Continuous casting process for reducing surface rolling cracks generated by hot charging of 20CrMo steel continuous casting billet

Abstract

The invention discloses a continuous casting process for reducing surface rolling cracks generated by hot charging of a 20CrMo steel continuous casting billet, which comprises the following steps of: passing 20CrMo molten steel meeting requirements through a crystallizer to obtain the continuous casting billet, and cooling the obtained continuous casting billet by stages to convert a metallographic structure on the surface layer of the continuous casting billet from austenite into bainite and martensite, and a grain boundary AlN phase is controlled to be separated out, then the treated continuous casting billet is fed into a heating furnace to be subjected to follow-up treatment, and the 20CrMo steel continuous casting billet is obtained. The continuous casting process has the beneficial effects that the separation amount of AlN on the surface layer of the continuous casting billet can be effectively reduced through the partition surface rapid cooling process after the continuous casting billet is discharged out of the crystallizer, the appearance of grain boundary film ferrite is avoided, and the purpose of reducing hot charging surface rolling cracks is achieved. The process is easy to implement, and the rapid production rhythm can be guaranteed.

Description

Technical Field

[0001] The invention mainly belongs to the technical field of metallurgy, and in particular relates to a continuous casting process for reducing surface rolling cracks generated by hot charging of a 20CrMo steel continuous casting billet. Background Art

[0002] The main alloy components in 20CrMo steel are as follows by mass percentage: 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%, N: 0.01-0.03%. This type of steel has good mechanical properties, high hardenability and wear resistance, and is mainly used to manufacture gears. Gear steel requires high hardness, wear resistance and contact fatigue strength on the surface, while the core has high strength and good plasticity and toughness. In production, the second phase AlN is often used to pin the austenite grain boundary to obtain the above properties. Therefore, production requires that there must be a certain amount of Al and N in the gear steel. In the production process, in order to speed up the production rhythm and improve thermal efficiency, the gear steel continuous casting billet is hot-sent and hot-charged into the heating furnace. The hot delivery and hot loading method refers to the use of online high-temperature continuous casting billets directly into the heating furnace through the conveyor roller. However, after adopting this process, a large number of surface cracks are prone to appear on the rolled material, and the yield is low. The large amount of AlN precipitation phase and grain boundary film ferrite that appear on the surface of the continuous casting billet during the continuous casting process are the causes of such cracks. How to reduce the amount of AlN precipitation in the continuous casting process and avoid the appearance of grain boundary film ferrite while ensuring a fast production rhythm is the difficulty of the current gear steel continuous casting process.

[0003] The existing Chinese patent CN116174668A "A production method for improving the hot charging rate of large square billet low-carbon aluminum killed steel billet" proposes to control the amount of AlN precipitation by strictly controlling the N content in the steel and the three-time quenching method during the continuous casting process. However, this method is not suitable for the production of 20CrMo steel. The main reason is that 20CrMo steel requires the mass percentage of N to be between 0.01 and 0.03%, and must meet Al×N≥4.5×10 -8 The amount of AlN precipitation in 20CrMo steel during continuous casting cannot be reduced by reducing the N content. Secondly, due to the high requirements for the aluminum nitrogen product (Al×N) of 20CrMo steel, the single three-cooling process has limited effect on inhibiting AlN precipitation on the surface of the continuous casting billet, and cannot avoid the occurrence of surface cracks to a large extent.

[0004] The existing Chinese patent CN114635027A "A Normalization Process for Stabilizing AlN Inhibition of Low-Temperature High-Magnetic Induction Oriented Silicon Steel" 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 processes. In addition, the Al and N content in oriented silicon steel is also much lower than that of 20CrMo steel.

[0005] For 20CrMo steel with high aluminum nitrogen content, the precipitation of AlN during the continuous casting process cannot be reduced by reducing the N content. A single three-stage quenching process may be useful for controlling the grain boundary film 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 to suppress AlN has too high cooling intensity and cannot be successfully applied to the continuous casting process. There are no reported processes for simultaneously controlling the precipitation of AlN on the surface of the continuous casting billet during the continuous casting process and suppressing the grain boundary film ferrite. Summary of the invention

[0006] The invention discloses a continuous casting process for reducing surface rolling cracks generated by hot charging of a 20CrMo steel continuous casting billet, so as to solve any of the above and other potential problems in the prior art.

[0007] In order to solve the above technical problems, the present invention discloses a continuous casting process for reducing surface rolling cracks generated by hot charging of a 20CrMo steel continuous casting billet. The continuous casting process comprises the following steps: passing a 20CrMo steel liquid that meets the requirements through a crystallizer to obtain a continuous casting billet; subjecting the obtained continuous casting billet to a staged cooling treatment so that the metallographic structure of the surface layer of the continuous casting billet is transformed from austenite to bainite and martensite, and the precipitation of AlN phase at the grain boundary is controlled; and then sending the treated continuous casting billet into a heating furnace for subsequent treatment to obtain a 20CrMo steel continuous casting billet.

[0008] Furthermore, the molten steel smelting and composition control before continuous casting require that the cleanliness of the molten steel meets the standard, and all the components are hit, and the continuous casting superheat is controlled at about 25°C. The continuous casting speed can be adopted according to the cross-sectional size of the continuous casting billet. The cooling intensity of the crystallizer can be adopted according to the cross-sectional size of the continuous casting billet.

[0009] Furthermore, the staged cooling includes a solidification stage and a hot delivery and hot charging stage; the solidification stage includes at least three cooling zones; and the surface temperature of the continuous casting billet during the hot delivery and hot charging process does not exceed 600°C.

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

[0011] In the first stage cooling zone, the surface temperature of the continuous casting billet is reduced from 1200°C to A1 temperature using the first cooling rate;

[0012] In the second stage cooling zone, the surface temperature of the continuous casting billet is reduced from the A1 temperature to the A2 temperature using the second cooling rate;

[0013] In the third stage cooling zone, the surface temperature of the continuous casting billet is reduced from the A2 temperature to the A3 temperature using the third cooling rate.

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

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

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

[0017] Further, the composition of the 20CrMo steel is as follows by 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%, the balance is Fe and unavoidable impurities, and the above composition satisfies: Al×N≥4.5×10 -8 .

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

[0019] Furthermore, the number density of the AlN precipitate particles is 1.8×10 5 Pieces / cm 2 , with an average diameter of 58.5nm.

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

[0021] Compared with the prior art, the beneficial effects of the process method of the present invention on reducing the surface rolling cracks caused by hot charging of 20CrMo steel continuous casting billets are as follows:

[0022] (1) The present invention can significantly reduce the amount of AlN precipitation on the surface of the continuous casting billet only through the continuous casting process stage, and is not subject to the influence of high aluminum nitrogen content. The change based on the original process is small and easy to implement.

[0023] (2) The continuous casting process of the present invention can not only significantly reduce the amount of AlN precipitation, but also has an inhibitory effect on niobium-titanium carbonitrides during the continuous casting process, thereby improving the surface quality problems caused by niobium-titanium carbonitrides.

[0024] (3) The present invention can also avoid the occurrence of grain boundary film ferrite only through the continuous casting process stage.

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

[0026] First, the continuous casting process of the present invention can significantly reduce the amount of AlN precipitation on the surface of 20CrMo steel during continuous casting and avoid the appearance of filmy ferrite at the grain boundary, thereby reducing the occurrence rate of surface rolling cracks after hot charging. Secondly, the continuous casting process of the present invention can also refine the surface structure of the continuous casting billet after recrystallization by obtaining martensite and bainite structures, thereby improving the continuous casting billet with good surface quality for the hot rolling process and reducing the occurrence of surface quality problems during the rolling process. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 It is a schematic diagram of the continuous casting cooling system, metallographic structure, and AlN precipitation phase of the invention example;

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

[0029] Figure 3A This is a picture of AlN precipitates on the surface of the continuous casting billet obtained in the invention example;

[0030] Figure 3B is the morphology and energy spectrum of AlN precipitate particles;

[0031] Figure 4 This is a photo of the surface metallographic structure obtained in the invention example;

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

[0033] Figure 6 This is a photo of the surface metallographic structure obtained in Comparative Example 1;

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

[0035] Figure 8This is a photo of the surface metallographic structure obtained in Comparative Example 2. DETAILED DESCRIPTION

[0036] In order to make the purpose, technical solution and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0037] On the contrary, the present invention covers any substitution, modification, equivalent method and scheme made on the essence and scope of the present invention as defined by the claims. Further, in order to make the public have a better understanding of the present invention, some specific details are described in detail in the detailed description of the present invention below. Those skilled in the art can fully understand the present invention without the description of these details.

[0038] The present invention discloses a continuous casting process for reducing surface rolling cracks generated by hot charging of a 20CrMo steel continuous casting billet.

[0039] (1) Zone I

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

[0041] The 20CrMo steel with high AlN content begins to precipitate and grow at 1100℃. Since the surface temperature of the continuous casting billet is close to 1200℃ after leaving the crystallizer, rapid cooling begins immediately after leaving the crystallizer. In addition, a cooling rate of 5℃ / s can significantly reduce the amount of AlN precipitation in 20CrMo steel in the range of 850℃~1200℃, such as Figure 1 As shown, the higher the cooling rate, the better the effect. In this range, the surface structure must be fully austenite.

[0042] (2) Zone II

[0043] In the second stage cooling zone, the surface temperature of the continuous casting billet is reduced from the A1 temperature to the A2 temperature at a second cooling rate of 0.4-0.6°C / s, and the A2 temperature is 810-800°C.

[0044] Continuous cooling transformation curve CCT of supercooled austenite of 20CrMo steel Figure 2 ) shows that the precipitation temperature Ar3 of the ferrite phase during the cooling process is 760℃, and the appearance of ferrite can only be avoided when the cooling rate is greater than 15℃ / s. Considering the limitations of the continuous casting secondary cooling process, the appearance of ferrite phase should be avoided as much as possible at this stage. Therefore, the surface temperature of the continuous casting billet is controlled between 810℃-850℃.

[0045] (3) Zone III

[0046] The surface temperature of the continuous casting billet is reduced from A2 temperature to A3 temperature by adopting a third cooling rate, wherein 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 the cooling process is 760°C, and the appearance of ferrite can only be avoided when the cooling rate is greater than 15°C / s. Therefore, the appearance of grain boundary film ferrite is avoided by strong cooling.

[0048] (4) Zone IV

[0049] The continuous casting billet is transported and loaded into the heating furnace hot. During this process, the surface of the continuous casting billet will warm up, but the surface temperature does not exceed 600℃ during the whole process.

[0050] After the surface of the continuous casting billet is rapidly cooled in zone III, the heat inside the continuous casting billet will be conducted outward, causing the surface temperature to rise and the surface temperature to rise. If the temperature is too high, ferrite will reappear, so the surface temperature of this process is required not to exceed 600℃.

[0051] Example

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

[0053] The first step is molten steel smelting before continuous casting

[0054] The smelting method of electric furnace + LF + VD is adopted to ensure that the components of the molten steel are all hit and the cleanliness of the molten steel meets the requirements.

[0055] The second step: Tundish and crystallizer

[0056] The superheat of the tundish steel liquid is 25℃, and the cooling process of the crystallizer is the same as the conventional cooling process to ensure the normal continuous casting. The cross section of the continuous casting billet is 325mm×420mm.

[0057] Step 3, Zone I

[0058] After the continuous casting billet leaves the crystallizer, the surface temperature of the continuous casting billet drops rapidly to 843°C, with an average cooling rate of 5.4°C / s.

[0059] Step 4, Zone II

[0060] The surface temperature of the continuous casting billet dropped rapidly to 804°C, with an average cooling rate of 0.55°C / s.

[0061] Step 5, Zone III

[0062] The surface temperature of the continuous casting billet was rapidly cooled to 494°C, with an average cooling rate of 15.8°C / s.

[0063] Step 6, Zone IV

[0064] The highest temperature of the continuous casting billet surface was 586℃, and the temperature when entering the heating furnace was 550℃.

[0065] In the inventive examples, the alloying element content in the molten steel before casting reached the target value. The continuous casting process of the present invention was added on the basis of the conventional process, and the surface quality of the final continuous casting billet was good. The AlN precipitates on the surface of the final continuous casting billet were granular, such as Figure 3A As shown by the small white particles in Figure 3B is the energy spectrum of AlN precipitates. The number density of AlN precipitate particles in the inventive example is 1.8×10 5 Pieces / cm 2 The average diameter is 58.5nm. The surface metallographic structure of the continuous casting billet finally obtained in the invention example is bainite and martensite, without ferrite phase, such as Figure 4 As shown. The surface of the continuous casting billet is made to quickly pass through the temperature range of 1200℃ to 850℃, which effectively reduces the precipitation of AlN at the austenite grain boundary. The surface of the continuous casting billet is made to quickly pass through the temperature range of 800℃ to 600℃, and the ferrite on the surface of the continuous casting billet is effectively suppressed, successfully avoiding the appearance of film ferrite at the grain boundary. In the invention example, a total of 10 furnaces were cast continuously, and a total of 110 bars were rolled, of which one bar had surface cracks, and the crack occurrence rate was 0.91%.

[0066] Comparative Example 1

[0067] In this comparative example, the average cooling rate of the continuous casting billet surface in zone I is 2.9°C / s, which is lower than that of the inventive example.

[0068] In Comparative Example 1, due to the reduced cooling intensity of Zone I, the amount of AlN precipitation on the surface of the final continuous casting billet increases. Figure 5 The number density of AlN precipitate particles in Comparative Example 1 is 6.8×10 5 Pieces / cm 2 The average diameter is 134.6nm. The surface microstructure of the continuous casting billet finally obtained in Comparative Example 1 is bainite and martensite, without ferrite phase. Figure 6 As shown. The speed of the surface layer of the continuous casting billet passing through the temperature range of 1200℃~850℃ slows down, and the precipitation amount of AlN in the austenite temperature range increases. Comparative Example 1 has a total of 9 furnaces of continuous casting, and a total of 99 bars are rolled, of which 15 bars 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 rate of the continuous casting billet surface in zone III is 5.8°C / s, which is lower than the cooling rate of the inventive example.

[0071] In Comparative Example 2, due to the reduced cooling intensity of Zone III, grain boundary film ferrite appeared on the surface of the final continuous casting billet, such as Figure 7 The number density of AlN precipitate particles in Comparative Example 2 is 2.0×10 5 Pieces / cm 2 The average diameter is 68.4 nm. The surface metallographic structure of the continuous casting billet finally obtained in Comparative Example 2 is ferrite, bainite and martensite, and film ferrite appears, such as Figure 8 In Comparative Example 2, 10 continuous castings were performed and 110 bars were rolled, of which 10 bars had surface cracks, with a crack occurrence rate of 10.10%.

[0072] The above is a detailed introduction to a continuous casting process for reducing surface rolling cracks generated by hot charging of 20CrMo steel continuous casting billets provided in the embodiment of the present application. The description of the above embodiment is only used to help understand the method and core idea of ​​the present application; at the same time, for those skilled in the art, according to the idea of ​​the present application, there will be changes in the specific implementation method and application scope. In summary, the content of this specification should not be understood as a limitation on the present application.

[0073] For example, certain words are used in the specification and claims to refer to specific components. Those skilled in the art should understand that hardware manufacturers may use different nouns to refer to the same component. This specification and claims do not use differences in names as a way to distinguish components, but use differences in the functions of components as the criteria for distinction. As mentioned throughout the specification and claims, "including" and "comprising" are open-ended terms, so they should be interpreted as "including / including but not limited to". "Approximately" means that within an acceptable error range, those skilled in the art can solve the technical problem within a certain error range and basically achieve the technical effect. The subsequent description of the specification is a preferred embodiment of the present application, but the description is for the purpose of illustrating the general principles of the present application, and is not used to limit the scope of the present application. The scope of protection of the present application shall be determined by the definition of the attached claims.

[0074] It should also be noted that the terms "include", "comprises" or any other variations thereof are intended to cover non-exclusive inclusion, so that a product or system including a series of elements includes not only those elements, but also other elements not explicitly listed, or also includes elements inherent to such a product or system. In the absence of more restrictions, the elements defined by the sentence "comprises a ..." do not exclude the existence of other identical elements in the product or system including the elements.

[0075] It should be understood that the term "and / or" used in this article is only a description of the association relationship of associated objects, indicating that there can be three relationships. For example, A and / or B can represent: A exists alone, A and B exist at the same time, and B exists alone. In addition, the character " / " in this article generally indicates that the associated objects before and after are in an "or" relationship.

[0076] The above description shows and describes several preferred embodiments of the present application, but as mentioned above, it should be understood that the present application is not limited to the form disclosed herein, and should not be regarded as excluding other embodiments, but can be used in various other combinations, modifications and environments, and can be modified within the scope of the application concept described herein through the above teachings or the technology or knowledge of the relevant field. The changes and modifications made by those skilled in the art do not deviate from the spirit and scope of the present application, and should be within the scope of protection of the claims attached to the present application.

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 to pass the 20CrMo steel liquid that meets the requirements through a crystallizer to obtain a continuous casting billet, and then subject the obtained continuous casting billet to a staged cooling treatment so that the metallographic structure of the surface layer of the continuous casting billet is transformed from austenite to bainite and martensite, and the precipitation of AlN phase at the grain boundary is controlled, and then the treated continuous casting billet is sent to a heating furnace for subsequent treatment, so as to obtain a 20CrMo steel continuous casting billet.

2. The continuous casting process according to claim 1, characterized in that: The staged cooling includes a solidification stage and a hot delivery and hot charging stage; the solidification stage includes at least three stage cooling zones; the surface temperature of the continuous casting billet during the hot delivery and hot charging process does not exceed 600°C.

3. The continuous casting process according to claim 2, characterized in that: The specific process of the solidification stage is: In the first stage cooling zone, the surface temperature of the continuous casting billet is reduced from 1200°C to A1 temperature using the first cooling rate; In the second stage cooling zone, the surface temperature of the continuous casting billet is reduced from the A1 temperature to the A2 temperature using the second cooling rate; In the third stage cooling zone, the surface temperature of the continuous casting billet is reduced from the A2 temperature to the A3 temperature using the third cooling rate.

4. The continuous casting process according to claim 3, characterized in that: The first cooling rate is 4.5-6°C / s, and the A1 temperature is 860-840°C.

5. The continuous casting process according to claim 3, characterized in that: The second cooling rate is 0.4-0.6°C / s, and the A2 temperature is 810-800°C.

6. The continuous casting process according to claim 3, characterized in that: The third cooling rate is 14-20°C / s, and the A3 temperature is 510-490°C.

7. The continuous casting process according to claim 1, characterized in that: The composition of the 20CrMo steel is as follows by 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%, the balance is Fe and unavoidable impurities, and the above composition satisfies: Al×N≥4.5×10 -8 .

8. The continuous casting process according to claim 1, characterized in that: The surface layer of the continuous casting billet refers to the thickness range of 15 mm from the surface of the continuous casting billet.

9. The continuous casting process according to claim 1, characterized in that: The number density of the AlN precipitate particles is 1.8×10 5 Pieces / cm 2 , with an average diameter of 58.5nm.

10. A 20CrMo steel continuous casting billet, characterized in that: The 20CrMo steel continuous casting billet is prepared by the continuous casting process as described in any one of claims 1 to 9.

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