Preparation method of stainless steel with low dislocation density nano lamellar microstructure

Abstract

The invention discloses a preparation method of stainless steel with a low dislocation density nano lamellar microstructure. A stainless steel plate prepared through deep cooling rolling, high-temperature short-time annealing, warm rolling and rapid annealing has a uniform nano lamellar microstructure. Firstly, an initial stainless steel plate is subjected to deep cooling rolling, and grains are severely refined in a low-temperature induced twinning and phase change mode; performing high-temperature short-time annealing to obtain a stainless steel plate with nano-scale grains with low dislocation density; the deformation resistance is reduced through warm rolling, and a stainless steel plate with nanosheet layers distributed is obtained; and finally, quickly annealing to form a microstructure of a low-dislocation density nanosheet layer in the plate blank, so as to obtain the stainless steel plate with low-dislocation density uniform nanosheet layer characteristics in a large volume range. The low dislocation density characteristic of the microstructure is beneficial to theoretical research and structural design of novel alloy in the future, the material utilization rate is high, the characteristic structure, namely the lamellar thickness, can be obviously refined to the nanometer level in the whole volume, and a uniform nanometer lamellar layer is obtained.

Description

Technical Field

[0001] The invention belongs to the technical field of structural material preparation, and in particular relates to a method for preparing stainless steel with a low dislocation density nano-lamellar microstructure. Background Art

[0002] Alloys with layered microstructures have attracted much attention in the field of modern engineering technology. The well-known Damascus steel has high strength and toughness due to its microstructure of overlapping layered ferrite and cementite, and is therefore widely used to make sharp and durable knives. Layered alloys have attracted great research interest from materials scholars due to their excellent properties of high strength and hardness. A large number of studies have shown that the strength of layered alloys is directly related to the change in the thickness of the layer. When the thickness of the layer is above submicron, the relationship between the strength and thickness of the material satisfies the general Hall-Petch relationship: the yield strength is proportional to the inverse of the square root of the thickness. However, when the thickness of the layer drops to the nanometer level, there is no complete mechanism study on the constitutive relationship between the strength and thickness of the material. The main limitation is that even with complex and cumbersome preparation processes, large-scale materials with uniform nano-layer structures cannot be obtained. Therefore, the constitutive relationship between performance and layer thickness is limited to a very small range.

[0003] At present, the mainstream methods for preparing alloys with layered structures are rolling, hot forging and hot pressing, welding, cumulative lamination, powder metallurgy and vapor deposition. Chinese invention patent CN103773984B introduces a powder metallurgy method for preparing ultrafine-grained alloys using micro-nano laminations. The thickness of the lamination structure obtained by sintering this method is hundreds of nanometers. The characteristics of this method are (1) it can fully exert the ultrafine grain strengthening and alloy strengthening on the basis of maintaining good plasticity; (2) it can achieve inter-layer diffusion and thus uniform alloying. Its limitations are (1) in order to achieve the uniformity of the alloy composition, it must be kept warm for a long time; (2) the thickness of the lamination is between 100nm-5μm, which does not reach the thickness range of nano-lamellar (<100nm). Invention patent CN114029356A uses a combination of multi-directional hot forging, unidirectional hot forging and cold rolling to prepare layered stainless steel plates with ultrafine grains / nano-grains. The advantages of the preparation method are simple process, high utilization rate of raw materials, and controllable thickness. The thickness of the stainless steel plate microstructure prepared by this method is between 200-500nm, which does not reach the thickness range of nanosheets. Therefore, it is urgent to explore new technical routes to prepare alloys with sheet thickness less than 100nm as the basic material for research in the post-nanostructure material era. Summary of the invention

[0004] Based on the above background, the object of the present invention is to provide a method for preparing stainless steel with a low dislocation density nano-lamellar microstructure. Another object of the present invention is to provide a stainless steel sheet with a uniform nanostructure of larger size.

[0005] In order to achieve the above objectives, the technical solution adopted by the present invention is: a method for preparing a stainless steel plate with a low dislocation density nano-layer microstructure, by combining four processes of deep cold rolling, high temperature short-time annealing, warm rolling and rapid annealing to obtain a stainless steel plate with a uniform nano-layer microstructure with low dislocation density.

[0006] The cryogenic rolling process is as follows: commercial 306L stainless steel plates are selected, fully immersed in liquid nitrogen and then cryogenically rolled, dislocation movement, dynamic recovery and recrystallization are suppressed at low temperature, and coarse parent material grains are violently divided by inducing twinning and phase transformation, thereby obtaining initially refined 306L rolled plates;

[0007] The high temperature short time annealing is specifically as follows: the deformation-induced martensite phase transformation is restored to ultrafine grain austenite by high temperature short time annealing, and the severely deformed residual austenite is recrystallized and dislocations are annihilated, so as to obtain a uniform nano-scale 304 stainless steel sheet;

[0008] Specifically, the warm rolling is as follows: the nanostructured 306L stainless steel sheet is heated to a temperature range above the recovery temperature and below the recrystallization temperature for rolling, which reduces the deformation resistance. At the same time, the dynamic recovery weakens the work hardening, so that the nano-scale annealed grains are elongated along the rolling direction, and the nano-layered 306L stainless steel sheet with a high dislocation density is obtained;

[0009] Rapid annealing is specifically as follows: subsequent rapid annealing treatment is performed at a relatively high temperature for a very short time to allow dislocations to recover and annihilate, while preventing the coarsening and growth of the nanosheet layer, thereby obtaining a 306L stainless steel sheet with a low dislocation density nanosheet microstructure.

[0010] The specific steps include:

[0011] Step 1: Prepare 304 stainless steel sheet, perform surface treatment and pre-grinding of edges and corners.

[0012] Step 2: Immerse the initial stainless steel plate in liquid nitrogen for a long time (3h-10h) to achieve a uniform cryogenic temperature.

[0013] Step 3: The stainless steel plate that has reached the cryogenic temperature is placed in a cold and hot two-roll rolling mill for repeated cryogenic rolling. The roller speed is 35HZ, and the reduction is 0.2mm per pass. After each rolling, the sample is put back into liquid nitrogen and soaked for 10-30 minutes to ensure the cryogenic temperature. The total reduction is 30%-50%.

[0014] Step 4: The deep cold rolled sample is annealed at 900°C for 120s, and the cooling method is air cooling.

[0015] Step 5: After annealing, the stainless steel sheet is reheated to 400℃, kept warm for 10-20min, and then placed in a two-roller rolling mill for repeated warm rolling. The roller speed is 35HZ, and the reduction is 0.1-0.3mm per pass. After each rolling, the sample is returned to the annealing furnace at 400℃ and kept warm for 8-15min to ensure the warm rolling temperature. The warm rolling reduction is 75-85%.

[0016] Step 6: The warm-rolled stainless steel sheet is placed in an induction coil heating furnace for rapid annealing at 800-850°C for 8-10 seconds, and the cooling method is air cooling.

[0017] The principle of the preparation method proposed in the present invention is: first, deep cold rolling uses low temperature to inhibit dislocation movement, thereby forcing the grains to undergo deformation twinning and martensitic phase transformation, and drastically subdividing the coarse grains of the original parent material. At the same time, low temperature hinders dynamic recovery and recrystallization, so that the deformed structure accumulates a large amount of distortion energy. The total reduction is less than 50% of the plate thickness to avoid excessive martensitic phase transformation, which reduces plasticity and leads to the formation of through cracks and crack edges, which is conducive to ensuring the integrity of the sample. The subsequent high-temperature short-time annealing treatment releases the distortion energy of the deformed structure. 316L stainless steel belongs to metastable austenitic stainless steel. The martensitic phase is unstable at high temperature. At 900°C, the martensite undergoes shear reverse phase transformation and quickly transforms into austenite. At the same time, the residual austenite undergoes recovery and recrystallization refinement. During the annealing process, recrystallization densely nucleates, and competitive growth controls the grain size at the nanometer level. In addition, short-time annealing cannot provide enough time for grain growth, thereby obtaining austenitic stainless steel with a nano-equiaxed grain structure. Then, warm rolling at 400°C was used to reduce the deformation resistance of the material, so that the nano-scale annealed grains were elongated into nanosheets along the rolling direction. The dynamic recovery during warm rolling is active, which weakens the work hardening. The higher stacking fault energy also inhibits the occurrence of martensitic phase transformation and prevents the layered grains from being divided. In addition, 400°C avoids the brittle temperature of 316L stainless steel, avoiding the formation of precipitates caused by long-term heat preservation, which reduces the corrosion resistance of the material or makes it brittle. The final rapid treatment is to use high temperature to annihilate the dislocations generated by rolling, reduce the dislocation density, and at the same time, the small amount of martensite generated by warm rolling reverses to austenite, and short-term heat preservation inhibits the coarsening of the nanosheet. Finally, a single-phase austenitic stainless steel plate with a nanosheet microstructure with low dislocation density in the entire volume is obtained.

[0018] Compared with the prior art, the present invention has the following significant advantages:

[0019] (1) The stainless steel plate prepared by the present invention through deep cold rolling + high temperature short time annealing + warm rolling + rapid annealing has a uniform nano-layer microstructure. Compared with the nano-layers (≥100nm) prepared by other means, the 316L stainless steel nano-layers prepared by the preparation method of the present invention are finer in size (<80nm) and have a very low dislocation density (≤0.3×10 13 / m 2 ), with clear grain boundaries and high density, its low-density uniform nano-lamellar microstructure is beneficial to theoretical research and structural design of future new alloys.

[0020] (2) The nanosheet stainless steel plate prepared by the present invention has a high material utilization rate and can realize its nanosheet structural characteristics throughout the entire volume. Different rolling deformation times and annealing times can be adjusted to obtain a desired large nanosheet size. The preparation method described in the present invention can be implemented using simple equipment such as a rolling mill + annealing furnace + induction coil heating furnace, is easy to control and has high repeatability, and can achieve industrial mass production.

[0021] (3) The nano-lamellar microstructure 316L stainless steel obtained by the present invention has a lamellar grain size at the nanometer level and a low dislocation density, so that its mechanical properties ensure high strength while taking into account plasticity. Different from the structural inhomogeneity of traditional processing and preparation methods, the nano-lamellar microstructure provides high strength for 316L stainless steel, and the clean lamellar grains provide space for the proliferation and entanglement of dislocations during deformation to bear tensile deformation, thereby showing high strength and high toughness. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 The technical roadmap of the method for preparing a stainless steel having a low dislocation density nano-lamellar microstructure according to the present invention; wherein: 1 is an initial stainless steel plate sample, 2 is an insulation barrel filled with liquid nitrogen, 3 is a two-roller mill for hot and cold rolling, 4 is a deep cold rolling sample, 5 is a box-type annealing furnace, 6 is a sample after warm rolling, and 7 is an induction coil heating furnace.

[0023] Figure 2 This is a transmission electron microscope (TEM) bright field image of a nanosheet stainless steel plate with low dislocation density obtained by the preparation method of the present invention in Example 1;

[0024] Figure 3 This is a TEM bright field image of a nanosheet stainless steel plate with low dislocation density obtained by the preparation method of the present invention in Example 2;

[0025] Figure 4 The engineering stress-strain curve of Example 1-2 after being processed by the method of the present invention. DETAILED DESCRIPTION

[0026] The present invention is further described in detail below with reference to the accompanying drawings.

[0027] The present invention achieves the purpose and effect of obtaining low dislocation density nano-lamellar microstructure 316L stainless steel through a simple process of deep cold rolling + high temperature short-time annealing + warm rolling + rapid annealing. The nano-lamellar structure of the present invention is obtained by eliminating internal defects through warm rolling extrusion deformation and rapid annealing on the basis of nano-equiaxed crystals obtained by deep cold rolling severe refinement and high temperature short-time annealing martensitic reverse phase transformation. The preparation method adopted by the present invention can refine the grain size to the nanometer level, and can realize a uniform nano-lamellar structure in a larger-sized block 316L stainless steel.

[0028] In the present invention, the number of pressing times, the pressing amount and the annealing time can be adjusted according to the required grain sheet size to obtain finer nanosheet grains. After repeated experiments, the inventors of the present application found that the ideal pressing amount for each pass of 316L stainless steel plate is 0.2mm, and the deep cold rolling pressing amount is not more than 50% of the plate thickness. In order to ensure the nano-grain size, the high temperature 900°C annealing time is best when it is 120s. In order to make the nano-sheet thickness <80nm, the warm rolling pressing amount should not be less than 70%, and the rapid annealing time within 10s can ensure the best effect of the present invention.

[0029] In fact, based on the principles and cases of the present invention, those skilled in the art can flexibly adjust the rolling reduction and annealing parameters according to existing technologies and specific circumstances, which will not be elaborated here.

[0030] The present invention is described in detail below through illustrative examples. It should be pointed out that the following embodiments only give some exemplary embodiments of the method of the present invention by way of example, and do not mean any limitation to the present invention. All the following implementation cases use commercial 304 stainless steel plates as the initial parent material, and its chemical composition is shown in Table 1. The parent material is processed to a plate sample of a predetermined thickness (see the embodiment for the specific thickness), and is processed according to the preparation method of this embodiment. The equipment used in the embodiment of the present invention is a hot and cold two-roll mill, a box annealing furnace, and an induction coil heating furnace. For the specific preparation process, please refer to Figure 1 The flow chart of the preparation method of the present invention is shown. According to different parameters of the preparation method, the following Examples 1-2 are constructed.

[0031] Table 1: Chemical composition of 306L stainless steel (wt.%)

[0032]

[0033] Example 1

[0034] The initial sample size is 50 mm×15 mm×15 mm (length×width×thickness). The processing flow of the stainless steel sheet with low dislocation density nano-lamellar microstructure prepared in this embodiment is as follows:

[0035] (1) The initial sample is surface treated and the edges are polished, which is beneficial to the cleanliness of the stainless steel plate and the bite of the rollers during the subsequent repeated rolling process.

[0036] (2) The sample is immersed in liquid nitrogen for more than 3 h to ensure a uniform cryogenic temperature.

[0037] (3) The soaked samples were rolled. The reduction was 0.2 mm per pass. After each rolling pass, the rolled samples were immersed in liquid nitrogen for 10 min to ensure that the next rolling pass was at a deep cooling temperature. The total reduction was 4.5 mm. The thickness of the rolled plate after treatment was about 10.5 mm, and the total reduction was 30%.

[0038] (4) The above-mentioned deep cold rolled stainless steel plate is subjected to high temperature short-time annealing treatment. When the box-type annealing furnace is heated to 900°C, the rolled plate is placed in it, and the temperature is kept at 900°C for 120 seconds, and then the stainless steel plate is taken out and air-cooled. A uniform stainless steel plate with nano-scale equiaxed grains is obtained.

[0039] (5) The stainless steel plate after nano-treatment was reheated to 400℃, kept warm for 10 minutes, and then repeatedly rolled in a hot and cold two-roll mill, with a reduction of 0.2mm per pass. After each rolling, the rolled sample was placed in a 400℃ box annealing furnace for 8 minutes to ensure the warm rolling temperature, with a total reduction of 7.9mm. The thickness of the rolled plate after treatment was about 2.6mm, and the total reduction was 75%.

[0040] (6) The warm-rolled stainless steel sheet was placed in an induction coil heating furnace for rapid annealing at 800 °C for 10 s, and the cooling method was air cooling.

[0041] Example 1 The TEM bright field image of the stainless steel plate with low dislocation density nanosheet layer obtained by the preparation method of the present invention is shown in the attached figure. Figure 2 As shown, the average thickness of the austenite layer distributed along the rolling direction is about 66.73nm and is uniform over the entire volume. Room temperature tensile tests show that its tensile strength is 980MPa and its uniform elongation is 23%.

[0042] Example 2

[0043] The initial sample size is 50 mm×15 mm×15 mm (length×width×thickness). The processing flow of the stainless steel sheet with low dislocation density nano-lamellar microstructure prepared in this embodiment is as follows:

[0044] (1) The initial sample is surface treated and the edges are polished, which is beneficial to the cleanliness of the stainless steel plate and the bite of the rollers during the subsequent repeated rolling process.

[0045] (2) The sample is immersed in liquid nitrogen for more than 3 h to ensure a uniform cryogenic temperature.

[0046] (3) The soaked samples were rolled. The reduction was 0.2 mm per pass. After each rolling pass, the rolled samples were immersed in liquid nitrogen for 10 min to ensure that the next rolling pass was at a deep cooling temperature. The total reduction was 4.5 mm. The thickness of the rolled plate after treatment was about 10.5 mm, and the total reduction was 30%.

[0047] (4) The above-mentioned deep cold rolled stainless steel plate is subjected to high temperature short-time annealing treatment. When the box-type annealing furnace is heated to 900°C, the rolled plate is placed in it, and the temperature is kept at 900°C for 120 seconds, and then the stainless steel plate is taken out and air-cooled. A uniform stainless steel plate with nano-scale equiaxed grains is obtained.

[0048] (5) The stainless steel plate after nano-treatment was reheated to 400℃, kept warm for 10 minutes, and then repeatedly rolled in a hot and cold two-roll mill, with a reduction of 0.2mm per pass. After each rolling pass, the rolled sample was placed in a 400℃ box annealing furnace for 8 minutes to ensure the warm rolling temperature, with a total reduction of 8.9mm. The thickness of the rolled plate after treatment was about 1.5mm, and the total reduction was 85%.

[0049] (6) The warm-rolled stainless steel sheet was placed in an induction coil heating furnace for rapid annealing at 800 °C for 10 s, and the cooling method was air cooling.

[0050] Example 2 The TEM bright field image of the stainless steel plate with low dislocation density nanosheet layer obtained by the preparation method of the present invention is shown in the attached figure. Figure 3 As shown in Figure 1, the average thickness of the austenite lamellae distributed along the rolling direction is about 56.42 nm and is uniform over the entire volume. Figure 4 ,The room temperature tensile test shows that its tensile strength is ~1000MPa and the uniform elongation is ~19%.

[0051] The above embodiments clearly show that the present invention provides a method for preparing a stainless steel plate with a uniform nano-lamellar microstructure with low dislocation density, which is easy to operate, highly repeatable and economical. The nano-lamellar microstructure obtained by the preparation method of the present invention is uniform and has a low dislocation density, exhibits good mechanical properties, and can achieve a higher strength-plasticity match. It can be widely used in various structural components to reduce the size restrictions on the application of nano-lamellar structure materials. The descriptions and drawings of the above embodiments are exemplary and not limitations of the present invention. For example, different strain amounts can be applied or annealing parameters can be adjusted during the rolling process of the stainless steel plate according to the thickness or composition of the stainless steel plate, the pretreatment state before rolling, etc. These variations and modifications obviously do not depart from the scope of the present invention.

Claims

1. A method for preparing stainless steel having a low dislocation density nano-lamellar microstructure, characterized in that: A stainless steel sheet with a uniform nanosheet microstructure with low dislocation density is obtained by deep cold rolling, high temperature short-time annealing, warm rolling and rapid annealing. First, the initial stainless steel plate is subjected to deep cold rolling to drastically refine the grains by inducing twinning and phase transformation at low temperature; Then, a stainless steel plate with nano-scale grains having a low dislocation density is obtained by high-temperature short-time annealing; Then, the stainless steel plate with nanosheet distribution is obtained by reducing the deformation resistance through warm rolling; Finally, rapid annealing forms a microstructure of nanosheets with low dislocation density in the slab, thereby obtaining a stainless steel plate with uniform nanosheet characteristics over a large volume range.

2. The method for preparing stainless steel having a low dislocation density nano-lamellar microstructure according to claim 1, characterized in that: The stainless steel material is 306L stainless steel.

3. The method for preparing stainless steel having a low dislocation density nano-lamellar microstructure according to any one of claims 1 to 2, characterized in that: The thickness of the stainless steel plate is ≤15 mm.

4. The method for preparing stainless steel having a low dislocation density nano-lamellar microstructure according to claim 1, characterized in that: The reduction amount of deep cold rolling is 30%-50%, preferably, the reduction amount is 30%.

5. The method for preparing stainless steel having a low dislocation density nano-lamellar microstructure according to claim 1, characterized in that: The temperature of high temperature short time annealing is 900℃ and the holding time is 120s.

6. The method for preparing stainless steel having a low dislocation density nano-lamellar microstructure according to claim 1, characterized in that: The warm rolling temperature is 400° C., and the total reduction of the warm rolling is 75%-85%, preferably, the total reduction is 85%.

7. The method for preparing stainless steel having a low dislocation density nano-lamellar microstructure according to claim 1, characterized in that: The rapid annealing temperature is 800-850° C., and the annealing time is 8-10 s. Preferably, the annealing temperature is 800° C. and the annealing time is 10 s.

8. The method for preparing stainless steel having a low dislocation density nano-lamellar microstructure according to claim 1, characterized in that: The specific steps include: Step 1: Prepare 304 stainless steel sheet, perform surface treatment and pre-process the edges and corners; Step 2: Immerse the initial stainless steel plate in liquid nitrogen for a long time of 3h-10h to reach a uniform cryogenic temperature; Step 3: The stainless steel plate that has reached the cryogenic temperature is placed in a cold and hot two-roll rolling mill for repeated cryogenic rolling, with a roller speed of 35HZ and a reduction of 0.2mm per pass; after each rolling pass, the sample is returned to liquid nitrogen for 10-30 minutes to ensure the cryogenic temperature; the total reduction is 30%-50%; Step 4: anneal the deep cold rolled sample at 900°C for 120s, and the cooling method is air cooling; Step 5: After annealing, the stainless steel sheet is reheated to 400°C, kept warm for 10-20 minutes, and then placed in a two-roller rolling mill for repeated warm rolling; the roller speed is 35HZ, and the reduction is 0.1-0.3mm per pass; after each rolling, the sample is returned to the annealing furnace at 400°C and kept warm for 8-15 minutes to ensure the warm rolling temperature; the warm rolling reduction is 75-85%; Step 6: The warm-rolled stainless steel sheet is placed in an induction coil heating furnace for rapid annealing at 800-850°C for 8-10 seconds, and the cooling method is air cooling.

9. A stainless steel having a low dislocation density nanolamellar microstructure, characterized in that: The method is prepared by any one of claims 1 or 8.

10. The stainless steel having a low dislocation density nano-lamellar microstructure according to claim 9, characterized in that: In the low dislocation density nanolamellar structure of the stainless steel sheet, the average thickness of the austenite lamellae can be refined to less than 80nm.

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