the step of this embodiment.

Claims (2)

When performing continuous casting of steel, component values of five or more elements including C, Si, Mn, P, and S are analyzed for each charge as the component values of steel after secondary refining, and microsegregation is performed from these values. The embrittlement zone width η is calculated from the solid phase ratio obtained by calculating, the internal crack limit strain εcr is calculated, and the maximum value of the total strain εt calculated at each point of the slab during continuous casting is calculated A continuous casting method for steel that prevents internal cracks, wherein a maximum casting speed Vc that does not exceed the internal crack limit strain εcr is determined, and continuous casting is performed at the maximum casting speed Vc. The microsegregation calculation is performed by the numerical calculation method considering the solute distribution between δ / γ and solute diffusion in the solid phase, and the concentration of solute elements between dendrite trees during solidification. 2. The steel according to claim 1, wherein the embrittlement zone width η is determined from the solid phase ratio at the center of the dendritic tree at an arbitrary temperature, and the internal crack limit strain εcr is calculated based on the width η. Continuous casting method.

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