TWI785903B - Method for eliminating center segregation of steel slab - Google Patents

Abstract

A method for eliminating center segregation of a steel slab is described. In this method, a continuous cast process is performed on molten steel. The performing of the continuous cast process includes performing a solidification operation on the molten steel to form a semi-solidified steel slab; and a soft reduction operation is performed on the semi-solidified steel slab to form the steel slab. The performing of the soft reduction operation includes when a central solid fraction of the semi-solidified steel slab is equal to or smaller than about 20%, a first soft reduction step is performed to stabilize a liquid core size inside the semi-solidified steel billet slab; and after the first soft reduction step is performed and when the central solid fraction of the semi-solidified steel slab is from about 40% to about 100%, a second soft reduction step and a third soft reduction step are sequentially performed. A first pressure applied on the semi-solidified steel slab by the first soft reduction step is smaller than a second pressure and a third pressure applied on the semi-solidified steel slab by the second soft reduction step and the third soft reduction step respectively.

Description

Elimination method of core segregation of billet

The present disclosure relates to a continuous casting technology of steel billets, and in particular to a method for eliminating core segregation of steel billets.

During the solidification and cooling process of alloy steel, due to the developed dendrite structure, it is easy to form dendrite bridges inside the continuous casting billet. Coupled with the separation and crystallization during the solidification process, a large amount of solutes with low melting points are enriched between the dendrites. At the end of solidification, the strong suction force is generated due to the shrinkage of the solidified volume, which causes the residual liquid enriched in impurities between the dendrites to flow towards the center and fill it, thereby causing segregation defects in the center of the casting billet and resulting in a Loose organization and residual shrinkage cavity and other problems.

Traditionally, soft reduction technology and electromagnetic stirring technology (electromagnetic stirring, EMS) are mostly used to improve the shrinkage cavity or segregation in the core of the billet, and the soft reduction technology is the mainstream technology. At present, there is a dynamic soft reduction technology for billet continuous casting, which controls the reduction amount through the change of the solid fraction (Fs) in the center of the billet, so as to reduce the central segregation, loose structure and shrinkage cavity of the billet. and other defects. However, in the soft reduction device design of this technology, the soft reduction area is 16187mm to 24649mm below the mold liquid surface, and the total length is 8462mm. A total of 7 sets of roller sets are required for the soft reduction operation, and the maintenance cost of the equipment is high. In addition, the multi-section soft-pressing design of this technology is also likely to cause operational difficulties.

Therefore, one purpose of the present disclosure is to provide a method for eliminating segregation at the core of the billet, which performs three soft reduction steps on the semi-solidified steel billet at the end of solidification, wherein the first soft reduction step is for the semi-solidified steel billet. The pressure exerted by the steel billet is smaller than the pressure exerted on the semi-solidified steel billet by the second soft reduction step and the third soft reduction step. Thereby, the central segregation and V-shaped segregation at the center of the billet can be improved.

Another object of the present disclosure is to provide a method for eliminating segregation at the core of the billet, which can perform electromagnetic stirring at the end of the solidification of the semi-solidified billet before performing a light reduction operation on the semi-solidified billet, thereby improving The phenomenon of uneven solidification composition of the billet can greatly improve the V-shaped segregation of the core of the billet.

According to the above purpose of the present disclosure, a method for eliminating segregation at the core of a steel billet is proposed. In this method, molten steel is subjected to a continuous casting process. The continuous casting process includes solidifying molten steel to form a semi-solidified steel billet; and lightly reducing the semi-solidified steel billet to obtain a steel billet. Carrying out the soft reduction operation includes performing a first soft reduction step on the semi-solidified steel billet when the central solid phase fraction of the semi-solidified steel billet is less than or equal to about 20%, so as to stabilize the semi-solidified steel billet. The size of the internal liquid core; and after the first light reduction step, when the central solid fraction of the semi-solidified steel billet is about 40% to about 100%, the semi-solidified steel billet is sequentially subjected to a second soft reduction step with a third light depressing step. The first pressure applied to the semi-solidified steel slab in the first light reduction step is smaller than the second pressure and the third pressure applied to the semi-solidified steel slab in the second light reduction step and the third light reduction step respectively.

According to an embodiment of the present disclosure, the first pressure applied to the semi-solidified steel slab in the first light reduction step is less than that applied to the semi-solidified steel slab in the second light reduction step and the third light reduction step respectively. The second pressure and the third pressure.

According to an embodiment of the present disclosure, the above method further includes performing a final electromagnetic stirring (FEMS) operation on the semi-solidified steel billet before performing the light reduction operation on the semi-solidified steel billet.

According to an embodiment of the present disclosure, the above-mentioned terminal electromagnetic stirring operation includes performing alternate stirring treatment on the position of 1/4 to 1/2 of the thickness of the semi-solidified steel billet.

According to an embodiment of the present disclosure, when performing the terminal electromagnetic stirring operation, the central solid phase fraction of the semi-solidified steel billet is about 5% to about 20%.

According to an embodiment of the present disclosure, performing the terminal electromagnetic stirring operation includes setting the current value to about 500A to about 650A, and setting the frequency to about 4 Hz to about 8 Hz.

According to an embodiment of the present disclosure, the above-mentioned first pressure is about 10 kg/cm ² to about 25 kg/cm ² .

According to an embodiment of the present disclosure, the above-mentioned second pressure and third pressure are about 20 kg/cm ² to about 50 kg/cm ² .

According to an embodiment of the present disclosure, the above-mentioned first pressure is about 25 kg/cm ² , and the second pressure and the third pressure are about 30 kg/cm ² .

According to an embodiment of the present disclosure, the above-mentioned solidification operation of the molten steel includes controlling the specific water volume of the secondary cooling to about 0.33 L/kg to about 0.78 L/kg.

During the solidification and cooling process of medium-carbon or medium-high-carbon alloy steel, due to the microscopic segregation of the distance between the secondary branch arms of columnar crystals, the composition segregation is uneven, and coarse niobium carbide particles are crystallized. In this way, the effect of grain refinement will be reduced, resulting in the coarsening of the grain structure in the core of the subsequent rolled material. In view of this, this disclosure proposes a method for eliminating core segregation of steel billets, which can improve the internal quality of continuous casting billets in the continuous casting process of steel billets, so as to solve defects such as central segregation of steel billets and loose structures in the center.

Please refer to FIG. 1 and FIG. 2 , which are respectively a flowchart of a method for eliminating core segregation of a steel billet according to an embodiment of the present disclosure, and a method for eliminating core segregation of a steel billet according to an embodiment of the present disclosure. Schematic diagram of the device. In this embodiment, the step 100 may be performed first to provide molten steel when manufacturing the casting billet, and the continuous casting equipment 200 is used to perform a continuous casting process on the molten steel to convert the molten steel into a billet. The molten steel can be, for example, medium-carbon alloy molten steel or medium-high carbon alloy molten steel. Therefore, the steel billet that can be obtained is a medium-carbon alloy steel billet or a medium-high carbon alloy steel billet.

In some embodiments, the continuous casting equipment 200 mainly includes a casting mold 210, an electromagnetic stirring device 220 at the end of solidification, a first soft reduction roller set 230, a second soft reduction roller set 240, and a third soft reduction roller Wheel set 250. In some exemplary examples, the electromagnetic stirring device 220 at the solidification end may be disposed below the mold liquid surface and about 10 meters to about 15 meters away from the mold liquid surface. Based on the traveling direction of molten steel in the continuous casting process, the first soft reduction roller set 230, the second slight reduction roller set 240, and the third slight reduction roller set 250 are located at the end of the solidification end electromagnetic stirring device 220. downstream. That is to say, the same position of the steel billet first passes through the electromagnetic stirring device 220 at the end of solidification, and then passes through the first light-pressing roller set 230 , the second light-pressing roller set 240 , and the third light-pressing roller set 250 .

In some exemplary cases, the first soft-reduction roller set 230 , the second soft-reduction roller set 240 , and the third soft-reduction roller set 250 may be disposed below the mold liquid surface and about 19° away from the mold liquid surface. meters to about 22 meters. Based on the traveling direction of the molten steel in the continuous casting process, the first soft reduction roller set 230 , the second slight reduction roller set 240 , and the third slight reduction roller set 250 are arranged in sequence. That is, the second soft reduction roller set 240 is located downstream of the first soft reduction roller set 230 , and the third soft reduction roller set 250 is located downstream of the second soft reduction roller set 240 . The same position of the steel billet passes through the first light-pressing roller set 230 , the second light-pressing roller set 240 , and the third light-pressing roller set 250 sequentially. In some exemplary examples, the first soft-reduction roller set 230 , the second soft-reduction roller set 240 , and the third soft-reduction roller set 250 are pressure-controlled soft-reduction roller sets.

In some embodiments, in the continuous casting process of the molten steel, step 110 may be performed first to solidify the molten steel so that the molten steel forms a semi-solidified steel billet. In the semi-solidified steel billet, the surface of the semi-solidified steel billet has solidified to form a solidified shell, and the molten steel in the core of the semi-solidified steel billet has not yet solidified. When performing the solidification operation of the molten steel, the molten steel can be poured into the mold 210 first, and the molten steel begins to cool and solidify to form a semi-solidified steel billet with a solidified shell on the outside and unsolidified molten steel inside.

In some examples of the continuous casting process, after the solidification operation, step 120 can be selectively performed according to product requirements, so as to perform terminal electromagnetic stirring operation on the semi-solidified steel billet by using the solidification terminal electromagnetic stirring device 220 . In some exemplary cases, during the terminal electromagnetic stirring operation, the alternate stirring process can be performed on the position of about 1/4 to about 1/2 of the thickness of the semi-solidified steel billet. Electromagnetic stirring at the end of the semi-solidified billet can promote the flow of molten steel rich in solutes, increase the equiaxed crystal ratio in the core, and improve the phenomenon of uneven solidification components of the billet. In some exemplary cases, the central solid phase fraction of the semi-solidified steel slab is about 5% to about 20% during the terminal electromagnetic stirring operation. In some examples, terminal electromagnetic stirring may be performed with a current value of about 500A to about 650A and a frequency of about 4 Hz to about 8 Hz.

Then, step 130 can be carried out, so that the semi-solidified steel billet is lightly depressed by using the first lightly depressing roller set 230, the second lightly depressing roller set 240, and the third lightly depressing roller set 250, And made steel embryo. In some examples, the soft reduction operation includes three steps 132 , 134 , and 136 performed in sequence, namely, a first soft reduction step, a second soft reduction step, and a third soft reduction step. In some demonstrative examples, when the central solid fraction of the semi-solidified steel billet is less than or equal to about 20%, step 132 is performed to perform a first light pressure on the semi-solidified steel billet by using the first light pressing roller set 230 next step. In the first light reduction step, the first light reduction roller set 230 provides appropriate control of the amount of reduction, so as to stabilize the liquid core size inside the semi-solidified steel billet and provide subsequent roller sets with an adapted reduction. standard range.

After the first soft reduction step, step 134 can be performed to use the second soft reduction roller set 240 to pair the semi-solidified steel billet when the central solid phase fraction of the semi-solidified steel billet is about 40% to about 100%. Embryos are subjected to a second light reduction step. Then, step 136 can be carried out, so that when the central solid phase fraction of the semi-solidified steel billet is about 40% to about 100%, utilize the third light-pressing roller set 250 to carry out the third light reduction to the semi-solidified steel billet step. In the light reduction operation, the first light reduction step is mainly used to fix the liquid core thickness of the billet, and the second light reduction step and the third light reduction step play the role of light reduction.

In this embodiment, the first pressure applied to the semi-solidified steel billet in the first soft reduction step is smaller than the second pressure applied to the semi-solidified steel billet in the second soft reduction step, and is also lower than the second pressure applied to the semi-solidified steel billet in the third soft reduction step. The third pressure exerted by the semi-solidified steel billet. In some examples, the first pressure is from about 10 kg/cm ² to about 25 kg/cm ² . The second pressure and the third pressure may be, for example, about 20 kg/cm ² to about 50 kg/cm ² . In some exemplary cases, the first pressure is about 25 kg/cm ² , and the second pressure and the third pressure are about 30 kg/cm ² .

Please refer to FIG. 3 , which is a diagram showing the relationship between the position of the electromagnetic stirring and soft pressing rollers and the distribution of the core solid fraction according to an embodiment of the present disclosure. In some examples, as shown in Figure 3, the electromagnetic stirring operation FEMS at the solidification end of the semi-solidified steel billet is carried out between 14 meters and 15 meters below the surface of the mold liquid. At this time, the central solid phase of the semi-solidified steel billet The percentage is quite small, much less than 20%. The first soft reduction step SR1 of the semi-solidified steel billet is carried out about 19 meters below the surface of the mold liquid. At this time, the central solid fraction of the semi-solidified steel billet is between 10% and 20%. The second soft reduction step SR2 of the semi-solidified steel billet is carried out at a distance of about 20.4 meters below the surface of the mold liquid. At this time, the central solid fraction of the semi-solidified steel billet is about 50%. The third soft reduction step SR3 of the semi-solidified steel billet is carried out at a distance of 21.8 meters below the surface of the mold liquid. At this time, the central solid fraction of the semi-solidified steel billet is about 100%.

Please refer to Fig. 4, which is a diagram showing the distribution of solid phase fractions in the core at light pressure positions with different specific water volumes. By designing the specific water volume of the secondary cooling, the central solid fraction of the billet can be controlled. This disclosure uses a dynamic temperature tracking (DTT) two-cooling heat transfer system to calculate an average superheat of 35 degrees and a pouring speed of 1.3m/min, and at different specific water volumes of 0.33L/kg, 0.65 L/kg, and 0.78 L /kg, the solid phase fraction distribution in the center of the process position under light pressure. In Fig. 4, the horizontal axis is the position of the three soft-pressing roller groups in the sprue respectively from the mold liquid surface. After the experiment, it was found that the solidified solid phase fraction at the position of the second lightly pressing down roller group is preferably more than 40%, which can provide a more obvious effect of improving segregation. Therefore, although the soft reduction process can improve the segregation problem in the case of the specific water volume of 0.33 L/kg and 0.65 L/kg, the segregation problem can be improved more obviously than the soft reduction process of the water volume of 0.78 L/kg. Since the segregation problem can be improved with a specific water volume of 0.33L/kg or more combined with a light reduction process, the specific water volume of the secondary cooling can be controlled from about 0.33L/kg to about 0.78L/kg when the molten steel is solidified.

The comparative examples and examples are listed below to illustrate the technical content of the method for eliminating segregation at the core of the billet in more detail, but they are not intended to limit the present disclosure.

The process parameters adopted in the comparative example are secondary cooling specific water volume 0.33L/kg, and soft reduction mode 111. Wherein, the soft reduction mode 111 indicates that three soft reduction steps are performed, and the pressure applied in each soft reduction step is 10 kg/cm ² . The steel billets obtained in the comparative example were pickled and sulfur stamped. From the etching results of the billet, it can be clearly observed that there are micro-shrinkage cavities in the center of the billet, so the central segregation problem is likely to occur at the end of the solidification of the billet. And from the cross-section of the billet, the obvious aggregation of segregation points in the core is also observed. In addition, based on the evaluation of the sulfur printing results, there are center and V-shaped segregation defects of grade 3 in the steel billet.

The process parameters adopted in the embodiment are secondary cooling specific water volume 0.78L/kg, soft reduction mode 233, and electromagnetic stirring alternate mode. Wherein, the soft reduction mode 233 indicates that three soft reduction steps are performed, and the pressure applied in the first soft reduction step is 25kg/cm ² , and the pressure applied in the second soft reduction step and the third soft reduction step are The pressure is 30kg/cm ² . Similarly, the steel billets obtained in the examples were pickled and sulfur printed. From the etching results of the billet, it can be observed that the micro-shrinkage cavity defects in the core of the billet have been significantly improved, and the V-shaped segregation points are small and evenly distributed. From the cross-section of the steel billet, it is also observed that the segregation point in the center becomes smaller, and the segregation defects are improved. In addition, based on the evaluation of sulfur printing results, the central segregation rating inside the steel billet is grade 0.

The results of the examples show that the center and V-shaped segregation defects in the core of the steel billet are greatly improved, and the overall pass rate can be greatly increased to over 99%.

It can be seen from the above-mentioned implementation that one of the advantages of the present disclosure is that the method for eliminating segregation at the core of the billet in the present disclosure performs three light reduction steps on the semi-solidified steel billet in sequence at the end of solidification, and the first step of light reduction The pressure applied to the semi-solidified steel billet in the step is smaller than the pressure applied to the semi-solidified steel billet in the second soft reduction step and the third soft reduction step. Thereby, the central segregation and V-shaped segregation at the center of the billet can be improved.

Another advantage of the present disclosure is that the method for eliminating segregation at the core of the billet can perform electromagnetic stirring at the end of the solidification of the semi-solidified billet before lightly pressing the semi-solidified billet, thereby improving the quality of the steel billet. The phenomenon of uneven solidification composition of the billet can greatly improve the V-shaped segregation in the core of the billet.

Although the present disclosure has been disclosed above with embodiments, it is not intended to limit the present disclosure. Any person with ordinary knowledge in this technical field may make various modifications and modifications without departing from the spirit and scope of the present disclosure. Therefore, the scope of protection of this disclosure should be defined by the scope of the appended patent application.

100: steps 110: Steps 130: Step 132: Step 134: step 136: Step 200: Continuous casting equipment 210: casting mold 220: Electromagnetic stirring device at the end of solidification 230: The first light pressing roller group 240: The second light pressing roller group 250: The third light pressing roller set FEMS: electromagnetic stirring operation at the end of solidification SR1: First soft depressing step SR2: Second soft depressing step SR3: Third soft depressing step

In order to make the above and other purposes, features, advantages and embodiments of the present disclosure more comprehensible, the accompanying drawings are described as follows: [Fig. 1] is a flowchart illustrating a method for eliminating core segregation of a billet according to an embodiment of the present disclosure; [Fig. 2] is a schematic diagram of a device for eliminating core segregation of steel billets according to an embodiment of the present disclosure; [Fig. 3] is a graph showing the distribution relationship between the position of the electromagnetic stirring and light pressing rollers and the solid fraction in the center according to an embodiment of the present disclosure; and [Fig. 4] is a diagram showing the distribution of the solid phase fraction in the core at the light pressure position with different specific water volumes.

Domestic deposit information (please note in order of depositor, date, and number) none Overseas storage information (please note in order of storage country, institution, date, and number) none

200: Continuous casting equipment

210: casting mold

220: Electromagnetic stirring device at the end of solidification

230: The first light pressing roller group

240: The second light pressing roller group

250: The third light pressing roller group

Claims (8)

A method for eliminating core segregation of a billet, comprising: performing a continuous casting process on a molten steel, wherein performing the continuous casting process includes: performing a solidification operation on the molten steel to form a half-solidified steel billet; Carrying out an end electromagnetic stirring operation on the steel billet, wherein performing the end electromagnetic stirring operation includes performing an alternate stirring process at a position of 1/4 to 1/2 of a thickness of the semi-solidified steel billet; and performing an alternating stirring process on the semi-solidified steel billet After performing the terminal electromagnetic stirring operation, performing a soft reduction operation on the semi-solidified steel billet to obtain a steel billet, wherein performing the soft reduction operation includes: a solid fraction in the center of one of the semi-solidified steel billets When it is less than or equal to 20%, carry out a first light reduction step to the semi-solidified steel billet to stabilize the liquid core size inside the semi-solidified steel billet; When the central solid phase fraction of the solidified steel billet is 40% to 100%, a second soft reduction step and a third soft reduction step are performed on the semi-solidified steel billet in sequence, wherein the first soft reduction step A first pressure applied to the semi-solidified steel billet in the step is lower than a second pressure and a third pressure respectively applied to the semi-solidified steel billet in the second light reduction step and the third light reduction step. The method for eliminating segregation at the center of a steel billet as described in Claim 1, wherein the steel billet is a medium-carbon alloy steel billet or a medium-high carbon alloy steel billet. The method for eliminating segregation at the center of the steel billet as described in Claim 1, wherein when performing the terminal electromagnetic stirring operation, the central solid phase fraction of the semi-solidified steel billet is 5% to 20%. The method for eliminating segregation at the core of the billet as described in Claim 1, wherein performing the terminal electromagnetic stirring operation includes setting a current value to 500A to 650A, and setting a frequency to 4Hz to 8Hz. The method for eliminating segregation at the core of the billet as claimed in Claim 1, wherein the first pressure is 10kg/cm ² to 25kg/cm ² . The method for eliminating core segregation of steel billets as described in claim 1, wherein the second pressure and the third pressure are 20kg/cm ² to 50kg/cm ² . The method for eliminating segregation at the core of the billet as described in Claim 1, wherein the first pressure is 25kg/cm ² , the second pressure and the third pressure are 30kg/cm ² . The method for eliminating segregation at the core of the billet as described in Claim 1, wherein performing the solidification operation on the molten steel includes controlling the ratio of primary and secondary cooling water to 0.33L/kg to 0.78L/kg.

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