A method for manufacturing high-strength, high-toughness, and plastic austenitic low-density steel

Abstract

A method for manufacturing high-strength, high-toughness and plastic austenitic low-density steel, wherein the initial rolling temperature of the hot rolling finishing stage is controlled to be ≤1000°C and the final rolling temperature range is 800°C to 870°C during the steps of smelting, casting, hot rolling and cooling, so that the microstructure of the steel plate material after final rolling has the organizational characteristics of fine grain size, high-density dislocation and deformation nano twins, thereby achieving fine grain strengthening, deformation strengthening and dislocation strengthening of the austenitic low-density steel and ensuring plasticity and toughness.

Description

Technical Field

[0001] The invention relates to the technical field of low-density steel manufacturing, in particular to a method for manufacturing high-strength, high-toughness and plastic austenite low-density steel. Background Art

[0002] As people's demand for equipment efficiency continues to increase, low-density materials are attracting more and more attention. Compared with other lightweight materials, low-density steel has the advantages of low cost and high strength. Austenitic FeMnAlC steel has the highest addition of lightweight elements such as Al and C, and has the advantages of minimum density and low magnetism compared to other types of low-density steel. However, the atomic arrangement of the austenite phase determines that the yield strength of austenitic low-density steel is relatively low, usually less than 400MPa. Increasing the strength through methods such as aging is likely to cause loss of plasticity and toughness. How to improve the strength level while ensuring a high level of toughness and plasticity is an important scientific issue facing austenitic low-density steel. At the same time, low-density steel used as structural parts should have good toughness and plasticity. Therefore, it is of great practical significance to study the method of obtaining austenitic low-density steel with high strength, high toughness and plasticity.

[0003] Many experts have conducted a lot of creative research and development work to improve the strength level of austenitic low-density steel and obtain high-strength, high-plasticity low-density steel. In summary, there are cold rolling deformation, precipitation strengthening using nano-carbides, microalloying, controlled hot rolling and other aspects. For example, patent document CN110205459B uses cold rolling and annealing to obtain high-strength, high-plasticity austenitic low-density steel through ultra-fine organization; more experts use the method of adding alloying elements to improve the strength level of low-density steel. Patent document CN114561517A uses Cr, Cu and N for precipitation strengthening. Patent document CN114480988A adds Nb and Cr elements to achieve grain refinement and improve corrosion resistance. Patent document CN114395732A proposes precipitation strengthening and grain refinement using microalloying elements such as Ti, Nb, Mo, V and Zr. Patent document CN104711494B steel proposes low-density, high-plasticity NiAl reinforced ultra-high strength steel and preparation method. Patent document CN105838995A and others use nano-sized carbides precipitated during aging treatment to achieve strengthening. Patent document CN111663085B proposes a hot-rolled austenitic low-density steel with ultra-high strength and plasticity and a production method, which produces a yield strength of 1307-1398MPa, a tensile strength of 1653-1721MPa, and an elongation of 49-56%, which can be used to make structural parts with more complex molding requirements. The process flow is smelting and continuous casting into billets, heating the continuous casting, rough rolling, finishing rolling, coiling, and natural cooling to room temperature, but the impact toughness of the steel is not involved.

[0004] In summary, austenitic low-density steel with high strength, high toughness and high plasticity has important application and research value. However, the strength level of austenitic low-density steel is currently low. Currently, alloying or cold rolling deformation are mainly used to improve the strength and plasticity of the material, but there is no effective method to control high toughness. For austenitic low-density steel used as structural parts, it is of great significance to obtain high strength, high toughness and high plasticity. Summary of the invention

[0005] The present invention aims at the development demand of high strength, high toughness and high plasticity of austenitic low-density steel and the current situation of lack of effective means for toughness control, and invents a method for manufacturing high-strength, high-toughness and plasticity austenitic low-density steel.

[0006] The technical solution of the present invention is as follows:

[0007] A method for manufacturing austenite type low-density steel with high strength, high toughness and plasticity, characterized in that the method comprises the following manufacturing steps for the chemical composition of FeMnAlC alloy low-density steel with austenite as the matrix:

[0008] Step 1, smelting;

[0009] Step 2, casting;

[0010] Step 3, hot rolling, the hot rolling includes rough rolling and finishing rolling stages, the rough rolling stage uses the characteristics of easy deformation at high temperature to complete the slab rolling by large deformation to achieve the expected thickness. The initial rolling temperature of the finishing rolling stage is ≤1000℃, the final rolling temperature range is 800℃~870℃, the cumulative deformation is ≥20%, and the microstructure of the steel plate material after final rolling has the organizational characteristics of fine grain size, high density dislocation and deformation nano twins, so as to achieve fine grain strengthening, deformation strengthening and dislocation strengthening of austenitic low-density steel, and ensure plasticity and toughness;

[0011] Step 4, cool.

[0012] The mechanical properties of the austenitic low-density steel are as follows: yield strength ≥ 650 MPa, elongation ≥ 30%, and -40°C Charpy impact energy KV2 ≥ 80 J.

[0013] The step 4 includes water cooling.

[0014] The step 1 includes smelting in a converter, an electric furnace or an induction furnace, LF refining can be used according to the cleanliness requirements, and RH or VD degassing treatment can be used.

[0015] The step 2 includes continuous casting or mold casting.

[0016] The chemical composition of the FeMnAlC alloy low-density steel is as follows in wt%: C=0.50-1.20, Mn=20.0-35.0, Al=6.0-12, Nb=0-0.5, V=0-0.5, Ti=0-0.5, W=0-0.5, Mo=0-0.5, P≤0.015, S≤0.01, and the balance is Fe.

[0017] It also includes one or more of the following elements with limited wt% content: Si=0-3.0, Cr=0-5.0, Cu=0-2.0, B=0.0005-0.01, rare earth RE=0.001-0.10, Ca=0.005-0.050.

[0018] The final rolling temperature in step 3 is 800°C to 870°C. The microstructure of the final rolling at 820°C is as follows: the average grain size according to the EBSD statistical results is 10.5 μm, and the dislocation density according to X-ray measurement is 4.8×10 14 According to the characterization of transmission electron microscopy, it has deformation nanotwins.

[0019] The method may further include step 5, solutionizing.

[0020] The technical effects of the present invention are as follows: A method for manufacturing high-strength, high-toughness and plastic austenitic low-density steel according to the present invention controls the initial rolling temperature of the hot rolling finishing stage to ≤1000°C and the final rolling temperature range to 800°C to 870°C during the steps of smelting, casting, hot rolling and cooling, so that the microstructure of the steel plate material after final rolling has the organizational characteristics of fine grain size, high-density dislocations and deformation nano-twins, thereby achieving fine grain strengthening, deformation strengthening and dislocation strengthening of the austenitic low-density steel, and ensuring plasticity and toughness.

[0021] Compared with the existing technologies of improving the strength of austenitic low-density steel such as alloying, cold rolling, aging, etc., the present invention obtains high-strength, high-plasticity, and high-toughness austenitic low-density steel, and the difference from the technical means previously disclosed in the field is: 1. The present invention adopts the method of hot rolling + rapid cooling to achieve the improvement of the strength level; 2. The microstructure of the steel is based on austenite, and high-density dislocations and fine grains are obtained after controlled hot rolling + rapid cooling, and the organizational characteristics of fine deformation twins can be obtained under certain deformation conditions; 3. Due to the strengthening effect of the deformation organization, the addition amount of other alloy elements can be reduced when obtaining the same strength level; 4. The high-strength, high-plasticity, and high-toughness austenitic low-density steel of the present invention can achieve a yield strength of ≥650MPa, an elongation of ≥30%, and a Charpy impact energy KV2 (-40°C) ≥80J. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1The present invention is a transmission electron microscope photograph of high-density dislocations in low-density steel obtained by implementing a method for manufacturing high-strength, high-toughness and plastic austenitic low-density steel of the present invention.

[0023] Figure 2 The EBSD analysis result of the effective grain size of the low-density steel in the hot-rolled water-cooled state obtained by implementing the method for manufacturing a high-strength, high-toughness and high-plasticity austenite low-density steel of the present invention. EBSD is electron back-scattered diffraction.

[0024] Figure 3 The invention relates to a nano twin structure of a low-density steel in a hot-rolled water-cooled state obtained by implementing a method for manufacturing austenite low-density steel with high strength, high toughness and plasticity of the invention. DETAILED DESCRIPTION

[0025] Below is the attached figure ( Figure 1-Figure 3 ) and Examples illustrate the present invention.

[0026] Figure 1 The present invention is a transmission electron microscope photograph of high-density dislocations in low-density steel obtained by implementing a method for manufacturing high-strength, high-toughness and plastic austenitic low-density steel of the present invention. Figure 2 The EBSD analysis result of the effective grain size of the low-density steel in the hot-rolled water-cooled state obtained by implementing the method for manufacturing the high-strength, high-toughness and high-plasticity austenitic low-density steel of the present invention. Figure 3 The nano twin structure of the low-density steel in the hot-rolled water-cooled state is obtained by implementing the method for manufacturing a high-strength, high-toughness and high-plasticity austenite low-density steel of the present invention. Figures 1 to 3 As shown, a method for manufacturing high-strength, high-toughness and plastic austenitic low-density steel is characterized in that, for the chemical composition of FeMnAlC alloy-based low-density steel with austenite as the matrix, the method comprises the following manufacturing steps: step 1, smelting; step 2, casting; step 3, hot rolling, the hot rolling includes a finishing rolling stage, the initial rolling temperature of the finishing rolling stage is ≤1000℃, the final rolling temperature range is 800℃~870℃, the cumulative deformation amount is ≥20%, and the microstructure of the steel plate material after final rolling has the organizational characteristics of fine grain size, high-density dislocation and deformation nano-twins, so as to achieve fine grain strengthening, deformation strengthening and dislocation strengthening of austenitic low-density steel, and ensure plasticity and toughness; step 4, cooling.

[0027] The mechanical properties of the austenitic low-density steel are: yield strength ≥ 650MPa, elongation ≥ 30%, -40°C Charpy impact energy KV2 ≥ 80J. The step 4 includes water cooling. The step 1 includes smelting in a converter, an electric furnace or an induction furnace, LF refining, and RH or VD degassing. The step 2 includes continuous casting or die casting.

[0028] The FeMnAlC alloy low-density steel has the following chemical composition in wt%: C=0.50-1.20, Mn=20.0-35.0, Al=6.0-12, Nb=0-0.5, V=0-0.5, Ti=0-0.5, W=0-0.5, Mo=0-0.5, P≤0.015, S≤0.01, and the balance is Fe. It also includes one or more of the following elements with limited wt% content: Si=0-3.0, Cr=0-5.0, Cu=0-2.0, B=0.0005-0.01, rare earth RE=0.001-0.10, and Ca=0.005-0.050. The final rolling temperature in step 3 is 800°C to 870°C. The microstructure of the final rolling at 820°C is as follows: the average grain size according to the EBSD statistical results is 10.5 μm, and the dislocation density according to X-ray measurement is 4.8×10 14 According to the characterization of transmission electron microscopy, the present invention has deformation nano twins. The present invention also includes step 5, solid solution.

[0029] The present invention relates to a method for manufacturing a high-strength, high-toughness and plastic austenite type low-density steel. The present invention is applicable to low-density steel with an austenite matrix, and is applicable to a composition range of: C = 0.50-1.20, Mn = 20.0-35.0, Al = 6.0-12, Nb = 0-0.5, V = 0-0.5, Ti = 0-0.5, W = 0-0.5, Mo = 0-0.5, P ≤ 0.015, S ≤ 0.01, the balance is Fe, and also includes one or more of the following elements, in terms of wt% content: Si = 0-3.0, Cr = 0-5.0, Cu = 0-2.0, B = 0.0005-0.01, rare earth RE = 0.001-0.10, Ca = 0.005-0.050. Austenitic low-density steel billet, after rough rolling or forging, starts at a temperature of ≤1000℃ in the finishing rolling stage, with a cumulative deformation of ≥20%, final rolling in the non-recrystallization temperature zone, and water cooling after rolling. The microstructure characteristics of fine grain size, high-density dislocation and deformation nano twins are obtained, and the mechanical properties of yield strength ≥650MPa, elongation ≥30%, and Charpy impact energy KV2 (-40℃) ≥80J can be obtained.

[0030] A method for manufacturing high-strength, high-toughness and plastic austenitic low-density steel, characterized in that it is applicable to austenitic low-density steel, and is applicable to the composition range of: C = 0.50 ~ 1.20, Mn = 20.0 ~ 35.0, Al = 6.0 ~ 12, Nb = 0 ~ 0.5, V = 0 ~ 0.5, Ti = 0 ~ 0.5, W = 0 ~ 0.5, Mo = 0 ~ 0.5, P ≤ 0.015, S ≤ 0.01, and the balance is Fe. It also includes one or more of the following elements, in wt% content: Si = 0 ~ 3.0, Cr = 0 ~ 5.0, Cu = 0 ~ 2.0, B = 0.0005 ~ 0.01, rare earth RE = 0.001 ~ 0.10, Ca = 0.005 ~ 0.050;

[0031] The microstructure of the high-strength, high-toughness, and high-plasticity austenitic low-density steel is characterized by obtaining the structural characteristics of fine grain size, high-density dislocations, and deformation nano twins.

[0032] The high-strength, high-toughness, and high-plasticity austenitic low-density steel has the following mechanical properties: yield strength ≥650MPa, elongation ≥30%, and Charpy impact energy KV2 (-40°C) ≥80J.

[0033] The high-strength, high-toughness and high-plasticity austenitic low-density steel is characterized in that the austenitic low-density steel billet, after rough rolling or forging, has a starting temperature of ≤1000°C in the finishing rolling stage, a cumulative deformation of ≥20%, final rolling in the non-recrystallization temperature zone, and water cooling after rolling.

[0034] A hot rolling method for high-strength, high-toughness and plastic austenite low-density steel is applicable to low-density steel of FeMnAlC alloy system with austenite as the matrix, and its composition characteristics are: C = 0.50-1.20, Mn = 20.0-35.0, Al = 6.0-12, Nb = 0-0.5, V = 0-0.5, Ti = 0-0.5, W = 0-0.5, Mo = 0-0.5, P ≤ 0.015, S ≤ 0.01, and the balance is Fe. It also includes one or more of the following elements, in terms of wt% content: Si = 0-3.0, Cr = 0-5.0, Cu = 0-2.0, B = 0.0005-0.01, rare earth RE = 0.001-0.10, Ca = 0.005-0.050.

[0035] The hot rolling method of the high-strength, high-toughness, high-plasticity, austenitic low-density steel described in the present invention has the matrix structure of austenite.

[0036] In order to achieve the invention purpose of high-strength, high-toughness, high-plasticity, austenitic low-density steel, the present invention discloses a method for preparing the high-strength, high-toughness, high-plasticity austenitic low-density steel, which comprises the following steps in sequence: (1) smelting; (2) casting; (3) (blank opening +) hot rolling; (4) cooling; (5) solution treatment (+ aging) and other subsequent treatments.

[0037] In the above manufacturing method, in the step (1), the smelting method adopts converter, electric furnace or induction furnace smelting, and LF refining (LF, Ladle Furnace, ladle refining furnace) and RH or VD degassing methods (RH, Ruhrstahl-Heraeus vacuum degassing, steel liquid vacuum cycle degassing method; VD, vacuum degassing, vacuum degassing) can be adopted.

[0038] In the above manufacturing method, in step (2), the casting method is continuous casting or mold casting.

[0039] In the above manufacturing method, in step (3), the research results show that since the Mn element significantly reduces the melting point of steel and the segregation phenomenon causes the local melting point to be low, the heating temperature of the ingot should be less than 1200° C. After the ingot is heated to achieve full solid solution, it can be directly rolled to open the ingot, or it can be opened by forging to prepare for the subsequent hot rolling process. Hot rolling is divided into rough rolling and finishing rolling stages. In the rough rolling stage, the surface oxide scale of the billet should be effectively removed before rough rolling, and a large cumulative deformation should be completed under high temperature conditions to achieve the thickness of the billet required for finishing rolling; in the finishing rolling stage, the initial rolling temperature should be less than 1000°C, the cumulative rolling deformation should not be less than 20%, and the final rolling temperature range should be controlled at 800-870°C. The inventor's research results show that a relatively low final rolling temperature can easily obtain a fine grain size and a high density of dislocations, and under certain conditions, deformation twins can also be obtained. These organizations are conducive to achieving fine grain strengthening, deformation strengthening, dislocation strengthening, etc. of austenitic low-density steel. At the same time, the fine grain size is conducive to ensuring the plasticity and toughness of the steel.

[0040] In the above manufacturing method, in the step (4), the steel plate after rolling should be cooled rapidly, such as by water cooling. The inventor's research results show that when the Al content in the steel is ≤8%, the cooling rate requirement is relatively small, while when the Al content in the steel is ≥9%, κ-carbide is easily precipitated under the condition of slow cooling rate. It is a coherent structure with the matrix, which is easy to form order and coarsening, thereby reducing the toughness of the steel. When cooling rapidly, it is easy to retain the high-density dislocations, fine recrystallized structures and other deformation-characteristic structures (such as deformation twins) produced by deformation, and at the same time, it can avoid and reduce κ-carbide during the cooling process. Therefore, rapid cooling should be performed after hot rolling.

[0041] In the above-mentioned manufacturing method, step (5) is a non-essential step and is adopted when judging the final performance requirements of the steel plate. For the steel plate obtained by steps (1) to (4), after the solution treatment in step (5), the strength will decrease while the toughness and plasticity will increase. This phenomenon is more obvious with the increase of the solution temperature and the extension of the solution time. For the steel plate obtained by steps (1) to (4), after the aging treatment in step (5), the strength level will increase, while the toughness and plasticity levels may decrease.

[0042] The test steels shown in Table 1 were smelted in a vacuum induction furnace. 1C25Mn7Al, 1C30Mn7Al, 1C30Mn8Al, 1C30Mn10Al and 1C30Mn11Al were smelted in a 50kg vacuum induction furnace. 1C30Mn9Al steel was smelted in a 1-ton vacuum induction furnace. After the ingots were solution treated at 1150°C, they were forged and hot rolled into slabs with a thickness of 35 mm for rolling.

[0043] The 35 mm thick slab was heated at 1150°C for 2 h for solution treatment and then hot rolled into a 12 mm thick steel plate through 5 passes. The cumulative deformation was greater than 20%, and the final rolling temperature was 800-870°C. Water cooling was used after hot rolling. The tensile properties and -40°C impact properties of the samples were evaluated according to GB / T228 and GB / T229, respectively. The results are shown in Table 2.

[0044] Table 1 Chemical composition of the invented steel

[0045] Steel Grade C Mn Al P S 1C25Mn7Al 1 25 7 0.0065 0.0035 1C30Mn7Al 1 30 7 0.0069 0.0038 1C30Mn8Al 1 30 8 0.0063 0.0063 1C30Mn9Al 1 30 9 0.0066 0.0063 1C30Mn10Al 1 30 10 0.0063 0.0066 1C30Mn11Al 1 30 11 0.0064 0.0064

[0046] Table 2 Heat treatment process and mechanical properties of the invented steel

[0047]

[0048]

[0049] From Table 2, it can be found that by simulating the cumulative deformation in the finishing rolling stage and controlling the final rolling temperature, that is, the controlled water cooling state, the yield strength has been significantly improved compared with the heat treatment solid solution state, and it also has higher plasticity and impact toughness results. By comparing the mechanical properties of steels with different Al contents, it can be found that under the same controlled rolling or solid solution conditions, the higher the Al content, the higher the strength level of the steel, while the plasticity and impact toughness levels are lower.

[0050] Taking 1C30Mn9Al steel as an example, the mechanical properties in the solid solution state, aging state, controlled rolling state and high final rolling temperature state were compared. The results show that the yield strength of the test steel in the solid solution state at 1050℃×2h is 492MPa, while after controlled rolling (800-870℃ final rolling, cumulative deformation>20%) and water cooling, the yield strength is greater than 800MPa and the impact absorption energy reaches 133J. When the final rolling is completed at a higher temperature, the strength level is lower than the controlled rolling water cooling process (800-870℃ final rolling) of the present invention. Aging treatment can also improve the strength level of steel, but the impact toughness of the steel after aging treatment is reduced by more than 50% compared with the controlled rolling water cooling state proposed in the present invention. Studies have shown that the ordering and coarsening behavior of κ-carbides precipitated in the aging state significantly deteriorates the impact toughness of the steel, while the water cooling state avoids the precipitation behavior of κ-carbides, so higher impact toughness can be obtained.

[0051] The microstructure of 1C30Mn9Al steel after final rolling at 1000℃ and 820℃ was compared. The EBSD statistical results (EBSD, electron back-scattered diffraction) showed that the average grain sizes at the two final rolling temperatures were 15μm and 10.5μm, respectively. The grain boundary diagram after final rolling at 820℃ is shown in Figure 2. Figure 1 As shown in the figure, the dislocation density was measured by X-ray. The dislocation density under 1000℃ and 820℃ final rolling was 1.24×10 14 and 4.8×10 14 ; The transmission electron microscopy characterization of the microstructure found that a high-density dislocation structure was obtained under the condition of final rolling at 820℃ ( Figure 2 ) and a large number of deformation nanotwins ( Figure 3 ).

[0052] It can be seen that through the controlled rolling + water cooling hot rolling process proposed in the present invention, the organizational characteristics of grain refinement, high-density dislocations and deformation twins are produced inside the steel, which effectively improves the strength level of austenitic low-density steel, especially the yield strength, while also having a higher level of plasticity and impact toughness.

[0053] Through a certain amount of cumulative deformation in the finishing rolling stage and relatively low final rolling temperature control, low-density austenitic steel can obtain high strength, high plasticity, and high toughness mechanical properties. For micro-alloyed low-density steel, combined with controlled rolling technology, the precipitation behavior of fine carbides can be regulated, which can further play a role in grain refinement and precipitation strengthening. For low-density steels with different compositions, the optimal mechanical property control can be achieved by regulating the cumulative deformation and final rolling temperature.

[0054] The contents not described in detail in the specification of the present invention belong to the prior art known to the professional and technical personnel in the field. It is pointed out here that the above description helps those skilled in the art to understand the invention, but does not limit the protection scope of the invention. Any equivalent replacement, modification and / or simplification of the above description without departing from the essence of the invention falls within the protection scope of the invention.

Claims

1. A method for manufacturing high-strength, high-toughness, and high-plasticity austenitic low-density steel, characterized in that: Aiming at the chemical composition of FeMnAlC alloy low-density steel with austenite as the matrix, the manufacturing steps include the following: Step 1, smelting; Step 2, casting; Step 3, hot rolling, the hot rolling includes a finishing rolling stage, the initial rolling temperature of the finishing rolling stage is less than 1000°C, the final rolling temperature is 820°C, and the final rolling is performed in a non-recrystallization temperature zone. The microstructure of the steel plate material after final rolling has the organizational characteristics of fine grain size, high-density dislocation and deformation nano twins, so as to achieve fine grain strengthening, deformation strengthening and dislocation strengthening of austenitic low-density steel, and ensure plasticity and toughness; The cumulative deformation in the finishing rolling stage is ≥20%, and the optimal mechanical property control is achieved by regulating the cumulative deformation and the final rolling temperature; Step 4, water cooling; The chemical composition of the FeMnAlC alloy low-density steel is as follows in terms of wt%: C=1-1.20, Mn=30.0-35.0, Al=6.0-12, P≤0.015, S≤0.01, and the balance is Fe; In step 3, the heating temperature of the ingot is less than 1200° C. After the ingot is heated to achieve full solid solution, it is rolled or forged to prepare for the subsequent hot rolling process. The microstructure of the steel after final rolling at 820℃ is as follows: the average grain size is 10.5 μm according to the EBSD statistical results, and the dislocation density is 4.8× 10 14 , according to the characterization of transmission electron microscopy, it has deformation nanotwins; The austenitic low-density steel has a -40°C Charpy impact energy KV2 ≥ 80J; The mechanical properties of the austenitic low-density steel are as follows: yield strength ≥ 650 MPa, and elongation ≥ 30%.

2. The method for manufacturing high-strength, high-toughness, and high-plasticity austenitic low-density steel according to claim 1, characterized in that: The step 1 includes smelting in a converter, an electric furnace or an induction furnace, refining in LF, and degassing in RH or VD.

3. The method for manufacturing high-strength, high-toughness, and high-plasticity austenitic low-density steel according to claim 1, characterized in that: The step 2 includes continuous casting or mold casting.

4. The method for manufacturing high-strength, high-toughness, and high-plasticity austenitic low-density steel according to claim 1, characterized in that: The process also includes step 5, solutionizing.

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