阅读说明：本技术 降低铸坯中心偏析的工艺方法 (Process for reducing center segregation of casting blank ) 是由 汪洪峰 于 2018-05-28 设计创作，主要内容包括：本发明涉及一种降低铸坯中心偏析的工艺方法,所述方法包括以下步骤：1)钢水纯净度和钢中夹杂物控制；2)压下前坯壳鼓肚和靠前大压下控制；该方法主要采用让铸坯先鼓肚后轻压下技术,提高轻压下技术的效果；通过钢水夹杂物控制从源头减少钢液凝固过程中选分结晶的杂质元素,不需增加任何设备和工艺步骤,满足高质量钢种的要求。(The invention relates to a process method for reducing center segregation of a casting blank, which comprises the following steps: 1) controlling the purity of molten steel and inclusions in steel; 2) the blank shell is expanded before pressing down and is controlled by the large pressure in front; the method mainly adopts the technology of bulging the casting blank firstly and then slightly reducing the casting blank, so that the effect of the slightly reducing technology is improved; the impurity elements of selective crystallization in the process of molten steel solidification are reduced from the source through molten steel inclusion control, no equipment and process steps are required to be added, and the requirement of high-quality steel grades is met.)

1. A process for reducing center segregation of a cast slab, comprising the steps of:

1) controlling the purity of molten steel and inclusions in steel;

The blank shell is expanded before pressing down and is controlled by the large pressure in front.

2. The process method for reducing the center segregation of the casting blank according to claim 1, wherein the purity of the molten steel and the inclusion content in the steel in the step 1) are controlled, and specifically, the purity of the molten steel is improved and the inclusion content in the steel is reduced mainly by the following steps in the actual production process of the mixed steel;

11) And (3) converter end point control: the converter adopts high-carbon-drawing end point control, the carbon content of tapping is more than 0.04 percent for low-carbon steel, more than 0.10 percent for medium-high-carbon steel, and the free oxygen at the end point of molten steel is controlled to be less than 700 ppm; the tapping of the converter adopts the slag discharge detection and sliding plate slag stopping technology, and the slag discharge of the molten steel of the converter is controlled to be less than 2 kg/ton;

12) And (3) secondary refining control: deoxidizing and desulfurizing white slag produced by an LF furnace, wherein the alkalinity of refined slag is more than 4.0; stirring the refined molten steel by adopting bottom blowing argon weak stirring, wherein the amount of the bottom blowing argon is based on the principle that the slag surface in a steel ladle slightly shakes but the molten steel is not exposed, and the flow of the bottom blowing argon is required to be between 40 and 70m3\ h;

13) Continuous casting protection pouring and steady state pouring control: carrying out continuous casting, namely carrying out molten steel protection pouring, controlling the nitrogen increase of the molten steel to be less than 3ppm and the burning loss of acid-soluble aluminum of the molten steel to be less than 5 ppm; making steady-state protection pouring for continuous casting, controlling the flow of argon of a stopper rod to be 2-4l/min, controlling the flow of argon of a water feeding port to be less than 2l/min, and controlling the surface flow rate of molten steel in a crystallizer to be 0.05-0.10 m/s;

14) Finally, the total oxygen content of the casting blank is controlled to be less than 15ppm, the P content of the molten steel is controlled to be less than 0.010 percent, the S content is controlled to be less than 0.005 percent, and the nitrogen content is controlled to be less than 35ppm, so that the contents of impurities and easily segregated elements of selective crystallization in the process of solidifying the molten steel are reduced from the source, and the degree of center segregation is reduced.

3. The process method for reducing the center segregation of the casting blank according to claim 2, wherein the step 2) of controlling bulging of the blank shell before reduction and large reduction before reduction comprises the following specific steps: 21) the front blank shell is pressed down and the belly is bulged,

the bulging of the shell of the blank before reduction is realized by artificially increasing the roll gap of the previous sector of the reduction sector by 0.2-0.5mm, and after the bulging of the casting blank with the liquid core, the reduction sector is reduced to reduce the bulged casting blank along with the running of the casting blank, so that a larger reduction is generated at the reduction position of the casting blank, namely near the tail end of the liquid core of the casting blank to compensate the solidified shrinkage of the casting blank, an internal gap formed by the solidified shrinkage of the casting blank is eliminated or reduced, the residual liquid phase enriched with solute between dendrites is still kept at the original position to prevent the segregation from moving transversely to the center of the casting blank, and meanwhile, the liquid core molten steel flows reversely along the drawing blank under the action of the larger reduction under the light reduction, so that the central molten steel is redistributed, the central solidified tissue is more compact, the components are more uniform, and the effects of reducing the central.

4. The process method for reducing the center segregation of the casting blank according to claim 3, wherein the step 2) of controlling bulging of the blank shell before reduction and large reduction before reduction further comprises the following steps: 22) the method comprises the following steps of (1) carrying out forward high reduction, wherein the reduction position is required to be properly forward, the reduction position is required to be positioned between the critical feeding rate and the critical flowing solid phase rate of a casting blank, namely the residual rate of a solidified liquid phase of a core part of the casting blank is between 40% and 60%, and the reduction is sufficiently large, so that the internal cracks of the process upper casting blank can not be generated and the equipment upper reduction can be realized, the total reduction of the continuous casting of the plate blank is 6-12 mm, the larger the thickness of the casting blank is, the larger the reduction is, the total reduction is distributed among 2-3 fan-shaped sections, the total reduction is non-uniformly distributed among the reduced fan-shaped sections, and the reduction distribution principle is 20%, 50%; the distribution principle of the reduction amount is 30 percent and 70 percent when 2 fan-shaped sections are pressed down.

Technical Field

The invention relates to a method, in particular to a process method for reducing center segregation of a casting blank, and belongs to the technical field of metallurgical continuous casting.

background

Center segregation is one of the main defects of continuous casting slabs, and seriously affects the properties of steel products, such as welding performance, drawing performance and the like. For example, a high-carbon steel casting blank used for rolling high-performance wire rods and petroleum pipelines often has a center segregation defect, and even if the high-carbon steel casting blank is rolled or forged in a subsequent process, the adverse effect on the performance of a final product is difficult to eliminate, so that the mechanical property of the product is deteriorated. Therefore, the center segregation defect is a technical problem of improving the quality of high-carbon steel and alloy steel continuous casting billets in actual production, the development and application of certain high-quality and high-added-value steel grades with special requirements on center segregation are limited, and how to effectively control the center segregation of casting billets is a target which is always pursued by metallurgical workers.

The central line segregation of the casting blank is mainly formed by the flow of a solute-enriched liquid phase caused by selective crystallization and solidification shrinkage during the solidification of the casting blank. In other words, the segregation occurs only when solidification occurs, and the center line segregation of the cast slab can be reduced and cannot be completely eliminated. At present, the casting blank soft reduction technology is one of effective measures for controlling and relieving the defects, but the control and relieving effect of the technology on the center segregation is limited, and the quality requirements of a plurality of high-quality and high-added-value steel grades with special requirements on the center segregation cannot be met, so that how to effectively control the center segregation of the casting blank to meet the quality requirements of the steel grades is a common problem of domestic and foreign continuous casting.

similar documents are searched, and patent publication No. CN101934357B discloses a process for effectively controlling center segregation of a continuous casting slab, which is characterized in that the effective length for effectively implementing light pressing is ensured by controlling the pressure value of a hydraulic cylinder at the lower section of light pressing, the cooling water quantity of a casting blank, the superheat degree of molten steel, the pulling speed and other process parameters. Patent publication No. CN102310178A "a method for solving center segregation of a cast slab": the method is characterized in that the central segregation of elements such as C, S, Mn and the like which are formed because solute-enriched element molten steel is closed and can not exchange with other liquids due to separation crystallization in the casting blank solidification process is improved through optimization of processes such as molten steel superheat degree control, upper steel S content control, continuous casting crystallizer wire feeding control, continuous casting pulling speed control, continuous casting secondary cooling system control, casting blank dump cooling control and the like. Therefore, a new technical scheme for solving the technical problem is urgently needed.

disclosure of Invention

The invention provides a process method for reducing the center segregation of a casting blank aiming at the technical problems in the prior art, which mainly adopts the technology of bulging the casting blank firstly and then slightly reducing the casting blank, thereby improving the effect of the slightly reducing technology; impurity elements of selective crystallization in the process of molten steel solidification are reduced from the source through molten steel inclusion control. The application of the invention obviously improves the technical effect of soft reduction, and the center segregation of the casting blank meets the quality requirement, which is not met by the traditional process; the method does not need to increase any equipment and process steps, obviously reduces the center segregation level of the casting blank, meets the requirement of high quality, improves the production and development capacity of high value-added steel and obtains obvious economic benefit.

In order to achieve the above object, the technical solution of the present invention is a process for reducing center segregation of a cast slab, the process comprising the steps of:

1) Controlling the purity of molten steel and inclusions in steel;

2) The blank shell is expanded before pressing down and is controlled by the large pressure in front.

as an improvement of the invention, the step 1) of controlling the purity of the molten steel and the inclusion in the steel is specifically characterized in that the purity of the molten steel is improved and the content of the inclusion in the steel is reduced by the following steps mainly in the actual production process of the steel mixture;

11) and (3) converter end point control: the converter adopts high-carbon-drawing end point control, the carbon content of tapping is more than 0.04 percent for low-carbon steel, more than 0.10 percent for medium-high-carbon steel, and the free oxygen at the end point of molten steel is controlled to be less than 700 ppm; the tapping of the converter adopts the slag discharge detection and sliding plate slag stopping technology, and the slag discharge of the molten steel of the converter is controlled to be less than 2 kg/ton;

12) And (3) secondary refining control: deoxidizing and desulfurizing white slag produced by an LF furnace, wherein the alkalinity of refined slag is more than 4.0; stirring the refined molten steel by adopting bottom blowing argon weak stirring, wherein the amount of the bottom blowing argon is based on the principle that the slag surface in a steel ladle slightly shakes but the molten steel is not exposed, and the flow of the bottom blowing argon is required to be between 40 and 70m3\ h;

13) continuous casting protection pouring and steady state pouring control: carrying out continuous casting, namely carrying out molten steel protection pouring, controlling the nitrogen increase of the molten steel to be less than 3ppm and the burning loss of acid-soluble aluminum of the molten steel to be less than 5 ppm; making steady-state protection pouring for continuous casting, controlling the flow of argon of a stopper rod to be 2-4l/min, controlling the flow of argon of a water feeding port to be less than 2l/min, and controlling the surface flow rate of molten steel in a crystallizer to be 0.05-0.10 m/s;

14) Finally, the total oxygen content of the casting blank is controlled to be less than 15ppm, the P content of the molten steel is controlled to be less than 0.010 percent, the S content is controlled to be less than 0.005 percent, and the nitrogen content is controlled to be less than 35ppm, so that the contents of impurities and easily segregated elements of selective crystallization in the process of solidifying the molten steel are reduced from the source, and the degree of center segregation is reduced.

as an improvement of the invention, the step 2) of pressing down the front blank shell to bulge and controlling the blank shell under the large front pressing condition comprises the following specific steps: 21) the front blank shell is pressed down and the belly is bulged,

the bulging of the blank shell before reduction is that the roll gap of the previous segment of the reduction segment is artificially increased by 0.2-0.5mm, and the blank shell with the liquid core naturally generates corresponding slight bulging at high temperature due to the maximum reduction roll gap value of the casting blank (the roll gap value is excessively increased, the bulging of the casting blank is insufficient, and the roll gap value is excessively increased, so that the bulging of the casting blank is excessively large, and the subcutaneous cracks of the casting blank are caused.

After the casting blank with the liquid core is expanded, the expanded casting blank is reduced by reducing the reduction fan-shaped section along with the operation of the casting blank, so that a larger reduction is generated at the reduction position of the casting blank (near the tail end of the liquid core of the casting blank) to compensate the solidified shrinkage of the casting blank, an internal gap formed by the solidified shrinkage of the casting blank is eliminated or reduced, a residual liquid phase with solute enrichment among dendrites is still kept at the original position to prevent the casting blank from moving transversely to form segregation, and meanwhile, the liquid core molten steel flows along the reverse direction of the drawing blank under the action of the larger reduction under the light reduction, so that the central molten steel is redistributed, the central solidified tissue is more compact, the components are more uniform, and the effects of reducing the central segregation and improving the central porosity are.

As an improvement of the invention, the step 2) of pressing the front blank shell to bulge and controlling the front blank shell under large pressure further comprises the following steps: 22) the method comprises the following steps of (1) carrying out forward high reduction, wherein the reduction position is required to be properly forward, the reduction position is required to be positioned between the critical feeding rate and the critical flowing solid phase rate of a casting blank, namely the residual rate of a solidified liquid phase of a core part of the casting blank is between 40% and 60%, and the reduction is sufficiently large, so that the internal cracks of the process upper casting blank can not be generated and the equipment upper reduction can be realized, the total reduction of the continuous casting of the plate blank is 6-12 mm, the larger the thickness of the casting blank is, the larger the reduction is, the total reduction is distributed among 2-3 fan-shaped sections, the total reduction is non-uniformly distributed among the reduced fan-shaped sections, and the reduction distribution principle is 20%, 50%; the distribution principle of the reduction amount is 30 percent and 70 percent when 2 fan-shaped sections are pressed down. In the process of casting blank solidification, the forward high pressure reduction can strengthen convection and stirring of the solidification front edge of the enriched solute, promote dendritic crystal to fall off and re-melt, increase crystallization core and solidification supercooling degree, and promote the uniformity of the organization and the components of the finally solidified central area, thereby obtaining the effect of reducing central segregation; in addition, the larger reduction can also compensate the volume shrinkage of the residual molten steel at the final stage of solidification and enhance the feeding and filling capacity of the casting blank, so that the central porosity can be reduced, the welding of center line cracks can be promoted, and the effect of increasing the central density of the casting blank is achieved.

Compared with the prior art, the invention has the following beneficial effects: 1) before reduction, the blank shell bulges, so that the casting blank slightly bulges, and then soft reduction is carried out, which is equivalent to further aggravating the macroscopic flow of molten steel, increasing the reduction amount and improving the reduction efficiency, so that the center solidification structure is more compact, the components are more uniform, and the effects of reducing center segregation and improving center porosity are achieved; 2) the reduction interval is advanced by the front high reduction, the reduction amount is increased, the internal gap formed by solidification and shrinkage of the casting blank is eliminated or reduced, and the casting blank is prevented from moving transversely to the center of the casting blank to form segregation. 3) Reducing the inclusions in the molten steel, improving the purity of the molten steel, and being beneficial to reducing segregation substances from the source so as to reduce the segregation degree. The high-quality requirement casting blanks such as high-performance tool and die steel, petroleum pipelines and the like produced by the process method have the center segregation grade reaching the A grade (best grade) above 80 percent, the level coincidence rate of the center segregation B above 1.0 being 100 percent, and the user quality requirement is completely met.

Drawings

FIG. 1 is a schematic view of a shell bulging and depressing apparatus;

fig. 2 is a schematic diagram of the bulging principle of the blank shell before pressing.

The specific implementation mode is as follows:

for the purpose of promoting an understanding of the invention, reference will now be made in detail to the present invention as illustrated in the accompanying drawings.