Method for greatly refining grains of thermal deformation steel and steel prepared through method

Abstract

In order to solve the problems existing in the prior art, common hot rolled steel, carbon manganese steel and low-carbon microalloyed steel are creatively subjected to Al alloying, the difference of free energy of transformation from austenite to ferrite is reduced through addition of the Al element, the free energy is about 140 mol / J, and the content of the Al element in the hot rolled steel is greatly reduced. During thermal deformation, austenite is subjected to phase transformation to induce ferrite phase transformation, so that the difficulty is reduced, and the possibility of deformation-induced ferrite phase transformation is improved. Besides, a certain content of ferrite delta ferrite is obtained within the thermal deformation temperature range (1200-800 DEG C) by adding the Al element, the ferrite delta ferrite is expected to be recrystallized during thermal deformation, fine crystal grains are obtained, and refining of the whole structure is achieved.

Description

Technical Field

[0001] This invention pertains to metal preparation technology and relates to a method for preparing steel, specifically a method for significantly refining the grain size of hot-deformed steel and the steel prepared therefrom. Background Technology

[0002] Grain refinement can simultaneously improve the strength and toughness of steel, avoiding the addition of precious elements such as Ni and Mo, thus saving resources and reducing costs. For hot-deformed steel, the principle of grain refinement is to refine austenite grains through austenite recrystallization, and then obtain refined ferrite grains by combining deformation of the non-recrystallized austenite region and controlling austenite phase transformation. However, due to the limited austenite refinement capacity and the need for deformation temperatures above 800℃ to reduce deformation resistance, the grain refinement effect of conventional hot-deformed steel is limited. For example, for steel thicker than 20 mm, the ferrite grain size is generally above 15 μm, resulting in insufficient strength and toughness. This is a key common problem faced in grain refinement of thick-gauge hot-deformed steel.

[0003] Existing methods for significantly refining grain size include deformation-induced ferrite transformation (DIFT), ferrite recrystallization, and cold rolling recrystallization. However, none of these methods can be directly applied to the production of thick-gauge hot-deformed steel. The reasons are as follows: (1) The temperature of deformation-induced ferrite transformation is low, generally around 750°C, and the deformation resistance is high, so it cannot be applied on the hot-deformation production line; (2) The maximum temperature of ferrite recrystallization is also below 800°C, so it cannot be directly applied; (3) The cold rolling recrystallization method requires more than 50% cold rolling, which is difficult to achieve for thick-gauge steel.

[0004] Therefore, it is necessary to introduce existing methods for significantly refining grains into the production of thick-gauge hot-deformed steel in order to achieve green and low-cost production of thick-gauge hot-rolled steel. Summary of the Invention

[0005] To address the problems of existing technologies, this invention innovatively alloys commonly used hot-rolled steel grades, carbon-manganese steel, and low-carbon microalloyed steel with Al. The addition of Al reduces the free energy difference for the transformation from austenite to ferrite to approximately 140 mol / J. This simplifies the process of austenite inducing ferrite transformation during hot deformation, increasing the likelihood of deformation-induced ferrite transformation. Furthermore, by adding Al within the hot deformation temperature range (1200~800℃), a certain content of δ-ferrite is obtained. This ferrite is expected to recrystallize during hot deformation, resulting in finer grains and refining the overall microstructure.

[0006] The present invention adopts the following technical solution.

[0007] A method for significantly refining the grain size of hot-deformed steel includes the following steps: alloying the steel with Al to achieve significant grain size refinement; the mass fraction of Al added is 1~2%.

[0008] A method for preparing hot-deformed steel includes the following steps: alloying the steel with Al to achieve a significant refinement of the grain size of the hot-deformed steel; the mass fraction of Al added is 1~2%.

[0009] Preferably, the added Al mass fraction is 1.2% to 1.8%, for example, the added Al mass fraction is 1.3%, 1.4%, 1.5%, 1.6%, 1.7% or any data within the range.

[0010] In this invention, the mass fraction of Al added is calculated based on the total mass of all raw materials of steel being 100%.

[0011] The present invention provides a method for significantly refining the grain size of hot-deformed steel, comprising the following steps: adding Al to steel raw materials, then smelting to prepare steel ingots, then hot forging to prepare steel billets, then hot homogenizing, and finally hot-deforming, thereby achieving significant grain size refinement of the hot-deformed steel and obtaining hot-deformed steel.

[0012] In this invention, the steel includes carbon manganese steel and low-carbon microalloyed steel.

[0013] In this invention, the temperature for thermal homogenization is 1100~1300℃, and the time is 1~3 hours.

[0014] In this invention, the temperature during hot deformation is 800℃~1100℃, and the deformation amount is 30%~50%.

[0015] This invention reduces the difference in free energy between austenite and ferrite by Al alloying in carbon manganese steel and low-carbon microalloyed steel, thereby reducing the stability of austenite. This is beneficial for deformation-induced ferrite phase transformation during high-temperature deformation, thus contributing to a significant refinement of the microstructure of high-temperature hot-deformed steel. This is one of the inventive aspects of this invention and has not been reported in the prior art.

[0016] This invention discloses the intragranular nucleation phenomenon of ferrite in deformation-induced phase transformation, which is very different from conventional deformation-induced ferrite phase transformation.

[0017] In the technical solution of the present invention, after Al alloying, the initial ferrite also undergoes a large degree of recrystallization during high-temperature deformation, which helps to refine the overall structure. Attached Figure Description

[0018] Figure 1 Equilibrium phase ratio diagram of Al-containing carbon manganese steel at various temperatures.

[0019] Figure 2 The effect of Al addition on the difference in free energy of austenite-ferrite transformation in carbon manganese steel, as calculated by thermodynamics.

[0020] Figure 3 Microstructure characteristics of Al-containing carbon-manganese steel before and after rolling at 1100℃: (a) Quenched microstructure before deformation; (b) Quenched microstructure after rolling at 5 r / min; (c) Quenched microstructure after rolling at 15 r / min; (d) Quenched microstructure after rolling at 25 r / min.

[0021] Figure 4 Microstructure characteristics of Al-containing carbon-manganese steel before and after rolling at 800℃: (a) Quenched microstructure before deformation; (b) Quenched microstructure after rolling at 5 r / min; (c) Quenched microstructure after rolling at 15 r / min; (d) Quenched microstructure after rolling at 25 r / min.

[0022] Figure 5 Elemental distribution diagrams of the two phases in the quenched microstructure of Al-containing carbon-manganese steel before hot deformation: (a) 1200℃; (b) 1100℃; (c) 800℃.

[0023] Figure 6 Thermal simulation experiment of Al-containing low-carbon microalloyed steel: (a) Experimental scheme; (b) Sampling method for microstructure observation; Figure a T For temperature, For strain rate, ε This represents the amount of deformation.

[0024] Figure 7 Experimental results of Al-containing low-carbon microalloyed steel: (a) Microstructural characteristics of different deformation amounts at 1000℃; (b) Effect of deformation temperature. Detailed Implementation

[0025] In this invention, for Al-containing carbon manganese steel, at higher temperature deformation, such as 1100℃, a lower strain rate helps to increase the proportion of deformation-induced ferrite phase transformation, which is beneficial to the overall refinement of the microstructure and the improvement of strength and toughness; at lower temperature deformation, such as 800℃, a higher strain rate helps to induce deformation-induced ferrite phase transformation and refine the ferrite grains generated.

[0026] The following experiments illustrate the technological advancements of this invention. The specific raw materials are existing products, and the specific preparation operations and performance testing are conventional techniques. The inventiveness of this invention lies in achieving significant grain refinement of hot-deformed steel through Al alloying, while the steelmaking process is a conventional technique. Example 1

[0027] This invention uses a commonly used carbon-manganese steel with a composition of 0.06% C, 1.5% Mn, and 0.17% Si, with 1.5% Al added by mass, and the remainder being Fe. The equilibrium ratio calculated using JMatPro is shown in the attached figure. Figure 1 As shown in the attached figure, the austenite content reaches a maximum of 77% at 1150℃, while the δ-ferrite content is a minimum of 23%. Thermo-Calc calculations show that the addition of Al reduces the difference in free energy between the two phases during the austenite-ferrite transformation, as illustrated in the attached figure. Figure 2 As shown.

[0028] A 50 kg steel ingot of this composition was prepared using vacuum smelting. After smelting, the ingot was heated to 1200℃ and held for 2 hours before multi-directional forging to produce a steel billet with a thickness of 75 mm and a width of 90 mm. Small steel plates with a thickness of 15 mm, a width of 45 mm, and a length of 100 mm were cut from the billet for hot deformation experiments. Before the deformation experiments, the small steel plates were heated to 1200℃ and held for 2 hours to homogenize the microstructure. The hot deformation experiments were then conducted. The steel plates were reheated to 1200℃ and held for 2 hours, then air-cooled to different temperatures (1100℃ or 800℃) before single-pass rolling experiments. The deformation amounts were 30%, 40%, and 50%, and the roll rotation speeds were 5, 15, and 25 r / min, corresponding to average strain rates of 1.67, 5.01, and 8.36 s⁻¹, respectively. -1 The steel plate was quenched to room temperature immediately before and after rolling to preserve its high-temperature microstructure before and after deformation.

[0029] For Al-containing carbon-manganese steel, when air-cooled from 1200℃ to 1100℃, the volume fraction of austenite in the microstructure is 75.7%, and the volume fraction of δ-ferrite is 24.3%, as shown in the attached figure. Figure 3 As shown in a. (Attached) Figure 3 The martensite in a is transformed from high-temperature austenite during quenching; therefore, the volume fractions of the two are the same.

[0030] At 1100℃, it took 1.67 s. -1 During deformation at the average strain rate, 75% of the austenite in the microstructure underwent deformation-induced ferrite phase transformation, converting into fine ferrite grains with an average ferrite grain size of 7.13 μm. The δ-ferrite grain size was refined from the initial 21.2 μm to 13.4 μm, as shown in the attached figure. Figure 3 As shown in b.

[0031] At 1100℃, it took 5.01 s -1 At the average strain rate of deformation, 65.2% of the austenite underwent deformation-induced ferrite phase transformation, resulting in ferrite grains with a size of 6.44 μm. However, the δ-ferrite did not undergo refinement, as shown in the attached figure. Figure 3As shown in c. When the strain rate is further increased to 8.36 s... -1 59.5% of the austenite underwent deformation-induced ferrite phase transformation, resulting in ferrite grains with a size of 5.22 μm. The δ-ferrite also did not undergo refinement, as shown in the attached figure. Figure 3 As shown in d, after hot deformation at 1100℃, the grain size was significantly refined.

[0032] When air-cooled to 800℃ for deformation, the volume fraction of austenite decreases to 22%, and the microstructure is dominated by δ-ferrite and ferrite transformed from austenite, collectively referred to as primary ferrite. The ferrite content is 1.67, 5.01, and 8.36 s⁻¹. -1 During deformation at strain rates of [specific values], the proportions of austenite undergoing deformation-induced ferrite phase transformation were 72.7%, 36.4%, and 31.8%, respectively. The ferrite grain sizes of the deformation-induced ferrite phase transformation were 2.36, 4.06, and 4.00 μm, respectively, and the grain size of δ-ferrite decreased from the initial 49.2 μm to approximately 11 μm, achieving a significant refinement. These research results are attached. Figure 4 As shown.

[0033] In addition, in the appendix Figure 3 and attached Figure 4 It was found that ferrite grains undergoing deformation-induced ferrite phase transformation primarily nucleate within the original austenite grains, especially the ferrite formed at 800℃. This differs from the conventional phenomenon of deformation-induced ferrite phase transformation, where ferrite preferentially nucleates at austenite grain boundaries due to the accumulation of significant deformation at these boundaries during deformation. Electron probe microanalysis was used to analyze the elemental distribution characteristics near the interface between ferrite and austenite phases before deformation, as shown in the attached figure. Figure 5 As shown in the figure, the results indicate that C and Al elements are enriched at the initial ferrite and austenite interface. The enrichment of C at the grain boundaries enhances austenite stability and suppresses deformation-induced ferrite phase transformation. The enrichment of Al at the two-phase interface hinders ferrite grain nucleation and growth. Therefore, although deformation still concentrates at the two-phase interface in this invention, the enrichment of C and Al elements suppresses deformation-induced ferrite phase transformation and ferrite nucleation at the two-phase interface, promoting nucleation primarily within the austenite grains, resulting in the unique intragranular nucleation phenomenon observed in this invention. Example 2

[0034] Since low-carbon microalloyed steel is also a commonly used steel grade, this invention also conducted rolling experiments with Al added to low-carbon microalloyed steel, verifying the effectiveness of Al alloying. Not only did deformation-induced ferrite phase transformation occur in austenite during high-temperature deformation, but δ-ferrite also underwent a certain degree of recrystallization, achieving overall microstructure refinement. Low-carbon microalloyed steel is also a commonly used hot-rolled steel grade. This invention further introduces the concept of Al alloying into low-carbon microalloyed steel, with a composition of 0.056% C, 0.17% Si, 1.54% Mn, 0.022% Nb, 0.019% Ti, and 1.5% Al, with the remainder being Fe. Following the invention's description of carbon-manganese steel, a 50 kg steel billet was prepared and hot-forged. Cylindrical specimens with a diameter of 8 mm and a length of 15 mm were cut along the length of the forged billet for experimental research using a thermal simulation testing machine. The effects of deformation amount and deformation temperature were studied; the experimental scheme is attached. Figure 6 When studying the effect of deformation, the deformation temperature was fixed at 1000℃ and the strain rate was fixed at 1 s. -1 The deformation amounts were 26%, 39%, and 55%, respectively; when studying the effect of deformation temperature, the strain rate and deformation amount were fixed at 1 s. -1 The deformation temperatures were 1100, 1000, 900, and 800℃, respectively, with a yield of 55% and a modulus of 15%. The experimental results are attached. Figure 7 .

[0035] At 1000℃ without deformation, the initial ferrite content in the microstructure was 48%. Applying a 26% deformation, the ferrite content increased to 50%, and deformation-induced ferrite phase transformation was observed in 4% of the austenite. When the deformation increased to 39% and 55%, the proportion of austenite undergoing deformation-induced ferrite phase transformation increased to 13% and 41%, respectively. The initial ferrite in the microstructure also recrystallized, achieving overall refinement. The overall grain sizes of the microstructure under different deformation amounts were 14.4, 11.0, and 11 μm, respectively. Therefore, the experimental results on the effect of deformation amount indicate that to induce deformation-induced ferrite phase transformation in austenite during high-temperature deformation, the deformation amount needs to exceed 39%, ideally reaching 55%.

Claims

1. A method for significantly refining the grain size of hot-deformed steel, characterized in that, The process includes the following steps: Al alloying is used in steel to achieve a significant reduction in grain size of hot-deformed steel; the mass fraction of Al added is 1-2%.

2. The method for significantly refining the grain size of hot-deformed steel according to claim 1, characterized in that, The added Al mass fraction is 1.2~1.8%.

3. The method for significantly refining the grain size of hot-deformed steel according to claim 1, characterized in that, Al is added to the steel raw material, then steel ingots are smelted and prepared, then steel billets are hot-forged, then hot-homogenized, and finally hot-deformed, so as to achieve a significant refinement of the grains of the hot-deformed steel and obtain hot-deformed steel.

4. The method for significantly refining the grain size of hot-deformed steel according to claim 3, characterized in that, The temperature for heat homogenization is 1100~1300℃, and the time is 1~3 hours; during heat deformation, the temperature is 800℃~1100℃, and the deformation is 30%~50%.

5. A method for preparing hot-deformed steel, characterized in that, The process includes the following steps: Al alloying is used in steel to achieve a significant reduction in grain size of hot-deformed steel; the mass fraction of Al added is 1-2%.

6. The method for preparing hot-deformed steel according to claim 5, characterized in that, Steel includes carbon manganese steel and low-carbon microalloyed steel.

7. Hot-deformed steel prepared by the method for preparing hot-deformed steel according to claim 5.

8. The application of the method for significantly refining the grain size of hot-deformed steel as described in claim 1 in improving the performance of steel.

9. The application of the hot-deformed steel of claim 7 in the preparation of high-performance steel.

10. The application according to claim 9, characterized in that, High-performance steels include hot-rolled steels.

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