A continuous casting process to reduce surface rolling cracks in hot-charged 20CrMo steel billets

Abstract

This invention discloses a continuous casting process to reduce surface rolling cracks in hot-charging 20CrMo steel continuously cast billets. The process involves passing molten 20CrMo steel that meets requirements through a crystallizer to obtain a continuously cast billet. The billet undergoes a staged cooling treatment to transform the microstructure of the surface layer from austenite to bainite and martensite, controlling the precipitation of AlN phase at grain boundaries. The treated billet is then fed into a heating furnace for further processing to obtain the final 20CrMo steel continuously cast billet. The advantages of this invention are that the staged rapid surface cooling process after the billet exits the crystallizer effectively reduces the amount of AlN precipitation on the billet surface, avoids the formation of grain boundary film ferrite, and thus reduces surface rolling cracks during hot charging. This process is not only easy to implement but also ensures a rapid production pace.

Description

TECHNICAL FIELD

[0001] The present application mainly belongs to the technical field of metallurgy, and particularly relates to a continuous casting process for reducing surface rolling cracks of 20CrMo steel continuous casting billets in hot charging. BACKGROUND

[0002] The main alloying components in the 20CrMo steel are as follows in percentage by mass: C: 0.19-0.23%, Mn: 0.75-0.95%, Si: 0.2-0.4%, S: 0.01-0.025%, Cr: 1.05-1.35%, Ni: 0.1-0.3%, Cu: 0.1-0.35%, Mo: 0.01-0.05%, Al: 0.03-0.05%, and N: 0.01-0.03%. The steel has good mechanical properties, high quenching property and wear resistance, and is mainly used for manufacturing gears. Gear steel requires high hardness, wear resistance and contact fatigue strength on the surface layer, and high strength, good plasticity and toughness in the core. In production, the second phase AlN is often used to pin the austenite grain boundary to obtain the above properties. Therefore, it is required that the gear steel must contain a certain amount of Al and N. In the production process, in order to speed up the production rhythm and improve the thermal efficiency, the gear steel continuous casting billet is used in the hot charging mode. The hot charging mode refers to that the high-temperature continuous casting billet produced on-line is directly put into the heating furnace through the conveying roller. However, after using this process, a large number of surface cracks are prone to occur on the rolled material, and the yield is low. A large number of AlN precipitates and grain boundary film-like ferrite on the surface layer of the continuous casting billet in the continuous casting process are the causes of such cracks. How to reduce the AlN precipitation amount in the continuous casting process and avoid the occurrence of grain boundary film-like ferrite while ensuring the fast production rhythm is the difficulty in the continuous casting process of gear steel.

[0003] The existing Chinese patent CN116174668A "A production method for improving the hot charging rate of low-carbon aluminum killed steel billets of large square billets" proposes to control the AlN precipitation amount by strictly controlling the N content in the steel and the three-time quenching method in the continuous casting process. However, this method is not suitable for the production of 20CrMo steel. The main reason is that the mass percentage of N in 20CrMo steel is required to be between 0.01-0.03%, and the AlxN must be greater than or equal to 4.5x10 -8 . The AlN precipitation amount of 20CrMo steel in the continuous casting process cannot be reduced by reducing the N content. Secondly, due to the high requirement of the aluminum-nitrogen product (AlxN) of 20CrMo steel, the single three-time cooling process has limited inhibitory effect on the AlN precipitation on the surface layer of the continuous casting billet, and cannot greatly avoid the occurrence of surface cracks.

[0004] The existing Chinese patent CN114635027A "A normalizing process for stabilizing low-temperature high-magnetic-susceptibility oriented silicon steel AlN inhibition force" proposes to control AlN precipitation through a strict heat treatment process. However, this method is applied to rolled steel strip products and is not suitable for continuous casting process. In addition, the Al and N content in oriented silicon steel is also far lower than that of 20CrMo steel.

[0005] For high aluminum-nitrogen content 20CrMo steel, AlN precipitation during continuous casting cannot be reduced by reducing N content. A single three-quenching process may be useful for controlling grain boundary film-like ferrite, but it is difficult to effectively reduce the amount of AlN precipitation under high aluminum-nitrogen content. The existing heat treatment rapid cooling process for inhibiting AlN has too high cooling intensity and cannot be successfully applied to continuous casting process. There is no reported process method for simultaneously controlling AlN precipitation on the surface layer of continuous casting billet during continuous casting process and inhibiting grain boundary film-like ferrite. SUMMARY

[0006] The present application discloses a continuous casting process for reducing surface rolling cracks of 20CrMo steel continuous casting billet hot charging to solve any of the above and other potential problems of the prior art.

[0007] To solve the above technical problems, the present application provides a continuous casting process for reducing surface rolling cracks of 20CrMo steel continuous casting billet hot charging. The continuous casting process is to obtain a continuous casting billet by passing the required 20CrMo steel liquid through a crystallizer, and then to convert the metallographic structure on the surface layer of the obtained continuous casting billet from austenite to bainite and martensite by adopting a staged cooling process, and to control the grain boundary AlN phase precipitation. Then, the treated continuous casting billet is sent to a heating furnace for subsequent treatment, and a 20CrMo steel continuous casting billet is obtained.

[0008] Further, the molten steel smelting and composition control before continuous casting requires that the cleanliness of the molten steel meets the standard, and all components hit the target. The continuous casting overheat degree is controlled at about 25℃. The continuous casting speed can be adopted according to the corresponding conventional speed according to the cross-sectional size of the continuous casting billet.

[0009] Further, the staged cooling includes a solidification stage and a hot charging stage; the solidification stage includes at least three stage cooling zones; and the temperature on the surface of the continuous casting billet during the hot charging process is not more than 600℃.

[0010] Further, the specific process of the solidification stage is as follows:

[0011] In the first stage cooling zone, the surface temperature of the continuous casting billet is reduced from 1200℃ to A1 temperature at a first cooling rate.

[0012] The second stage cooling zone reduces the surface temperature of the continuous casting billet from A1 temperature to A2 temperature by using a second cooling rate;

[0013] The third stage cooling zone reduces the surface temperature of the continuous casting billet from A2 temperature to A3 temperature by using a third cooling rate.

[0014] Further, the first cooling rate is 4.5-6℃ / s, and the A1 temperature is 860-840℃.

[0015] Further, the second cooling rate is 0.4-0.6℃ / s, and the A2 temperature is 810-800℃.

[0016] Further, the third cooling rate is 14-20℃ / s, and the A3 temperature is 510-490℃.

[0017] Further, the 20CrMo steel component is as follows in terms of mass percentage: C: 0.19-0.23%, Mn: 0.75-0.95%, Si: 0.2-0.4%, P: ≤0.02%, S: 0.01-0.025%, Cr: 1.05-1.35%, Ni: 0.1-0.3%, Cu: 0.1-0.35%, Mo: 0.01-0.05%, Al: 0.03-0.05%, N: 0.01-0.03%, and the balance is Fe and inevitable impurities, and the above components satisfy: Al*N≥4.5*10 -8 .

[0018] Further, the surface layer of the continuous casting billet refers to a thickness range of 15mm from the surface of the continuous casting billet.

[0019] Further, the number density of the AlN precipitate particles is 1.8*10 5 / cm 2 , and the average diameter is 58.5nm.

[0020] A 20CrMo steel continuous casting billet is prepared by using the above continuous casting process.

[0021] Compared with the prior art, the beneficial effects of the process method of the present application in reducing the surface rolling cracks of the hot charging of the 20CrMo steel continuous casting billet are embodied in that:

[0022] (1) The present application can greatly reduce the AlN precipitation amount of the surface layer of the continuous casting billet only by the continuous casting process stage, and is not affected by the high aluminum and nitrogen content. The change on the basis of the original process is small and easy to realize.

[0023] (2) The continuous casting process of the present application not only can greatly reduce the amount of AlN precipitates, but also has an inhibiting effect on the Nb-Ti carbonitride in the continuous casting process, and can improve the surface quality problem caused by the Nb-Ti carbonitride.

[0024] (3) The present application can avoid the appearance of grain boundary film-like ferrite only through the continuous casting process stage.

[0025] (4) The continuous casting process of the present application can obtain a continuous casting billet surface layer structure with martensite and bainite as the main phase. The surface layer recrystallized austenite grains after reheating in the subsequent heating furnace will be very small. Such fine recrystallized austenite can reduce the surface quality problems such as network cracks and scarring that may occur during rolling of the continuous casting billet, and improve the surface quality of the finished product after rolling.

[0026] Firstly, the continuous casting process of the present application can greatly reduce the surface layer AlN precipitates and avoid the appearance of grain boundary film-like ferrite during the continuous casting process of 20CrMo steel, and reduce the occurrence rate of surface rolling cracks after hot charging. Secondly, the continuous casting process of the present application can also obtain a continuous casting billet surface layer structure after the refinement of martensite and bainite, which can improve the continuous casting billet with good surface quality for the hot rolling process, and reduce the surface quality problems during rolling. BRIEF DESCRIPTION OF DRAWINGS

[0027] Figure 1 is a schematic diagram of the continuous casting cooling system, the metallographic structure and the AlN precipitates of the inventive example;

[0028] Figure 2 is a schematic diagram of the continuous cooling transformation curve (CCT) of the undercooled austenite of 20CrMo steel;

[0029] Figure 3A is a picture of the surface layer AlN precipitates of the continuous casting billet obtained by the inventive example;

[0030] Figure 3B is the morphology and energy spectrum diagram of the AlN precipitates;

[0031] Figure 4 is a surface layer metallographic structure photo obtained by the inventive example;

[0032] Figure 5 is a picture of the surface layer AlN precipitates of the continuous casting billet obtained by Comparative Example 1;

[0033] Figure 6 is a surface layer metallographic structure photo obtained by Comparative Example 1;

[0034] Figure 7 is a picture of the surface layer AlN precipitates of the continuous casting billet obtained by Comparative Example 2;

[0035] Figure 8is a surface metallographic structure photo obtained from Comparative Example 2. DETAILED DESCRIPTION

[0036] In order to make the objects, technical solutions and advantages of the present application clearer, the present application will be further described in detail below in combination with the drawings and examples. It should be understood that the specific examples described herein are only used to explain the present application and do not limit the present application.

[0037] On the contrary, the present application covers any substitution, modification, equivalent method and solution made within the essence and scope of the present application defined by the claims. Further, in order to make the public have a better understanding of the present application, some specific details are described in the following detailed description of the present application. The present application can also be completely understood without the description of these details by those skilled in the art.

[0038] The present application is a continuous casting process for reducing surface rolling cracks generated by hot charging of 20CrMo steel continuous casting billets,

[0039] (1) Zone I

[0040] The first stage cooling zone is to reduce the surface temperature of the continuous casting billet from 1200℃ to A1 temperature by using a first cooling rate. The first cooling rate is 4.5-6℃ / s, and the A1 temperature is 860-840℃.

[0041] The 20CrMo steel with high aluminum and nitrogen content starts to precipitate and grow at 1100℃. Since the surface temperature of the continuous casting billet is close to 1200℃ after leaving the crystallizer, fast cooling is started immediately after leaving the crystallizer. In addition, the cooling speed of 5℃ / s can greatly reduce the AlN precipitation amount of 20CrMo steel in the range of 850℃-1200℃, as shown in FIG. 1, and the higher the cooling speed, the better the effect. This interval should ensure that the surface layer structure is fully austenitic. Figure 1

[0042] (2) Zone II

[0043] The second stage cooling zone is to reduce the surface temperature of the continuous casting billet from A1 temperature to A2 temperature by using a second cooling rate. The second cooling rate is 0.4-0.6℃ / s, and the A2 temperature is 810-800℃.

[0044] The continuous cooling transformation curve CCT of the undercooled austenite of 20CrMo steel shows that the ferrite phase precipitation temperature Ar3 in the cooling process is 760℃, and the cooling speed should be greater than 15℃ / s to avoid the appearance of ferrite. Considering the limitations of the continuous casting secondary cooling process, the appearance of ferrite phase in this stage should be avoided as much as possible. Therefore, the surface temperature of the continuous casting billet is controlled between 810℃-850℃. Figure 2

[0045] ​​(3) Zone III

[0046] The surface temperature of the continuously cast slab is reduced from the A2 temperature to the A3 temperature using a third cooling rate. The third cooling rate is 14-20°C / s, and the A3 temperature is 510-490°C.

[0047] The ferrite phase precipitation temperature Ar3 during cooling is 760°C, and the cooling speed must be greater than 15°C / s to avoid the appearance of ferrite. Therefore, the appearance of grain boundary film ferrite is avoided by strong cooling.

[0048] (4) Zone IV

[0049] The continuously cast slab enters the heating furnace in a hot charging manner, and the surface of the continuously cast slab will appear a re-warming phenomenon during this process, but the surface temperature does not exceed 600°C during the entire process.

[0050] After the surface layer of the continuously cast slab is rapidly cooled in Zone III, the internal heat of the continuously cast slab will conduct outward, causing the surface temperature to rise, and a surface re-warming phenomenon appears. If the re-warming temperature is too high, ferrite will reappear, so the surface temperature of this process is required to be no more than 600°C.

[0051] Example

[0052] The continuous casting production is carried out according to the process standard of the invention, and the specific steps are as follows:

[0053] First step, steel liquid smelting before continuous casting

[0054] The smelting method of electric furnace + LF + VD is used to make the composition of the steel liquid hit all the targets, and the cleanliness of the steel liquid meets the requirements.

[0055] Second step, tundish and mold

[0056] The superheat of the steel liquid in the tundish is 25°C, the mold cooling process is the same as the conventional cooling process, and the continuous casting is ensured to proceed normally. The cross section of the continuously cast slab is 325mm x 420mm.

[0057] Third step, Zone I

[0058] After the continuously cast slab exits the mold, the surface temperature of the continuously cast slab is rapidly reduced to 843°C, and the average cooling speed is 5.4°C / s.

[0059] Fourth step, Zone II

[0060] The surface temperature of the continuously cast slab is rapidly reduced to 804°C, and the average cooling speed is 0.55°C / s.

[0061] Fifth step, Zone III

[0062] The surface temperature of the continuously cast slab is rapidly cooled to 494°C, and the average cooling speed is 15.8°C / s.

[0063] Sixth step, IV zone

[0064] The surface of the continuously cast slab is reheated to a maximum temperature of 586°C, and the temperature of the continuously cast slab when entering the heating furnace is 550°C.

[0065] In the inventive example, the content of the alloying elements in the molten steel before casting reaches the target value. On the basis of the conventional process, the continuously casting process of the present application is added, and the surface quality of the final continuously cast slab is good. The AlN precipitates in the surface layer of the final continuously cast slab are in the form of particles, as shown by the white small particles in Figure 3A , Figure 3B is an energy spectrum of the AlN precipitates. The number density of the AlN precipitate particles in the inventive example is 1.8 x 10 5 / cm 2 , and the average diameter is 58.5 nm. The metallographic structure of the surface layer of the final continuously cast slab in the inventive example is bainite and martensite, without ferrite phase, as shown in Figure 4 . The surface layer of the continuously cast slab is passed through the temperature range of 1200°C to 850°C at a high speed, effectively reducing the amount of AlN precipitated at the austenite grain boundaries. The surface layer of the continuously cast slab is passed through the temperature range of 800°C to 600°C at a high speed, effectively suppressing the ferrite in the surface layer of the continuously cast slab, and successfully avoiding the occurrence of the grain boundary film-like ferrite. The inventive example has a total of 10 continuous casting, and a total of 110 rods are rolled, of which 1 rod has a surface crack, and the crack occurrence rate is 0.91%.

[0066] Comparative Example 1

[0067] The average cooling speed of the surface of the continuously cast slab in this comparative example is 2.9°C / s in the I zone, which is smaller than the cooling speed in the inventive example.

[0068] In Comparative Example 1, due to the decrease in cooling intensity in the I zone, the amount of AlN precipitated in the surface layer of the final continuously cast slab increases, as shown in Figure 5 . The number density of the AlN precipitate particles in Comparative Example 1 is 6.8 x 10 5 / cm 2 , and the average diameter is 134.6 nm. The metallographic structure of the surface of the final continuously cast slab in Comparative Example 1 is bainite and martensite, without ferrite phase, as shown in Figure 6 . The speed of the surface layer of the continuously cast slab passing through the temperature range of 1200°C to 850°C is slow, and the amount of AlN precipitated in the austenite temperature range increases. Comparative Example 1 has a total of 9 continuous casting, and a total of 99 rods are rolled, of which 15 rods have surface cracks, and the crack occurrence rate is 15.15%.

[0069] Comparative Example 2

[0070] This comparative example is basically the same as the inventive example, except that the average cooling speed of the surface of the continuously cast slab in the III zone is 5.8°C / s, which is smaller than the cooling speed in the inventive example.

[0071] In Comparative Example 2, due to the reduced cooling intensity in Zone III, grain boundary film-like ferrite appeared on the surface of the final continuously cast billet, such as... Figure 7 As shown. The number density of AlN precipitate particles in Comparative Example 2 is 2.0 × 10⁻⁶. 5 pcs / cm 2 The average diameter was 68.4 nm. The final microstructure of the continuously cast billet obtained in Comparative Example 2 consisted of ferrite, bainite, and martensite, with film-like ferrite appearing, such as... Figure 8 As shown. Comparative Example 2 underwent 10 consecutive heats of casting, producing a total of 110 bars. Among them, 10 bars developed surface cracks, resulting in a crack occurrence rate of 10.10%.

[0072] The above provides a detailed description of a continuous casting process for reducing surface rolling cracks in hot-charging of 20CrMo steel continuous casting billets, as provided in the embodiments of this application. The descriptions of the embodiments above are merely for the purpose of helping to understand the method and core ideas of this application; furthermore, those skilled in the art will recognize that, based on the ideas of this application, there will be changes in specific implementation methods and application scope. Therefore, the content of this specification should not be construed as a limitation of this application.

[0073] Certain terms are used in the specification and claims to refer to specific components. Those skilled in the art will understand that hardware manufacturers may use different names to refer to the same component. This specification and claims do not distinguish components based on differences in name, but rather on differences in function. The terms "comprising" and "including" used throughout the specification and claims are open-ended and should be interpreted as "comprising / including but not limited to". "Approximately" means that within an acceptable margin of error, those skilled in the art can solve the technical problem and substantially achieve the technical effect within a certain margin of error. The following descriptions in the specification are preferred embodiments for carrying out this application; however, these descriptions are for the purpose of illustrating the general principles of this application and are not intended to limit the scope of this application. The scope of protection of this application shall be determined by the appended claims.

[0074] It should also be noted that the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a product or system comprising a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a product or system. Without further limitation, an element defined by the phrase "comprising one..." does not exclude the presence of other identical elements in the product or system that includes said element.

[0075] It should be understood that the term "and / or" as used herein merely describes associated objects, and can exist in three forms: A and / or B, A or B, and A and B. In addition, the character " / " generally represents an "or" relationship between the front and rear associated objects.

[0076] The above description illustrates and describes several preferred embodiments of the present application, but as previously noted, it is not intended to be exhaustive or to be limited to the precise form disclosed. It is intended to cover various modifications and alternate methods, and combinations thereof, falling within the scope of the application as defined by the appended claims, and their equivalents. Changes and modifications can be made to the application in light of the above teachings. It is therefore, intended to cover any and all changes and modifications of the application in accordance with the scope of the appended claims.

Claims

1. A continuous casting process for reducing surface rolling cracks caused by hot charging of 20CrMo steel continuous casting billets, characterized in that, The continuous casting process is that the qualified 20CrMo liquid steel is passed through a crystallizer to obtain a continuous casting billet, the obtained continuous casting billet is treated by stage cooling to convert the metallographic structure of the surface layer of the continuous casting billet from austenite to bainite and martensite, and the grain boundary AlN phase is controlled to precipitate, and then the treated continuous casting billet is sent into a heating furnace for subsequent treatment, so as to obtain the 20CrMo steel continuous casting billet; The 20CrMo steel component is as follows in percentage by mass: C: 0.19-0.23%, Mn: 0.75-0.95%, Si: 0.2-0.4%, P: ≤0.02%, S: 0.01-0.025%, Cr: 1.05-1.35%, Ni: 0.1-0.3%, Cu: 0.1-0.35%, Mo: 0.01-0.05%, Al: 0.03-0.05%, N: 0.01-0.03%, the balance of Fe and inevitable impurities, and the above components satisfy: Al x N ≥ 4.5 x 10 -8 ; The stage cooling includes a solidification stage and a hot charging stage; the solidification stage includes at least three stage cooling zones; the temperature of the surface of the continuous casting billet in the hot charging stage is not more than 600℃; The specific process of the solidification stage is that: In the first stage cooling zone, the surface temperature of the continuous casting billet is reduced from 1200℃ to A1 temperature by using a first cooling rate; In the second stage cooling zone, the surface temperature of the continuous casting billet is reduced from A1 temperature to A2 temperature by using a second cooling rate; In the third stage cooling zone, the surface temperature of the continuous casting billet is reduced from A2 temperature to A3 temperature by using a third cooling rate; The first cooling rate is 4.5-6℃ / s, and the A1 temperature is 860-840℃; The second cooling rate is 0.4-0.6℃ / s, and the A2 temperature is 810-800℃; The third cooling rate is 14-20℃ / s, and the A3 temperature is 510-490℃; The surface layer of the continuous casting billet refers to a thickness range of 15mm from the surface of the continuous casting billet.

2. The continuous casting process according to claim 1, characterized in that, The number density of the AlN precipitate particles was 1.8 x 10 5 cm 2 -3, and the average diameter was 58.5 nm.

3. A 20CrMo steel continuous cast billet, characterized in that, The 20CrMo steel continuous casting billet is prepared by using the continuous casting process according to any one of claims 1-2.

Images