阅读说明：本技术 一种连铸坯组合压下方法 (Combined pressing method for continuous casting billet ) 是由 董其鹏 长海博文 夏军 屈天鹏 于 2019-11-12 设计创作，主要内容包括：本发明揭示了一种连铸坯组合压下方法,通过对连铸坯实施压下前建立凝固模型,确定铸坯凝固终点位置及中心固相率,预测出铸坯中心宏观偏析最终形成位置的临界固相率和中心疏松开始形成位置的临界固相率,在铸坯凝固末端,中心固相率小于宏观偏析最终形成位置的临界固相率的区域进行动态轻压下,在中心固相率大于中心疏松开始形成位置的临界固相率的区域进行重压下。并且采用压下模型预测不同压下量对中心偏析和疏松的改善效果,以确定最佳压下位置和压下量。本发明能够有效改善铸坯中心偏析及疏松,同时可以预先确定最佳压下参数,实现最佳压下效果,降低实验成本。(The invention discloses a continuous casting billet combined reduction method, which comprises the steps of establishing a solidification model before carrying out reduction on a continuous casting billet, determining a casting billet solidification end point position and a central solid phase rate, predicting a critical solid phase rate of a final forming position of macrosegregation of the center of a casting billet and a critical solid phase rate of a starting forming position of center porosity, carrying out dynamic soft reduction on a region, at the solidification end of the casting billet, of which the central solid phase rate is smaller than the critical solid phase rate of the final forming position of macrosegregation, and carrying out heavy reduction on a region, of which the central solid phase rate is larger than the critical solid phase rate of. And predicting the improvement effect of different rolling reductions on center segregation and porosity by using a rolling model so as to determine the optimal rolling position and rolling reduction. The method can effectively improve the center segregation and the porosity of the casting blank, and can predetermine the optimal reduction parameters, realize the optimal reduction effect and reduce the experiment cost.)

1. A combined reduction method for a continuous casting billet is characterized by comprising the following steps:

s1, establishing a continuous casting billet solidification model, and determining a casting billet solidification end point position and a central solid phase rate fs;

s2, predicting the critical solid phase rate m of the final forming position of macrosegregation through a macrosegregation model; predicting the critical solid phase rate n of the position where the central porosity begins to form through a solidification shrinkage analysis model; wherein m is more than 0 and less than 1, and n is more than 0 and less than 1;

s3, at the solidification end of the casting blank, the dynamic soft reduction is carried out in the area where the central solid phase rate fs is less than the critical solid phase rate m of the final forming position of macrosegregation, and the heavy reduction is carried out in the area where the central solid phase rate fs is greater than the critical solid phase rate n of the initial forming position of central porosity.

2. The slab combined reduction method according to claim 1, wherein: in the S3, in the area of central solid phase rate fs < m, dynamic soft reduction is carried out by adopting a plurality of fan-shaped sections or pulling and straightening rollers.

3. The slab combined reduction method according to claim 2, wherein: the number of the fan-shaped sections is 1-3, and the number of the pulling and straightening rollers is 3-6.

4. The slab combined reduction method according to claim 1, wherein: in S3, the region having a center solid phase ratio fs > n is heavily reduced by a single roller.

5. The slab combined reduction method according to claim 4, wherein: and after the working roll is pressed down heavily, a plurality of press rolls are adopted to continuously apply a certain amount of pressing down to prevent the rebound of the heavy pressing down.

6. The slab combined reduction method according to claim 1, wherein: and in S1, after the continuous casting solidification model is established, predicting the temperature field of the continuous casting billet, and verifying and debugging the calculation result of the solidification model by adopting a nail-shooting experiment or a lead-sinking method.

7. The slab combined reduction method according to claim 1, wherein: in the step S3, the reduction amounts under dynamic light pressure and heavy pressure are used to predict the reduction amounts based on the reduction segregation model and the reduction porosity model.

8. The slab combined reduction method according to any one of claims 1 to 7, wherein: the continuous casting billet comprises a small square billet, a large square billet and a thick slab.

Technical Field

The invention relates to the technical field of continuous casting production, in particular to a combined pressing method for a continuous casting billet.

Background

Macrosegregation and center porosity are the main internal defects of the continuous casting billet, and improper control will seriously affect the quality of steel products. Mechanical reduction is a main technical means for improving the internal quality of a casting blank, particularly for macrosegregation, with the technical progress, the center segregation can be controlled in a certain range by adopting a dynamic soft reduction technology at present, but the improvement effect of the soft reduction technology on center porosity is little. This is mainly determined by the formation characteristics of macrosegregation and center porosity, and the applicant studies show that the macrosegregation final forming position is before the solidification end point, and the center porosity starting forming position is in the region with higher solid fraction at the solidification end point, so that the reduction interval executed by the existing soft reduction technology is not suitable for improving the center porosity.

In order to improve the central porosity of a casting blank, solidification tail end heavy reduction technology is developed successively at home and abroad in recent years, but the prior related technology is still limited to a certain type of continuous casting blank due to the limitation of a control method, for example, Chinese patent CN104874758A provides a billet continuous casting heavy reduction device and a control method; chinese patent CN106001476A discloses a two-stage pressing method for bloom and wide and thick slabs. In addition, in the reduction process, the setting of the reduction position and the reduction amount is very critical, the desired effect is difficult to obtain due to improper reduction position, and excessive reduction amount not only increases the equipment load and the production cost, but also aggravates the internal defect. The PosHarp technology reduction interval of Korea pump is 0.05-0.6 fs; CN106001476A proposes that the rolling start points of the bloom and the wide and thick slab are both at the position where the central solid phase rate fs is 0.6, the wide and thick slab and the second stage rolling start point are the solidification end point. Although few studies give exact pressing positions and pressing amount intervals, there is no correlation therebetween, and thus it is obvious that the process effect cannot be stably and reliably exerted.

Disclosure of Invention

The invention aims to overcome the defects of the prior art, and provides a combined reduction method for overcoming the defects of the prior art, aiming at simultaneously improving the macrosegregation and center porosity of a casting blank and improving the stability and reliability of process effect.

In order to achieve the purpose, the invention provides the following technical scheme: a combined reduction method for a continuous casting billet comprises the following steps:

s1, establishing a continuous casting billet solidification model, and determining a casting billet solidification end point position and a central solid phase rate fs;

s2, predicting the critical solid phase rate m of the final forming position of macrosegregation through a macrosegregation model; predicting the critical solid phase rate n of the position where the central porosity begins to form through a solidification shrinkage analysis model; wherein m is more than 0 and less than 1, and n is more than 0 and less than 1;

s3, at the solidification end of the casting blank, the dynamic soft reduction is carried out in the area where the central solid phase rate fs is less than the critical solid phase rate m of the final forming position of macrosegregation, and the heavy reduction is carried out in the area where the central solid phase rate fs is greater than the critical solid phase rate n of the initial forming position of central porosity.

Preferably, in S3, in the region where the central solid phase ratio fs < m, dynamic soft reduction is performed by using a plurality of sectors or pulling and straightening rollers.

Preferably, the number of the fan-shaped sections is 1-3, and the number of the pulling and straightening rollers is 3-6.

Preferably, in S3, the area with center solid phase ratio fs > n is heavily reduced by using a single roller.

Preferably, after the work roll is pressed down again, a plurality of press rolls are adopted to continue to apply a certain amount of pressing down so as to prevent the rebound of the pressing down.

Preferably, in S1, after the continuous casting solidification model is established, the temperature field of the continuous casting slab is predicted, and the calculation result of the solidification model is verified and debugged by a nail-shooting experiment or a lead-sinking method.

Preferably, in S3, the reduction amounts under the dynamic light pressure and the heavy pressure are predicted according to a reduction segregation model and a reduction porosity model.

Preferably, the continuous casting slab comprises a billet, a bloom and a thick slab.

Compared with the prior art, the combined pressing method provided by the invention can give consideration to both the improvement of macrosegregation and center porosity, namely, the macrosegregation is improved under the front-stage light pressing, and the center porosity is healed under the subsequent heavy pressing; in addition, the invention determines the pressing position and the pressing amount by taking numerical simulation as a control basis, and can ensure that the pressing effect is more obvious and accurate.

The invention can effectively improve the macrosegregation and center porosity defects of the continuous casting billet, particularly aims at an extra-thick slab, improves the internal quality of the casting billet, and realizes the production of thick plates and extra-thick plates by a low-compression ratio rolling process.

Drawings

FIG. 1 is a diagram of the combined pressing process proposed by the present invention;

FIG. 2 is a flow chart of the combined pressing control proposed by the present invention;

FIG. 3 is a schematic view of a reduction model of the continuous casting process of the present invention;

FIG. 4 is a graph showing the m value calculated by the segregation model in example 1;

FIG. 5 is a graph showing the value of n calculated by the porosity model in example 1;

FIG. 6 shows the results of the low power operation based on the combined reduction method proposed by the present invention.

Detailed Description

The technical solution of the embodiment of the present invention will be clearly and completely described below with reference to the accompanying drawings of the present invention.

The invention discloses a continuous casting billet combined reduction method, which is characterized in that a solidification model is established before the continuous casting billet is reduced, the critical solid phase rate of the forming positions of the center macrosegregation and the center porosity of the casting billet is predicted, the improvement effect of different reduction on the center segregation and the porosity is predicted by adopting the reduction model, and the optimal reduction position and the optimal reduction are determined.

Specifically, as shown in fig. 1, a process diagram for performing combined rolling of a slab using a plurality of rolls performs dynamic light rolling in a region where the central solid fraction fs is relatively small, that is, in a region where the central solid fraction fs is smaller than the critical solid fraction m of the final formation position of macrosegregation predicted from a solidification model, that is, fs < m, to uniformize the distribution of solute elements inside by causing forced convection in a two-phase region. And then, in a region with the central solid phase rate fs larger than the critical solid phase rate n of the position where the central porosity starts to form, namely fs is larger than n, which is predicted according to the solidification model, a single press roller is adopted to perform solidification tail end single-roller heavy reduction, and the central porosity is healed by forcibly deforming the casting blank, particularly in the central non-fully solidified region.

As shown in fig. 2, the combined pressing method proposed by the present invention comprises:

s1, establishing a continuous casting billet solidification model, tracking and calculating temperature field distribution of casting billets at different positions in real time, calculating the thickness of a billet shell at different positions of the casting billet and the central solid phase rate corresponding to the lower rollers under different pouring parameters based on the temperature field result, and determining the solidification end point position of the casting billet, wherein the equation of the control process is as follows:

equation of continuity

Equation of conservation of momentum

Wherein, the momentum source term

Can be expressed as:

in the formula

Acceleration of gravity, acting only on the z-direction equation of momentum, m/s²；

β_T-coefficient of thermal expansion, 1/K;

β_C,i-solute expansion coefficient of element i;

ξ -a very small positive number introduced to ensure that the denominator is not 0.

Energy conservation equation

In the formula, the enthalpy H can be expressed as a function of the temperature:

fraction of liquid phase f_lCan be calculated from the following formula:

in the formula, T_lAnd T_sLiquidus temperature and solidus temperature, respectively.

S2, forecasting the distribution rule of solute elements in the casting blank in the continuous casting process in real time through a macrosegregation model, obtaining a relation graph of solidification end macrosegregation and central solid phase rate, and determining the critical solid phase rate m corresponding to the final forming position of macrosegregation, wherein the control equation is as follows:

solute conservation equation:

redistribution of solute at the solidification front:

in the formula, X is the length of the solidification end microsegregation section, and is generally assumed to be one-half of the secondary dendrite spacing.

In the formula, α^CAn additional term to the fourier number.

Based on a solidification model, establishing a solidification shrinkage analysis model, predicting the internal solidification shrinkage change of a casting blank in the continuous casting process, and predicting the evolution condition of central porosity generated by solidification shrinkage to obtain the relation between the size of the central porosity and the central solid phase rate, and determining the critical solid phase rate n corresponding to the initial forming position of the central porosity, wherein the specific calculation details are as follows:

three variables are defined in the calculation process to describe the shrinkage at different positions, namely the total solidification volume shrinkage delta V of the casting blank_totalCasting blank surface solidification volume shrinkage delta V_surfAnd the internal volume shrinkage DeltaV of the casting blank_centThe volumetric shrinkage at the three different positions is related as:

ΔV_cent＝ΔV_total-ΔV_surf(11)

in formula (11), Δ V_centRepresents the internal volume shrinkage of the cast slab in m³；

ΔV_totalRepresents the total solidification volume shrinkage of the casting blank, and the unit is m³；

ΔV_surfRepresents the amount of the solidification volume shrinkage of the surface of the casting blank, and the unit is m³。

Neglecting heat transfer in the pulling speed direction, taking a slice with the thickness of delta L as a calculation area, so that the cross-sectional area of the shrinkage of the casting blank can be expressed as follows:

ΔA＝ΔV/ΔL (12)

ΔA_cent＝ΔA_total-ΔA_surf(13)

the temperature gradient difference can lead to the formation of shrinkage cavities at the coagulation tip. In the solving process, the total solidification shrinkage of the casting blank is solved by tracking solidification information of the solidification front edge of the casting blank. Therefore, assuming that the solidification process temperature gradient is linear, the total solidification shrinkage of the ingot can be expressed as:

in formula (14), x₀The size of the section of the casting blank is expressed in m;

t_endthe complete solidification time of the casting blank is expressed in the unit of s;

t_beginthe time of the casting blank center entering the mushy zone is expressed in the unit of s;

β_Tthe coefficient of thermal expansion of the metal is 4.0X 10^-41/K。

The amount of solidification shrinkage of the surface of the cast slab can be expressed as:

the equations (13), (14) and (15) are combined to obtain a solution equation of the solidification shrinkage of the center of the casting blank:

equation (16) is integrated over length x:

in the formula (17), dT (t)/dt represents a cooling rate of a solidification front of a cast slab, and the unit is K/s;

dT(t)_sthe/dt represents the cooling rate of the surface of the cast strand in K/s.

S3, at the solidification end of the casting blank, in the area with lower central solid phase ratio, namely the area with 0.1< fs < m, 1-3 fan-shaped sections (plate blanks or large square blanks) or 3-6 pulling and straightening rolls (square blanks) are adopted for dynamic soft reduction, in the area with higher central solid phase ratio, namely the area with fs > n, single rolls are adopted for heavy reduction, and in order to inhibit the rebound phenomenon after heavy reduction, 2-3 press rolls are adopted for continuously applying the reduction of about 2mm after heavy reduction after the working rolls are pressed.

S4, establishing a continuous casting process depressing model (as shown in figure 3) by using commercial software Thercast, namely a depressing segregation model and a depressing loosening model, predicting the improving effect of macrosegregation under light pressure at the front section and the healing effect of center loosening under heavy pressure at the rear section, and determining the optimal depressing amount, namely X mm under light pressure and Y mm under heavy pressure.

The invention also comprises the steps of verifying and debugging the calculation result of the solidification model through a nail shooting experiment or a lead sinking method and a temperature measurement experiment.

The process is suitable for the continuous casting process of steel, including small square billets, large square billets and thick plate billets, and is suitable for the continuous casting process from low-carbon steel to high-carbon steel and stainless steel. Aiming at the characteristics of blank shape and diversified continuous casting parameters of a continuous casting blank, a combined reduction method is provided, a universal combined reduction process is provided, and a determination method of specific reduction process implementation parameters is provided, wherein the determination method comprises selection of reduction positions and reduction amounts. Aiming at the differences of the forming positions of macrosegregation and center porosity of a continuous casting billet, the combined pressing method provided by the invention can give consideration to the improvement of macrosegregation and center porosity, namely improving macrosegregation under the front section light pressing and healing the center porosity under the subsequent heavy pressing; in addition, the invention determines the pressing position and the pressing amount by taking numerical simulation as a control basis, and can ensure that the pressing effect is more obvious and accurate.