CN107414049B - Refining control method for metallographic structure of surface layer of corner of continuous casting slab - Google Patents
Refining control method for metallographic structure of surface layer of corner of continuous casting slab Download PDFInfo
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- CN107414049B CN107414049B CN201610342455.0A CN201610342455A CN107414049B CN 107414049 B CN107414049 B CN 107414049B CN 201610342455 A CN201610342455 A CN 201610342455A CN 107414049 B CN107414049 B CN 107414049B
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- B22D11/00—Continuous casting of metals, i.e. casting in indefinite lengths
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- B22D11/22—Controlling or regulating processes or operations for cooling cast stock or mould
- B22D11/225—Controlling or regulating processes or operations for cooling cast stock or mould for secondary cooling
Abstract
The invention relates to a refining control method for a metallographic structure of a corner surface layer of a continuous casting slab, which is characterized by comprising the following steps of: (1) firstly, passing through a primary cooling zone of a crystallizer; (2) and secondly, supporting the wide surface of the casting blank with the liquid core by using continuous paired clamping rollers, spraying water between the clamping rollers while continuously drawing the casting blank to take away the heat inside the casting blank until the liquid core is completely solidified, wherein the cooling area is a secondary cooling area and is used for cooling in the secondary cooling area, and the refining control of the metallographic structure of the surface layer of the corner of the continuous casting blank is realized. The required cast metallographic structure of the surface layer of the slab is obtained by controlling the solidification and cooling process of the continuous casting slab within the specific steel component range, namely controlling the cooling strength of the surface of the slab.
Description
Technical Field
The invention relates to a control method, in particular to a refining control method for a metallographic structure of a surface layer of a corner of a continuous casting slab, and belongs to the technical field of steelmaking-continuous casting.
Background
In the steelmaking continuous casting production process, the corners of a continuous casting slab often have fine transverse crack defects, so that a large amount of surface edge quality defects are generated on subsequent steel rolling plate strip products, serious influence is brought to the manufacturing process, a large amount of economic loss is generated, and the method is a common technical problem to be solved urgently for realizing high-quality and high-efficiency production.
Under the condition of the conventional continuous casting process, coarse austenite grains are formed in the process of solidifying and cooling continuous casting billets, a film-shaped or net-shaped pro-eutectoid ferrite film is formed along austenite grain boundaries in a specific temperature range, and a precipitated phase of carbide, nitride or carbonitride exists in the ferrite film. The precipitation behavior at the austenitic grain boundaries leads to a reduction in the grain boundary strength, manifested by a drastic reduction in the macroscopic plasticity and strength of the metal structure, i.e. the high temperature mechanical phenomenon of the steel, customarily known as "plastic valley". The "plastic valley" is generally Ar which is in the process of solidification and cooling3One interval below the temperature. It is generally considered that when the surface temperature of the continuous casting blank enters a bending and straightening section in a plastic valley temperature range, under the action of bending or straightening stress, a grain boundary of a surface structure of a casting blank corner is easy to cause micro transverse cracks of the surface corner due to stress concentration. Under the condition of the conventional continuous casting billet production process, the surface temperature of the casting blank cannot be avoided to be completely outside the plastic valley temperature interval. Therefore, before the casting blank enters the bending and straightening area, the initial austenite grains are refined, the grain boundary precipitation of the grain boundary microalloy carbonitride of the corner structure of the casting blank is inhibited, the grain boundary of the corner structure is strengthened, and the plastic valley effect of the surface structure of the corner is eliminatedThe precipitation temperature interval of the main microalloy element carbonitride (such as Nb (CN), BN and the like) in the steel is generally in the temperature range of 850 ~ 1150 ℃ and 1150 ℃, and both practice and existing researches show that the precipitation of the microalloy element carbonitride on a steel structure grain boundary can be effectively inhibited by adopting a rapid cooling mode in the microalloy element carbonitride precipitation temperature interval, so that the grain boundary strength of the steel is favorably strengthened.
For example, the invention patent with the patent number 201010259985.1 discloses a method for integrally increasing the water quantity of the wide surface and the narrow surface of a continuous casting blank by 2 365 times in a vertical section (vertical bending section), so that the surface of the casting blank is cooled at the cooling speed of 3 ~ ℃/s, thereby achieving the purposes of reducing the precipitation of grain boundary microalloy element carbon nitride of a surface structure of the casting blank and reducing the angular transverse cracks.
Disclosure of Invention
The invention provides a refining control method of a metallographic structure of a surface layer of a corner of a continuous casting slab aiming at the technical problems in the prior art, and the required cast metallographic structure of the surface layer of the slab is obtained by controlling the solidification and cooling process of the continuous casting slab within a specific steel component range, namely controlling the cooling strength of the surface of the slab.
In order to achieve the above object, the technical solution of the present invention is a method for refining and controlling a metallographic structure of a surface layer of a corner of a continuous casting slab, comprising the steps of: (1) firstly, passing through a primary cooling zone of a crystallizer; (2) and secondly, supporting the wide surface of the casting blank with the liquid core by using continuous paired clamping rollers, spraying water between the clamping rollers while continuously drawing the casting blank to take away the heat inside the casting blank until the liquid core is completely solidified, wherein the cooling area is a secondary cooling area and is used for cooling in the secondary cooling area, and the refining control of the metallographic structure of the surface layer of the corner of the continuous casting blank is realized.
As an improvement of the invention, the secondary cooling area is divided into a vertical area, a bending area, an arc area, a straightening area and a horizontal area from front to back in sequence.
As a modification of the invention, the secondary cooling zone has a length of 30 ~ 40 m, the vertical zone has a length in the range of 2.0 ~ 3.0.0 m, the bend zone has a length in the range of 1.0 ~ 1.5 m, the first section of the arc zone has a length in the range of 1.5 ~ 2.0.0 m, the straightening zone has a length in the range of 3.5 ~ 4.5.5 m and the horizontal zone has a length in the range of 10.0 ~ 20.0.0 m.
As an improvement of the invention, in the step (2), the secondary cooling zone is controlled by 5 sets of control loops to obtain proper cooling intensity for each supporting roller zone, and the secondary cooling zone control loop and the corresponding matching relation with the supporting nip roller are as follows:
as an improvement of the invention, the forced cooling mode is that the density of secondary cold water flow is more than 100L/min2,The weak cooling mode is that the secondary cold water flow density is less than 100L/min2,。
As an improvement of the invention, the diameter of the wide-surface clamping roller of the secondary cooling area is 350 mm, and the interval of the clamping rollers is 350 mm.
Compared with the prior art, the method has the advantages that the required cast-state metallographic structure of the surface layer of the slab is obtained by controlling the continuous casting slab solidification cooling process within the specific steel component range, namely the control on the cooling strength of the surface of the slab, wherein the specific steel component range refers to the carbon content of 0.02-0.70%, and is hereinafter referred to as the specific component range, and simultaneously contains various necessary components which other carbon steels should contain, the method particularly plays a role in preventing cracks for crack sensitive steel containing microalloy elements such as niobium, vanadium, titanium, boron and the like, and the novel cast-state metallographic structure obtained by the method is characterized by fine ferrite and dispersed pearlite, and the average size of crystal grains is less than 20 microns. The cast structure of the conventional slab with the specific composition range is characterized by comprising coarse pro-eutectoid ferrite and pearlite. During the solidification and cooling process of steel, pro-eutectoid ferrite and carbon and nitride generated along austenite grain boundaries are precipitated as the root cause of crack generation, so the novel cast structure eliminates the root cause of the crack.
Drawings
FIG. 1 is a comparison of metallographic structures of new and old methods.
Detailed Description
For the purpose of enhancing an understanding of the present invention, the present embodiment will be described in detail below with reference to the accompanying drawings.
Example 1A refining control method of a metallographic structure of a corner surface of a continuous cast slab, comprising the steps of (1) passing through a primary cooling zone of a mold, cooling molten steel of a specific composition range through the mold called the primary cooling zone to form a cast slab with a liquid core having a shell thickness of 20 ~ 40 mm, cooling the molten steel of the specific composition range through a mold device to produce a cast slab with a shell thickness of less than 1300 ℃ and an austenite surface structure of the shell, and (2) supporting the broad surface of the cast slab with the liquid core by using a pair of nip rolls continuously and drawing the cast slab continuouslyMeanwhile, water is sprayed between the clamping rollers to take away heat inside the casting blank until the liquid core is completely solidified, the casting blank is a secondary cooling area and is cooled in the secondary cooling area, and the refining control of the metallographic structure of the surface layer of the corner of the continuous casting slab is realized. After primary cooling by the crystallizer, the wide surface of the casting blank with the liquid core is supported by using continuous paired clamping rollers (roller rows), and water is sprayed between the clamping rollers to take away the heat inside the casting blank while the casting blank is continuously drawn until the liquid core is completely solidified, so that the casting blank is a secondary cooling area (hereinafter referred to as a 'secondary cooling area'). And finishing the phase change control of the surface austenite of the casting blank while the liquid core in the casting blank in the secondary cooling area is solidified, wherein the steel types in the specific component range have the same austenite cooling phase change mode. In the conventional cooling mode, with the gradual reduction of the surface temperature of the shell, when the temperature is reduced to Ar3At temperature, ferrite begins to precipitate at austenite grain boundaries. The ferrite is increased along with the continuous reduction of the temperature, and when the temperature is reduced to Ar1At temperature, the residual austenite undergoes eutectoid transformation to form pearlite. The structure at room temperature is composed of proeutectoid ferrite and pearlite. In the steel grades having the above-specified composition ranges, Ar is increased with the increase of the carbon content3And Ar1The difference therebetween gradually decreases. That is, when the carbon content is 0.02%, Ar3And Ar1The difference between them is maximum; according to the theory of metal phase transition, Ar is present when the carbon content is 0.77%3And Ar1The difference between them is zero, called eutectoid transition point. Ar (Ar)3And Ar1The temperature interval in between is the crack sensitive zone.
The diameter of the wide-surface clamp rolls of the secondary cooling area of the slab continuous casting machine adopted in the technical scheme is 100 ~ mm, the roll diameter at the front part of the roll array is small, the roll diameter at the rear part of the roll array is thick, the spacing distance of the clamp rolls is 150 ~ mm, the roll distance at the front part of the roll array is small, the roll distance at the rear part of the roll array is large, the number of the secondary cold supporting clamp rolls of the slab continuous casting machine can reach more than 100 pairs according to the thickness specification and the maximum working pulling speed of a slab, the length of the whole supporting area can reach 40 m, the wide-surface support rolls of the secondary cooling area are divided into five areas, namely an ① vertical area, a ② bending area, an ③ arc area, a ④ straightening area and a ⑤ horizontal area, the length range of the vertical area is 2.0 ~.0 m, the length range of the bending area is 1.0 ~.5 m, and the length range of the first.
The cooling device is matched with wide-face clamping rollers, 2 ~ 5 pairs of short supporting rollers are also arranged on the narrow face of the lower opening of the crystallizer, the roller diameter is 100 ~ 150 mm, the length of a supporting interval is 400 ~ 600 mm, the cooling device is matched with wide-face and narrow-face supporting clamping rollers of a secondary cooling area, cooling water nozzles are arranged among the clamping roller intervals to provide secondary cooling water for secondarily cooling the casting blank, in order to achieve the control cooling effect, a control loop is adopted to control the secondary cooling water in a partitioning manner, so that each supporting roller area obtains proper cooling intensity, and the corresponding matching relationship between the secondary cooling control loop and the supporting clamping rollers is shown as the following table:
the narrow-face foot roller and the lower cooling nozzles are ingeniously arranged, and a plurality of nozzles are added for enhancing corner cooling. The newly added nozzle and the two cold foot roller supports are integrally designed and can move along with the width adjustment of the narrow surface of the crystallizer, so that the online width adjustment is ensured to have the same cooling effect on the corner of a casting blank with any section. The invention adopts the measure of mutually matching wide and narrow surface cooling, so that the surface structure of the corner part of the casting blank before entering the bending area is subjected to multiple phase changes, the control on the solidification structure is realized, the crystal grains are greatly refined, and the grain boundary precipitation is reduced, thereby eliminating the brittle valley effect, improving the macroscopic high-temperature strength of the steel and reducing the occurrence of cracks along the grain boundary. The secondary cooling nozzles and the water supply configuration of the existing continuous casting machine can not enable the whole casting blank surface to completely reach the preset target, and the temperature of the corner of the casting blank can reach the set target by adopting the additional secondary cooling nozzles and the water distribution.
By applying the roller array and the secondary cooling arrangement, the following control method is adopted to achieve the effect:
① sufficient cooling strength is ensured at the top and upper part of the secondary cooling vertical area to rapidly reduce the surface temperature of the casting blank corner to Ar3The following are A1In the above temperature range, the surface structure of the cast slab undergoes primary austenite decomposition transformation before entering the bending range. At the end of this phase, the microstructure of the surface layer of the slab corner is austenite and ferriteThe coexistence shape achieves the effects of inhibiting the growth of crystal grains and simultaneously inhibiting the precipitation of grain boundary carbonitride.
② at the lower part of the secondary cooling vertical zone, the sensible heat of the liquid core in the casting blank is used to quickly return the surface temperature to Ac3Keeping the temperature above; in this process, the structure of the steel undergoes a further transformation, i.e. the previously formed ferritic structure reverts to austenite, but retains its free-standing fine grain morphology.
③ in the secondary cold bending area, continuously keeping the weak cooling mode to make the bending deformation of the casting blank in Ar3The temperature is increased, namely the bending deformation is carried out in an austenite area with better plasticity so as to prevent cracks;
④ in the first section of the second cold arc area, the cooling intensity is increased again, and the surface corner temperature is reduced rapidly to Ar according to the target temperature set by the specific steel grade3Above the 100 ℃ interval, to control the austenite growth. Then, normal secondary cooling water distribution is maintained, the surface temperature is kept stable, and the corner temperature is reduced to Ar after the casting blank passes through the straightening area3Then, the fine austenite grains are transformed into fine ferrite grains, and the ferrite film on the coarse austenite grain boundaries is eliminated.
⑤ Another embodiment of step ④ is that the cooling intensity is increased again in the first stage of the secondary cooling arc zone, the surface corner temperature is rapidly decreased according to the target temperature set by the specific steel type, and the corner temperature is decreased to Ar before the cast slab passes through the straightening zone1Following and passing through the straightening zone, fine ferrite grains can also be obtained.
The grain size of the metallographic structure on the surface layer of the corner of the continuous casting slab is controlled by the method to be less than 20 micrometers and far less than 200 micrometers of the metallographic structure obtained by the conventional method at normal temperature.
Application example: to compare the effects of the old and new processes, comparative tests were carried out on two streams of the same caster, one stream using the new process and the other stream following the conventional process.
The compositions of the test steel grades are as follows:
casting blank section: 230mm is multiplied by 1650 mm; casting temperature: 1545 deg.C; production drawing speed: 1.1 m/min.
Pre-calculating and detecting to obtain Ar of the steel grade3The temperature is 717 ℃, Ar1The temperature was 612 ℃.
The position of the supporting pinch roll and the amount of secondary cooling water are configured as follows:
the key point values are as follows:
because the corners of the slab are cooled in two dimensions, the normal temperature is lower than in the middle of the broad face. From the above simulation calculations, the corner minimum temperature is about 370 ℃ lower than the broad face minimum temperature due to the further enhanced corner cooling in the upper portion of the secondary cooling vertical zone. Under the condition of the temperature field, the thermal stress of the blank shell is within an allowable range. The method further enhances the cooling of the corner part at the upper part of the vertical area by increasing the corner part cooling nozzle which is matched with the wide surface cooling, so that the temperature change of the surface layer of the corner part reaches the requirement of a phase change temperature interval.
After the test billet had cooled to ambient temperature, a sample was taken at the corner for metallographic examination, see fig. 1, for comparison as follows:
the original metallographic structure is characterized by being composed of coarse proeutectoid ferrite and pearlite, while the novel metallographic structure is characterized by being composed of fine ferrite and dispersed pearlite.
It should be noted that the above-mentioned embodiments are not intended to limit the scope of the present invention, and all equivalent modifications and substitutions based on the above-mentioned technical solutions are within the scope of the present invention as defined in the claims.
Claims (3)
1. A refining control method for a metallographic structure of a surface layer of a corner of a continuous casting slab is characterized by comprising the following steps: (1) firstly, passing through a primary cooling zone of a crystallizer;
(2) secondly, supporting the wide surface of the casting blank with the liquid core by using continuous paired clamping rollers, spraying water between the clamping rollers while continuously drawing the casting blank to take away the heat inside the casting blank until the liquid core is completely solidified, wherein the casting blank is a secondary cooling area and is cooled in the secondary cooling area, and the refining control of the metallographic structure on the surface layer of the corner of the continuous casting blank is realized;
the secondary cooling area is sequentially divided into a vertical area, a bending area, an arc area, a straightening area and a horizontal area from front to back;
the length of the secondary cooling area is 30-40 meters, the interval length of the vertical area is 2.0-3.0 meters, the length of the bending area is 1.0-1.5 meters, the length of the first section of the arc area is 1.5-2.0 meters, the length of the straightening area is 3.5-4.5 meters, and the length of the horizontal area is 10.0-20.0 meters;
in the step (2), the secondary cooling area is controlled by 5 groups of control loops to ensure that each supporting roller area obtains proper cooling strength, and the secondary cooling area control loops and the corresponding matching relation of the supporting rollers are as follows:
。
2. The method for refining and controlling the metallographic structure of the surface layer of the corner of a continuous casting slab as set forth in claim 1Characterized in that the forced cooling mode is that the secondary cooling water flow density is more than 100L/min2The weak cooling mode is that the secondary cold water flow density is less than 100L/min2。
3. The method for refining and controlling the metallographic structure of the surface layer of the corner of the continuously cast slab as claimed in claim 2, wherein the diameter of the wide-surface pinch rolls of the secondary cooling zone is 100-350 mm, and the interval between the pinch rolls is 150-350 mm.
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| TW202033292A (en) * | 2018-12-10 | 2020-09-16 | 日商日本製鐵股份有限公司 | Continuous casting method for steel |
| CN112605361B (en) * | 2020-12-11 | 2022-01-28 | 湖南华菱涟钢特种新材料有限公司 | Control method for transverse crack defect on surface of 75Cr1 steel |
| CN113414362B (en) * | 2021-05-31 | 2022-04-22 | 中南大学 | Cooling system method for simultaneously improving strength and plasticity of corner of high-carbon steel small square billet |
| CN114734014B (en) * | 2022-03-31 | 2024-01-19 | 东北大学 | Cooling method and system for controlling crack at corner of microalloy steel plate blank |
| CN115090847B (en) * | 2022-05-31 | 2024-03-08 | 天津钢铁集团有限公司 | Spray cooling control system and control method for corner of slab after exiting crystallizer |
| CN115415489B (en) * | 2022-09-05 | 2024-02-13 | 东北大学 | Continuous casting equipment and process for aluminum/aluminum alloy sheet billet |
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| JPS577370A (en) * | 1980-06-16 | 1982-01-14 | Nippon Steel Corp | Cooling method for continuously cast ingot |
| US20070251663A1 (en) * | 2006-04-28 | 2007-11-01 | William Sheldon | Active temperature feedback control of continuous casting |
| JP5419394B2 (en) * | 2008-06-24 | 2014-02-19 | 株式会社神戸製鋼所 | Slab manufacturing method |
| CN101733382B (en) * | 2008-11-12 | 2011-12-14 | 攀钢集团研究院有限公司 | Continuous casting method |
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| CN102380597B (en) * | 2011-10-28 | 2014-03-19 | 中冶赛迪工程技术股份有限公司 | Method for controlling secondary cooling water spraying width during continuous casting of plate blank |
| CN102861890A (en) * | 2012-09-19 | 2013-01-09 | 中冶南方工程技术有限公司 | Secondary cooling method for reducing transverse cracks of corners of microalloy sheet billet |
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