A method for preparing a bainite high-strength steel with rapid phase transformation induced by deformation

Abstract

The application discloses a preparation method of a bainite high-strength steel with a rapid phase change induced by deformation and precipitation, and belongs to the alloy field.The preparation method disclosed by the application comprises the following steps: S1, melting and casting;S2, hot rolling to refine grains;S3, warm rolling deformation to induce precipitation;S4, bainite phase change isothermal;S5, a small amount of martensite is introduced;S6, bainite phase change isothermal is performed again;the application utilizes the deformation to induce the precipitation of a submicron second phase, optimizes a heat treatment process, and obtains the bainite high-strength steel with no incubation period, and the phase change completion time is within 70 minutes, so that the isothermal transformation rate of the nano bainite is greatly improved, the preparation time is significantly shortened, and the production efficiency is improved.

Description

Technical Field

[0001] This invention relates to a method for preparing a deformation-induced rapid phase transformation bainitic high-strength steel, belonging to the field of alloys. Background Technology

[0002] Nanobainitic steels, with their nanoscale bainitic and retained austenitic structures, exhibit a better balance of strength, plasticity, and toughness. However, the long transformation time of nanobainitic phases restricts their production and application. Early nanobainitic steels were designed with high C and high Si compositions, and it took tens of hours of isothermal treatment to obtain nanobainite, resulting in poor toughness and weldability.

[0003] Ultrafine bainitic steel possesses mechanical properties similar to maraging steel, but its cost is only one-ninth that of maraging steel. Furthermore, it can be produced using conventional processing methods without the need for rapid cooling or mechanical processing, making it a promising new type of high-strength steel with significant development potential and promising applications. A low-temperature isothermal quenching process can be used to prepare a multiphase microstructure composed of nanoscale bainitic ferrite laths and carbon-rich thin-film retained austenite. This process is simple and offers a good balance of strength and plasticity. However, the low-temperature transformation process is time-consuming, with the phase transformation taking tens of hours, hindering its large-scale production and application. Therefore, shortening the isothermal quenching time and accelerating the bainitic phase transformation process are key to solving the industrialization problem of high-strength bainitic steel.

[0004] To accelerate the bainitic phase transformation process and obtain high-strength bainitic steel, Chinese invention patent CN201811347357.1 discloses a rapid phase transformation nano-bainitic steel and its preparation method. The production process involves heating the billet to the austenitizing temperature, followed by rapid cooling to the martensitic phase transformation temperature via salt bath quenching to obtain steel containing a small amount of martensite. The steel is then rapidly transferred to the nano-bainitic phase transformation temperature range for isothermal phase transformation. This process simplifies production and improves product performance and quality. However, the mechanical properties of the product are not high, and its microstructure contains a large amount of blocky retained austenite, which easily transforms into hard and brittle martensite during service, causing stress concentration and cracking. Chinese invention patent 202110466341.8 discloses a low-carbon nano-bainitic multiphase steel and its preparation method. The prepared product has excellent mechanical properties, but its preparation process is complex, making large-scale production difficult, and the preparation time is also long.

[0005] In summary, given the shortcomings of existing methods for preparing high-strength bainitic steel, there is an urgent need to design new alloy compositions and combine them with supporting controlled rolling and cooling processes to produce high-strength bainitic steel with excellent comprehensive performance in order to adapt to increasingly complex service conditions. Summary of the Invention

[0006] The purpose of this invention is to provide a method for preparing bainitic high-strength steel with rapid phase transformation induced by deformation. Through elemental regulation, combined with controlled rolling and cooling plus a multi-step isothermal process, deformation-induced precipitation is used to induce the precipitation of uniformly distributed submicron-sized carbides during the rolling process. Rolling not only refines the grains but also stores a large amount of deformation energy in the steel, accelerating the phase transformation rate during the bainitic isothermal process. This invention solves the problem of excessively long phase transformation time for ultrafine bainitic steel, which is detrimental to production. Specifically, it includes the following steps:

[0007] (1) Melting and casting: according to the design requirements of steel composition, melt and cast steel ingots in a vacuum induction furnace.

[0008] (2) Hot rolling to refine grains: The steel ingot is heated to 1160℃~1280℃ at a heating rate of 65-80℃ / h for homogenization treatment for 2h, and then cooled to 1150℃~1100℃ for hot rolling.

[0009] (3) Warm rolling deformation-induced precipitation: The hot-rolled billet obtained in step S2 is cooled to 750℃~800℃ and then warm rolled.

[0010] (4) Bainite phase transformation isothermal: The warm rolled billet obtained in step S3 is salt bath quenched to 400±30℃ and held for 10 to 30 minutes.

[0011] (5) Introducing a small amount of martensite: The steel billet obtained in step S4 is salt bath quenched and rapidly cooled to 10-15°C below Ms point and held for 5-10 seconds to obtain steel containing a small amount of martensite.

[0012] (6) Isothermal transformation of bainitic phase again: The steel obtained in step S5 is placed in a salt bath furnace at 400±30℃ and isothermal for 20 to 40 minutes. After isothermal transformation, it is air-cooled to room temperature.

[0013] The bainitic high-strength steel comprises the following elements by weight percentage: C: 0.30–0.40%, Si: 1.05–1.25%, Mn: 1.24–1.45%, Cr: 0.80–1.20%, Ni: 0.95–1.46%, Al: 0.85–1.25%, Nb: 0.08–0.18%, with the remainder being Fe and unavoidable impurities. The bainitic high-strength steel is a medium-carbon alloy steel.

[0014] Preferably, in step (2) of the present invention, the cooling rate is 10℃ / s, the initial rolling temperature is 1150℃~1100℃, 3~4 rolling passes are performed, the total hot rolling deformation is 60~80%, and the final rolling temperature is not lower than 850℃.

[0015] Preferably, in step (3) of the present invention, the cooling rate is 10℃ / s, the initial rolling temperature is 750℃~800℃, the total deformation after 2~3 rollings is 30~50%, and the final rolling temperature is not lower than 660℃.

[0016] The microstructure of the bainitic high-strength steel described in this invention is nanobainite + a small amount of martensite + submicron-sized second-phase carbides, with a tensile strength of 1530-1756 MPa, a yield strength of 1120-1260 MPa, and an elongation of 18-26%.

[0017] The principle of this invention:

[0018] This invention involves hot rolling a steel billet that has undergone high-temperature homogenization treatment in the single-phase austenitic temperature range. The extensive deformation promotes repeated recrystallization of austenite grains, refining the austenite grain structure and eliminating some casting defects. Warm rolling the billet at a lower temperature range generates numerous crystal defects in the deformed steel, leading to deformation-induced precipitation of Nb-containing carbide particles. Simultaneously, severe cold deformation of the austenite grains significantly increases the dislocation density in the steel, promoting bainitic ferrite nucleation. Furthermore, the precipitation of niobium-containing carbides refines the austenite grains, provides more nucleation sites for bainitic phase transformation, and ensures that the nano-bainitic steel retains high strength at high temperatures. Rapid cooling and holding after warm rolling are to prevent… Other phase transformations occur in steel, such as pearlitic transformation. After isothermal treatment for 10–30 minutes, the steel is quenched in a salt bath furnace at 10–15°C below Ms and held for 5–10 seconds. This is because after continuous bainitic transformation for a period of time, the transformation rate of bainitic transformation will decrease significantly. Introducing a small amount of martensite at this time can increase the nucleation points of bainitic transformation at the original austenitic grain boundaries where bainitic transformation has not yet occurred, thus accelerating the bainitic transformation time during the subsequent isothermal process within the bainitic transformation temperature range. Through the design of the steel composition, controlled rolling and cooling processes, and multi-step isothermal process technology, a phase transformation without incubation period was achieved in high-strength bainitic steel, with the transformation completion time within 70 minutes. This greatly improves the low-temperature isothermal transformation rate of bainite, significantly shortens the preparation time, and improves production efficiency.

[0019] The beneficial effects of this invention are:

[0020] (1) The method described in this invention uses a special alloy chemical composition design to ensure that no other phase transformations occur during the warm rolling process, and that the austenitic single phase is still maintained. In addition, the addition of Nb element can induce the precipitation of Nb-containing carbides during the rolling process. On the one hand, Nb-containing carbides can refine the austenitic grains and provide more nucleation sites for bainitic phase transformation. On the other hand, niobium-containing carbides with excellent thermal stability can suppress the recovery of bainitic ferrite laths, ensuring that nano-bainitic steel still has high strength at high temperatures (450-550℃).

[0021] (2) The method of the present invention can promote the austenite to undergo multiple recovery recrystallizations after homogenization treatment of the steel billet and hot rolling at a slightly lower temperature, which can fully refine the grains and eliminate casting defects. The hot rolling process stores a large amount of deformation distortion energy, increases the nucleation rate and growth rate of bainite, and can significantly shorten the bainite phase transformation time and improve production efficiency.

[0022] (3) Through controlled rolling and cooling process and multi-step isothermal process, a large amount of nano bainite structure is obtained in high-strength steel, and a large number of submicron-sized Nb-containing carbides are dispersed. By utilizing fine grain strengthening, precipitation strengthening and solid solution strengthening mechanisms, the comprehensive mechanical properties of steel are improved.

[0023] (4) The method described in this invention is feasible and has high production efficiency; this invention abandons the conventional low temperature isothermal quenching process, which is both energy-consuming and time-consuming, and shortens the bainitic phase transformation time to less than 70 minutes. Attached Figure Description

[0024] Figure 1 This is a process flow diagram of the present invention;

[0025] Figure 2 The metallographic structure diagram is shown in Example 1.

[0026] Figure 3 This is a SEM image of Example 1;

[0027] Figure 4 This is a high-magnification SEM image of Example 1. Detailed Implementation

[0028] The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments, but the scope of protection of the present invention is not limited to the content described.

[0029] Example 1

[0030] A deformation-induced rapid phase transformation bainitic high-strength steel and its preparation method are disclosed. The high-strength steel has the following chemical composition by mass percentage: C: 0.35%, Si: 1.18%, Mn: 1.35%, Cr: 1.08%, Ni: 1.14%, Al: 1.05%, Nb: 0.12%, with the remainder being Fe and unavoidable impurities. The preparation process includes:

[0031] (1) Melting and casting: according to the design requirements of steel composition, melt and cast steel ingots in a vacuum induction furnace.

[0032] (2) Hot rolling: The steel ingot is heated to 1280℃ at a heating rate of 80℃ / h and homogenized for 2h. Then it is cooled to 1160℃ at a cooling rate of 10℃ / s and hot rolling is carried out. The initial rolling temperature is 1125℃, the final rolling temperature is 880℃, and the total deformation of hot rolling is 70%.

[0033] (3) Warm rolling: The hot-rolled billet is cooled to 820°C at a cooling rate of 10°C / s and then warm-rolled. The initial rolling temperature is 780°C and the final rolling temperature is 680°C. The total deformation of the hot rolling is 40%.

[0034] (4) Isothermal: The rolled steel plate obtained by warm rolling is immediately placed in a 400℃ salt bath furnace and kept at that temperature for 20 minutes;

[0035] (5) Introduce a small amount of martensite: Immediately place the steel plate obtained in step (4) at 12°C (288°C) below Ms point for 8 seconds.

[0036] (6) Isothermal treatment again: The steel plate obtained in step (5) is immediately placed in a salt bath furnace at 400°C and kept at that temperature for 30 minutes. After the holding time is completed, it is air-cooled to room temperature.

[0037] Example 2

[0038] A deformation-induced rapid phase transformation bainitic high-strength steel and its preparation method are disclosed. The high-strength steel has the following chemical composition by mass percentage: C: 0.32%, Si: 1.09%, Mn: 1.30%, Cr: 0.85%, Ni: 1.02%, Al: 0.92%, Nb: 0.08%, with the remainder being Fe and unavoidable impurities. The preparation process includes:

[0039] (1) Melting and casting: according to the steel composition design requirements, melt and cast into steel ingots in a vacuum induction furnace;

[0040] (2) Hot rolling: The steel ingot is heated to 1200℃ at a heating rate of 65℃ / h and homogenized for 2h. Then it is cooled to 1160℃ at a cooling rate of 10℃ / s and hot rolling is carried out. The initial rolling temperature is 1100℃, the final rolling temperature is 850℃, and the total deformation of hot rolling is 60%.

[0041] (3) Warm rolling: The hot-rolled billet is cooled to 820°C at a cooling rate of 10°C / s and then warm-rolled. The initial rolling temperature is 750°C and the final rolling temperature is 660°C. The total deformation of the hot rolling is 30%.

[0042] (4) Isothermal: The rolled steel plate obtained by warm rolling is immediately placed in a 370℃ salt bath furnace and kept at that temperature for 30 minutes;

[0043] (5) Introduce a small amount of martensite: Immediately place the steel plate obtained in step (4) at 15°C (275°C) below Ms point and keep it for 5 seconds.

[0044] (6) Isothermal treatment again: The steel plate obtained in step (5) is immediately placed in a salt bath furnace at 370°C and kept at that temperature for 40 minutes. After the holding time is completed, it is air-cooled to room temperature.

[0045] Example 3

[0046] A deformation-induced rapid phase transformation bainitic high-strength steel and its preparation method are disclosed. The high-strength steel has the following chemical composition by mass percentage: C: 0.38%, Si: 1.20%, Mn: 1.40%, Cr: 1.15%, Ni: 1.35%, Al: 1.21%, Nb: 0.18%, with the remainder being Fe and unavoidable impurities. The preparation process includes:

[0047] (1) Melting and casting: according to the design requirements of steel composition, melt and cast steel ingots in a vacuum induction furnace.

[0048] (2) Hot rolling: The steel ingot is heated to 1160℃ at a heating rate of 75℃ / h and homogenized for 2h. Then it is cooled to 1150℃ at a cooling rate of 10℃ / s and hot rolling is carried out. The initial rolling temperature is 1150℃, the final rolling temperature is 880℃, and the total deformation of hot rolling is 80%.

[0049] (3) Warm rolling: The hot-rolled billet is cooled to 820°C at a cooling rate of 10°C / s and then warm-rolled. The initial rolling temperature is 800°C and the final rolling temperature is 690°C. The total deformation of the hot rolling is 40%.

[0050] (4) Isothermal: The rolled steel plate obtained by warm rolling is immediately placed in a 430℃ salt bath furnace and kept warm for 10 minutes;

[0051] (5) Introduce a small amount of martensite: Immediately place the steel plate obtained in step (4) at 10°C (295°C) below Ms point for 10 seconds.

[0052] (6) Isothermal treatment again: The steel plate obtained in step (5) is immediately placed in a salt bath furnace at 430°C and kept at that temperature for 20 minutes. After the holding time is completed, it is air-cooled to room temperature.

[0053] Comparative Example 1

[0054] Comparative Example 1 of this invention has the same chemical composition as Example 1 except that it does not contain niobium. The chemical composition by mass percentage is as follows: C: 0.35%, Si: 1.18%, Mn: 1.35%, Cr: 1.08%, Ni: 1.14%, Al: 1.05%, with the remainder being Fe and unavoidable impurities.

[0055] The preparation process is exactly the same as in Example 1.

[0056] Comparative Example 2

[0057] Comparative Example 2 of this invention has the same chemical composition as Example 1, and the preparation process includes:

[0058] (1) Melting and casting: according to the design requirements of steel composition, melt and cast steel ingots in a vacuum induction furnace.

[0059] (2) Hot rolling: The steel ingot is heated to 1280℃ at a heating rate of 80℃ / h and homogenized for 2h. Then it is cooled to 1160℃ at a cooling rate of 10℃ / s and hot rolling is carried out. The initial rolling temperature is 1125℃, the final rolling temperature is 880℃, and the total deformation of hot rolling is 70%.

[0060] (3) Isothermal: The rolled steel plate obtained by warm rolling is immediately placed in a 400℃ salt bath furnace and kept warm for 20 minutes.

[0061] (4) Introduce a small amount of martensite: Immediately place the steel plate obtained in step (3) below Ms point at 12°C (288°C) for 8 seconds.

[0062] (5) Isothermal treatment again: The steel plate obtained in step (4) is immediately placed in a salt bath furnace at 400°C and kept at that temperature for 30 minutes. After the holding time is over, it is air-cooled to room temperature.

[0063] Comparative Example 3

[0064] The chemical composition of this embodiment is exactly the same as that of Example 1, and the preparation process includes:

[0065] (1) Melting and casting: according to the design requirements of steel composition, melt and cast steel ingots in a vacuum induction furnace.

[0066] (2) Hot rolling: The steel ingot is heated to 1280℃ at a heating rate of 80℃ / h and homogenized for 2h. Then it is cooled to 1160℃ at a cooling rate of 10℃ / s and hot rolling is carried out. The initial rolling temperature is 1125℃, the final rolling temperature is 880℃, and the total deformation of hot rolling is 70%.

[0067] (3) Warm rolling: The hot-rolled billet is cooled to 820°C at a cooling rate of 10°C / s and then warm-rolled. The initial rolling temperature is 780°C and the final rolling temperature is 680°C. The total deformation of the hot rolling is 40%.

[0068] (4) Isothermal: The rolled steel plate obtained by warm rolling is immediately placed in a 400℃ salt bath furnace and kept warm for 50 minutes. After the warming is completed, it is air-cooled to room temperature.

[0069] The bainitic high-strength steels prepared in Examples 1-3 of this invention have microstructures consisting of fine and uniform bainite and a small amount of martensite, with a large number of submicron-sized niobium-containing carbides dispersed in the matrix. This ensures that the bainitic high-strength steel possesses high strength while also guaranteeing sufficient impact toughness. Furthermore, the presence of niobium-containing carbides can inhibit the recovery of bainitic ferrite laths at higher temperatures, ensuring that the bainitic high-strength steel maintains high strength even at high temperatures. A detailed explanation is provided using the bainitic high-strength steel prepared in Example 1 as an example:

[0070] Figure 2 The image shows the metallographic structure of Example 1. As can be seen from the image, the structure consists of fine bainite, retained austenite, and a small amount of martensite. The fine structure suggests that fine grain strengthening has made a significant contribution to the improvement of strength and elongation.

[0071] Figure 3 The image shows the microstructure of Example 1 under a scanning electron microscope. It can be seen from the image that the thickness of the bainite laths is on the nanoscale, and the retained austenite is mostly in the form of a thin film. The presence of a sufficient number of thin film-like retained austenite can ensure that the workpiece undergoes deformation-induced plasticity (TRIP) during deformation, thereby improving its strength and plasticity.

[0072] Figure 4 The image shows a higher magnification scanning electron microscope (SEM) image of the microstructure of Example 1. It can be seen from the image that there are many diffusely distributed submicron-sized precipitates in the matrix. The precipitates are mainly circular in shape. The submicron-sized precipitates can improve the strength of high-strength steel by hindering dislocation movement.

[0073] Table 1. Microstructure of Examples 1-3 and Comparative Examples 1-2

[0074] Sample number Microorganism Implementation Case 1 94% bainite + 6% martensite + a large amount of dispersed carbides Implementation Case 2 93% bainite + 7% martensite + a large amount of dispersed carbides Implementation Case 3 90% bainite + 10% martensite + a large amount of dispersed carbides Comparison Case 1 Bainite 94% + Martensite 6% Comparison Case 2 94% bainite + 6% martensite + small amount of carbides Comparison Case 3 68% bainite + 32% martensite + a large amount of dispersed carbides

[0075] Table 1 shows the microstructure results of Examples 1-3 and Comparative Examples 1-3. As can be seen from Table 1, the microstructure of Examples 1-3 consists of bainite + a small amount of martensite + a large amount of dispersed carbides, with the volume fraction of bainite exceeding 90%. No carbides were found in the microstructure of Comparative Example 1 because Nb was not added to its chemical composition, preventing deformation-induced precipitation of niobium-containing carbides during rolling. The chemical composition of Comparative Example 2 was identical to that of Example 1, but its microstructure contained only a very small amount of carbides. This was because it was not subjected to warm rolling treatment, preventing deformation-induced precipitation of carbides. The volume fraction of bainite in the microstructure of Comparative Example 3 was only 68%. Although its isothermal time was the same as that of Example 1, it was not quenched in a salt bath furnace at a temperature below Ms, resulting in incomplete bainitic transformation. During subsequent air cooling, the austenite grains that did not undergo bainitic transformation underwent martensitic transformation, generating a large amount of martensite. Therefore, the addition of Nb is beneficial to the formation of niobium-containing carbides induced by rolling deformation. During the bainitic isothermal process, quenching the steel suddenly into a salt bath furnace at a temperature slightly below Ms point and then continuing the isothermal process within the bainitic phase transformation temperature range can significantly shorten the transformation time of nanobainite and improve the production efficiency of nanobainite.

[0076] Table 2 shows the mechanical properties of Examples 1-3 and Comparative Examples 1-3. As can be seen from Table 2, the yield strength, tensile strength, and elongation of Comparative Examples 1-2 are all lower than those of Examples 1-3. This is because Comparative Example 1 does not contain Nb, thus failing to generate a large amount of dispersed niobium-containing carbides. Although Comparative Example 2 contains Nb in its chemical composition, it was not subjected to warm rolling to induce deformation and precipitate niobium-containing carbides, which could not hinder dislocation movement during stretching. The yield strength and tensile strength of Comparative Example 3 are both higher than those of Examples 1-3, but the elongation is significantly lower. This is because the bainitic phase transformation time in Example 3 was insufficient, resulting in 32% martensite, leading to increased strength but decreased toughness.

[0077] Table 2 Mechanical properties of Examples 1-3 and Comparative Examples 1-2

[0078]

[0079] This invention aims to obtain a high-strength bainitic rolled steel. It employs a special composition design combined with controlled rolling and cooling processes and a multi-step isothermal process to significantly reduce the bainite transformation time and improve the production efficiency of nano-bainitic steel. Furthermore, by adding a certain amount of Nb element during composition design, deformation-induced precipitation of dispersed submicron-sized particles is achieved in the bainitic high-strength steel, enhancing precipitation strengthening and further increasing the strength of the bainitic high-strength steel.

[0080] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any changes or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for preparing a deformation-induced rapid phase transformation bainitic high-strength steel, characterized in that: By controlling elements and combining controlled rolling and cooling with a multi-step isothermal process, deformation-induced precipitation is used to induce uniformly distributed submicron-sized carbides to precipitate in the steel during rolling, thereby accelerating the phase transformation rate of the steel during the bainitic isothermal process. The specific steps include: (1) Melting and casting: according to the design requirements of the steel composition, the steel is melted and cast into steel ingots in a vacuum induction furnace; (2) Hot rolling to refine grains: The steel ingot is heated to 1160℃~1280℃ at a heating rate of 65-80℃ / h for homogenization treatment for 2h, and then cooled to 1150℃~1100℃ for hot rolling. (3) Warm rolling deformation-induced precipitation: The hot-rolled billet obtained in step S2 is cooled to 750℃~800℃ and then warm rolled; (4) Bainite phase transformation isothermal: The warm rolled billet obtained in step S3 is salt bath quenched to 400±30℃ and held for 10 to 30 minutes. (5) Introducing a small amount of martensite: The steel billet obtained in step S4 is salt bath quenched and rapidly cooled to 10-15°C below Ms point and held for 5-10 seconds to obtain steel containing a small amount of martensite. (6) Isothermal transformation of bainitic phase again: The steel obtained in step S5 is placed in a salt bath furnace at 400±30℃ and isothermal for 20 to 40 minutes. After isothermal transformation, it is air-cooled to room temperature. The bainitic high-strength steel comprises the following elements by weight percentage: C: 0.30–0.40%, Si: 1.05–1.25%, Mn: 1.24–1.45%, Cr: 0.80–1.20%, Ni: 0.95–1.46%, Al: 0.85–1.25%, Nb: 0.08–0.18%, with the remainder being Fe and unavoidable impurities. The bainitic high-strength steel is a medium-carbon alloy steel.

2. The method for preparing deformation-induced precipitation rapid phase transformation bainitic high-strength steel according to claim 1, characterized in that: In step (2), the cooling rate is 10℃ / s, the initial rolling temperature is 1150℃~1100℃, 3~4 rolling passes are performed, the total hot rolling deformation is 60~80%, and the final rolling temperature is not lower than 850℃.

3. The method for preparing bainitic high-strength steel with deformation-induced precipitation and rapid phase transformation according to claim 2, characterized in that: In step (3), the cooling rate is 10℃ / s, the initial rolling temperature is 750℃~800℃, the total deformation is 30~50% after 2~3 rolling processes, and the final rolling temperature is not lower than 660℃.

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