A readily workable high hardenable austenitic stainless steel and a method of making the same

Abstract

The application belongs to the technical field of austenitic stainless steel, and relates to an easily processed high-hardening austenitic stainless steel and a manufacturing method thereof. The easily processed high-hardening austenitic stainless steel provided by the application comprises, in percentage by weight, C<=0.12%, Si<=1.00%, Mn: 2.50%-4.50%, P<=0.045%, S<=0.030%, Cr: 16.00%-18.00%, Ni: 5.50%-7.50%, N<=0.20%, Cu<=0.50%, and the rest is Fe and inevitable impurities. The austenitic stainless steel has a yield strength of >=245 MPa, a tensile strength of >=710 MPa, and a hardness (HV) of <=200 HV.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of austenitic stainless steel, and relates to an easily processable high-hardening austenitic stainless steel and a manufacturing method thereof, in particular to an austenitic stainless steel with processability and high hardening rate and a manufacturing method thereof. BACKGROUND

[0002] The shell material of the vehicle-mounted gas cylinder needs to maintain its structural integrity and safety in various harsh environments, so the requirements are very strict, mainly including the following aspects: high strength and excellent toughness, the gas cylinder shell must be able to withstand a certain impact to prevent deformation or rupture during use. Therefore, the selected material should have high yield strength and tensile strength, and at the same time have good toughness to cope with potential impact and vibration. Excellent corrosion resistance: the gas cylinder may come into contact with various chemicals and humid environments during use, so the shell material must have excellent corrosion resistance to prevent structural damage and gas leakage due to corrosion. Good welding performance: the gas cylinder shell is usually welded from multiple parts, so the selected material should have good welding performance to ensure high strength and good sealing of the welded joint, and reduce welding defects. High temperature resistance: the vehicle-mounted gas cylinder may be exposed to high temperature environments during use, so the shell material needs to have good high temperature resistance to prevent performance degradation and structural deformation of the material due to high temperature. Lightweight: in order to reduce the overall weight of the vehicle and improve fuel efficiency, the shell material of the vehicle-mounted gas cylinder should be as light as possible while maintaining sufficient strength and safety. Based on the above requirements, stainless steel (such as 304 stainless steel) and aluminum alloy are common shell materials for vehicle-mounted gas cylinders; they can usually meet the above requirements and perform well in actual applications.

[0003] With the development of society, the concept of efficient use and resource conservation is increasingly ingrained in people's minds. When 304 stainless steel and other materials are used as shell materials, they must not only resist atmospheric corrosion, but also have certain processability and strength; this requires considering a relatively thick wall thickness at the beginning of the design to avoid leakage hazards of the shell after an accident collision. From the perspective of the processing process and safety performance of the vehicle-mounted gas cylinder shell, it is necessary to develop an easily processable and high-hardening austenitic stainless steel to meet the corrosion and mechanical performance requirements, and further improve the safety of the vehicle-mounted gas cylinder. SUMMARY

[0004] In view of the defects of the prior art, the present application provides an easily processable high-hardening austenitic stainless steel and a manufacturing method thereof.

[0005] Specifically, the easy-to-process high-hardening austenitic stainless steel provided by the present application comprises, in percentage by weight: C≤0.12%, Si≤1.00%, Mn: 2.50%-4.50%, P≤0.045%, S≤0.030%, Cr: 16.00%-18.00%, Ni: 5.50%-7.50%, N≤0.20%, Cu≤0.50%, and the rest is Fe and inevitable impurities.

[0006] The easy-to-process high-hardening austenitic stainless steel provided by the present application comprises, in percentage by weight: C≤0.07%, Si: 0.2%-0.5%, Mn: 3.0%-4.0%, P≤0.045%, S≤0.030%, Cr: 16.5%-17.5%, Ni: 6.0%-6.5%, N: 0.04%-0.09%, Cu: 0.02%-0.04%, and the rest is Fe and inevitable impurities.

[0007] The easy-to-process high-hardening austenitic stainless steel provided by the present application comprises, in percentage by weight: C: 0.04%, Si: 0.4%, Mn: 3.3%, P≤0.045%, S≤0.030%, Cr: 17.0%, Ni: 6.2%, N: 0.06%, Cu: 0.025%, and the rest is Fe and inevitable impurities.

[0008] The easy-to-process high-hardening austenitic stainless steel provided by the present application has a metal dislocation energy γ SFE of 24-29 J / m 2 .

[0009] In another aspect, the present application also provides a method for preparing the easy-to-process high-hardening austenitic stainless steel, comprising the following steps: a smelting step→a continuous casting step→a hot rolling step→a hot annealing and pickling step→a cold rolling step→a cold annealing and pickling step→a flattening step→a longitudinal cutting step; wherein the molten steel obtained in the smelting step has the same composition as the easy-to-process high-hardening austenitic stainless steel.

[0010] The method for preparing the easy-to-process high-hardening austenitic stainless steel provided by the present application, in the hot rolling step, the heating temperature of the slab before rolling is controlled to be between 1200-1300 ℃, and the heating time is 9.5-12.5 min / 10 mm; and the rolling temperature of the hot continuous rolling process is controlled to be above 950 ℃.

[0011] The method for preparing the easy-to-process high-hardening austenitic stainless steel provided by the present application, in the hot annealing and pickling step, the annealing temperature is controlled to be between 1100-1200 ℃.

[0012] The method for preparing the easy-to-process high-hardening austenitic stainless steel provided by the present application, in the cold rolling step, the total deformation amount of the cold rolling is 40-70%.

[0013] The preparation method of the easy-to-process high-hardening austenitic stainless steel, in the cold reduction and pickling process, the annealing temperature is controlled between 1050-1150 DEG C.

[0014] The technical scheme of the present application has the following beneficial effects:

[0015] (1) The manufacturing method of the austenitic stainless steel of the present application, by selecting reasonable component system and matching the component and content of alloying elements, adjusting the elements of chromium, nickel, manganese, nitrogen and copper, ensures the stability of austenitic structure of the material, greatly improves the tensile strength under the yield strength of the material, and further improves the work hardening rate by adjusting the alloying elements, improves the safety performance in the use process under the requirement of the workability of the material.

[0016] (2) The austenitic stainless steel of the present application, the yield strength is greater than or equal to 245 MPa, the tensile strength is greater than or equal to 710 MPa, and the hardness (HV) is less than or equal to 200 HV. BRIEF DESCRIPTION OF DRAWINGS

[0017] By reading the detailed description of the preferred embodiments below, various other advantages and benefits will become apparent to those of ordinary skill in the art. The drawings are for the purpose of illustrating the preferred embodiments only and are not to be considered as limiting of the present application.

[0018] Figure 1 The microstructure diagram of the austenitic stainless steel cold-rolled plate prepared in Example 1 after cold reduction and pickling. DETAILED DESCRIPTION

[0019] In order to fully understand the purpose, features and effects of the present application, the following specific embodiments are used to describe the present application in detail. The process method of the present application adopts the conventional method or device in the art except the following content. Unless otherwise specified, the following terms have the meanings commonly understood by those skilled in the art.

[0020] When a numerical range is disclosed herein, the above range is considered to be continuous and includes the minimum value and the maximum value of the range, and each value between the minimum value and the maximum value. Further, when the range refers to an integer, each integer between the minimum value and the maximum value of the range is included. In addition, when multiple ranges are provided to describe a feature or characteristic, the ranges can be combined. In other words, unless otherwise specified, all ranges disclosed herein should be understood to include any and all sub-ranges incorporated therein.

[0021] The present application provides a workable high-hardening austenitic stainless steel in the first aspect, which has the following chemical components: C≤0.12%, Si≤1.00%, Mn: 2.50-4.50%, P≤0.045%, S≤0.030%, Cr: 16.00-18.00%, Ni: 5.50-7.50%, N≤0.20%, Cu≤0.50%, and the rest is Fe and inevitable impurities.

[0022] In the present application, the content of Ni is 5.50-7.50%, the content of Mn is 2.05-4.50%, and the content of Cr is 16.00-18.00%, which can ensure that the material is austenitic after solid solution annealing, and the yield strength meets the processing requirements, and the adjustment of the content of Mn increases the dislocation energy of the material and improves the work hardening capacity of the material. Preferably, the content of Ni is 6.0-6.5%, the content of Mn is 3.0-4.0%, and the content of Cr is 16.5-17.5%; most preferably, the content of Ni is 6.2%, the content of Mn is 3.3%, and the content of Cr is 17.0%.

[0023] C: C is an element that can strongly form and stabilize and expand the austenite zone, which can act together with other austenitizing elements such as Mn and N to promote the formation of austenite. However, when the content of C is too high, C will form Cr-rich carbide with Cr, which reduces the corrosion resistance of the material. Therefore, the content of C in the present application is limited to ≤0.12%, preferably the content of C is limited to ≤0.07%, and most preferably 0.04%.

[0024] Si: Si is a ferrite-forming element, which generally plays a deoxidizing role during smelting; it can also enhance the high-temperature non-scaling property of the austenitic stainless steel and ensure the safety of use; Si can also improve the elastic limit, yield point and tensile strength of the steel, thereby significantly improving the overall performance of the stainless steel. However, too much Si will reduce the impact toughness and corrosion resistance of the steel. Therefore, the content of Si in the present application is limited to ≤1.00%, preferably the content of Si is limited to 0.2-0.5%, and most preferably 0.4%.

[0025] Cr: for austenitic stainless steel, chromium is one of the important stable ferrite elements, which can reduce the phase region of austenite in the phase diagram, and is not conducive to the nucleation of austenite. Therefore, in the smelting process of high chromium austenitic stainless steel, a large amount of nickel, manganese, nitrogen, carbon and other forming elements are added to stabilize the austenite structure. Cr improves the performance of the steel mainly in enhancing the corrosion resistance of the steel, and the yield performance of the steel decreases with the decrease of Cr content. When the chromium content in the steel is greater than 12%, the steel has good corrosion resistance. In order to ensure the corrosion resistance of the material, the Cr content is 16.00-18.00%, the Cr content is 16.5-17.5%, and the most preferred Cr content is 17.0%.

[0026] Ni: the role of nickel is similar to that of nitrogen, which is also an element for forming and stabilizing austenite. In addition to strongly forming and stabilizing austenite, nickel can also increase the phase region of austenite. Increasing the content of nickel can completely eliminate the residual ferrite in austenitic stainless steel, and can very favorably reduce the sigma phase. Nickel can reduce the transformation temperature of martensite, and can also prevent the steel from undergoing martensite transformation at very low temperatures. Nickel plays a significant role in reducing the cold forming hardening tendency of austenitic stainless steel, ensuring that the steel has excellent strength, toughness, plasticity, cutting performance and weldability. Both strength and single organization stability are considered. Therefore, the content of the present application is limited to 5.50-7.50%, preferably 6.0-6.5%; most preferably, the Ni content is 6.2%.

[0027] N: nitrogen has a strong ability to stabilize austenite, so nitrogen is added to the steel to expand the phase region of austenite to obtain a completely austenitic structure, thereby reducing the nickel content in the steel to save steelmaking costs. Nitrogen is a relatively special element. The addition of nitrogen increases the strength of the steel without significantly reducing its plasticity and toughness. It generally enhances strength through solid solution strengthening. The corrosion resistance of nitrogen is about 30 times that of chromium, and the effect of nitrogen on the corrosion resistance of stainless steel, especially local corrosion resistance, is very obvious. Therefore, the nitrogen content is ≤0.20%, preferably 0.04-0.09%, and most preferably 0.06%.

[0028] Mn: Manganese is an important component of stainless steel, as a very strong stable austenite element, manganese added to the general alloy steel can improve its hardenability. Although manganese is an austenite forming element, compared with the austenite forming ability of nickel, its ability is less than one tenth of nickel. The important role of manganese element in nitrogen-containing stainless steel is to enhance the solubility of nitrogen together with chromium. Manganese can not significantly enhance the electrode and potential ability of the iron-based solid solution, so the effect of manganese addition on the corrosion resistance of stainless steel is not obvious, and the protection ability of the oxide film is also low. To some extent, Mn replaces Ni, but too high Mn content will reduce the impact toughness of the steel. The increase of Mn content will increase the stacking fault energy of the material, and improve the work hardening ability of the material. Therefore, the content of manganese is 2.50-4.50%, preferably 3.0-4.0%, most preferably 3.3%.

[0029] Cu: Copper element as an austenite forming element, the austenite forming ability is about 30% of nickel element. Copper element can improve the corrosion resistance and cold working ability of the austenitic stainless steel. However, the melting point of copper is 1083℃, which may form undetectable liquid copper during hot working process, causing grain boundary cracking. Therefore, the content of copper is controlled to be Cu≤0.50%, preferably 0.02-0.04%, most preferably 0.025%.

[0030] P, S: P, S are harmful elements in stainless steel, the content of P in the steel of the present application is ≤0.045%, and the content of S is ≤0.030%.

[0031] In order to further determine its high hardening characteristics, the metal stacking fault energy γ SFE of the austenitic stainless steel is 24-29 J / m 2 .

[0032] The metal stacking fault energy γ SFE of the austenitic stainless steel is 24-29 J / m SFE , which indicates the difficulty of austenite deformation-induced formation of martensite during deformation, and γ SFE is calculated according to the following formula:

[0033] γ SFE = 16.7 + 2.18 × (Ni% + 3 × Cu%) - 0.9 × Cr% + 3.2 × Mn%.

[0034] In the austenitic stainless steel of the present application, chromium element is an important component for the corrosion resistance of the material; nickel, manganese and nitrogen elements as austenite stabilizing elements play a role together to ensure the uniformity of the organization; appropriate manganese and nitrogen elements can increase the work hardening rate of the material without significantly affecting the yield strength of the material; a small amount of copper element is beneficial to the cold working performance of the material; the coupling and synergistic effect of various elements form such a high hardening austenitic phase stainless steel which is easy to process.

[0035] In another aspect, the application also provides a manufacturing method of the easy-to-process high-hardening austenitic stainless steel and the manufacturing method thereof, comprising the following steps: smelting, continuous casting, hot rolling, hot pickling, cold rolling, cold pickling, flattening and longitudinal cutting.

[0036] Smelting, casting

[0037] The smelting is performed by using an electric furnace / middle frequency furnace+AOD+LF refining method to obtain a stainless steel melt with qualified components, and then the continuous casting is performed to form a billet.

[0038] The components and the proportion of the melt are the same as those of the austenitic stainless steel provided in the first aspect of the application, and the application does not repeat the description here.

[0039] Hot rolling

[0040] The continuous casting billet is heated to complete the hot rolling, and the hot rolling is followed by coiling.

[0041] Preferably, in order to ensure complete austenitization of the structure and reduce the rolling load, the heating temperature of the slab before the rolling of the continuous casting billet is controlled to be between 1200-1300℃, and the heating time is 9.5-12.5min / 10mm thickness.

[0042] Preferably, in order to avoid excessively high rolling load and reduce the precipitation of carbides in the rolling process, the rolling temperature of the hot continuous rolling process is controlled to be above 950℃.

[0043] Hot pickling

[0044] The hot rolling coil is heated and then subjected to a "sulfuric acid+mixed acid" pickling process.

[0045] Preferably, the annealing temperature is controlled to be between 1100-1200℃, thereby causing the austenite to occur in the crystallization, the decomposition of carbides and high-temperature ferrite, and avoiding the rolling cracking.

[0046] Cold rolling cold pickling

[0047] Preferably, the total deformation amount of the cold rolling is 40-70%, thereby completely breaking the austenitic structure of the slab; and then the heating and pickling are performed.

[0048] The pickling process is a "sodium sulfate+nitric acid+mixed acid" pickling process.

[0049] Preferably, the annealing temperature is controlled to be between 1050-1150℃, thereby causing the austenite to occur in the crystallization, the decomposition of carbides and high-temperature ferrite, and avoiding excessively coarse grains to affect the performance.

[0050] It should be noted that the processes and process parameters not described in detail in the present application are carried out according to the prior art, and the present application does not make specific limitations thereto.

[0051] The cold-rolled finished product of the stainless steel obtained according to the method of the present application is of austenitic structure, the yield strength is ≥245 MPa, the tensile strength is ≥710 MPa, and the hardness (HV) is ≤200 HV, which meets the requirements of the shell material for producing on-board gas cylinders.

[0052] Examples

[0053] The present application will be further described by way of examples below, but the present application is not limited to the scope of the examples described. The experimental methods not specified in the following examples are carried out according to conventional methods and conditions.

[0054] Example 1

[0055] An austenitic stainless steel is smelted by using an electric furnace / middle frequency furnace+AOD+LF refining method, and a casting blank with a thickness of 200 mm is obtained by continuous casting; the casting blank is heated to 1250±10℃ in a walking beam furnace, and the temperature is maintained for 220 min; after being discharged, the hot-rolled coil with a thickness of 8 mm is rolled on a hot continuous rolling mill, and the finish rolling temperature is 990℃; the hot-rolled coil is subjected to annealing and surface pickling in a continuous annealing and pickling line, and the annealing temperature is 1110℃; and then the cold-rolled coil with a thickness of 4 mm is rolled on a cold rolling mill, and the annealing temperature is 1080℃.

[0056] The cold-rolled coil prepared in this example is of single-phase austenitic structure. The chemical composition of the austenitic stainless steel of this example is shown in Table 1, and the mechanical property and corrosion performance results are shown in Table 2.

[0057] Example 2

[0058] A stainless steel is smelted by using an electric furnace / middle frequency furnace+AOD+LF refining method, and a casting blank with a thickness of 180 mm is obtained by continuous casting; the casting blank is heated to 1250±10℃ in a walking beam furnace, and the temperature is maintained for 190 min; after being discharged, the hot-rolled coil with a thickness of 3 mm is rolled on a hot continuous rolling mill, and the finish rolling temperature is 1010℃; the hot-rolled coil is subjected to annealing and surface pickling in a continuous annealing and pickling line, and the annealing temperature is 1120℃; and then the cold-rolled coil with a thickness of 1.2 mm is rolled on a cold rolling mill, and the annealing temperature is 1110℃.

[0059] The cold-rolled coil prepared in this example is of single-phase austenitic structure. The chemical composition of the austenitic stainless steel of this example is shown in Table 1, and the mechanical property and corrosion performance results are shown in Table 2.

[0060] Example 3

[0061] The stainless steel is smelted by adopting the electric furnace / middle frequency furnace+AOD+LF refining mode, and a casting blank with a thickness of 180 mm is obtained through continuous casting; the casting blank is heated to 1250±10 DEG C in a walking beam furnace and is kept for 180 min; after being discharged, the hot-rolled coil plate with a thickness of 2.9 mm is rolled on a hot continuous rolling mill set, and the finish rolling temperature is 1020 DEG C; the hot-rolled coil plate is annealed and surface pickled in a continuous annealing and pickling line, and the annealing temperature is 1140 DEG C; then the cold-rolled coil plate with a thickness of 1.0 mm is rolled on a cold rolling mill set, and is post-cold-rolled and pickled, and the annealing temperature is 1120 DEG C.

[0062] The cold-rolled coil plate prepared in the embodiment has a single-phase austenitic structure. The chemical composition of the austenitic stainless steel in the embodiment is shown in Table 1, and the mechanical property and corrosion property results are shown in Table 2.

[0063] Comparative Example 1 and Comparative Example 2 are both 06Cr19Ni10 cold plates prepared according to a conventional method and meeting the requirements of the national standard GB / T 3280-2015.

[0064] Table 1 Chemical composition of alloys for testing

[0065]

[0066] Mechanical property tests are carried out on the stainless steels in Table 1, i.e., Example 1 to Example 3 and Comparative Example 1 to Comparative Example 2. Among them, the room temperature tensile test adopts the standard GB / T 228.1-2021. The corrosion test adopts the standard GB / T 17989, and the corrosion medium is 6% FeCl3, and the test temperature is 35 DEG C.

[0067] Table 2 Comparison of material properties

[0068]

[0069] As can be seen from Table 2, compared with Comparative Example 1 and Comparative Example 2, the yield strength of the austenitic stainless steel prepared by Example 1 to Example 3 is slightly higher, and the tensile strength is obviously improved compared with the comparative steel; from the yield strength and tensile strength of the material after 25% deformation, it can be seen that the work hardening of the austenitic stainless steel in Example 1 to Example 3 is more obvious; from the pitting rate, the corrosion resistance of the prepared austenitic stainless steel is basically equivalent to that of the comparative example.

[0070] The application has been disclosed in the foregoing by preferred embodiments, but those skilled in the art should understand that these embodiments are only used to depict the application and should not be understood as limiting the scope of the application. It should be noted that any changes and substitutions equivalent to these embodiments should be considered as covered by the scope of the claims of the application. Therefore, the protection scope of the application should be subject to the scope defined in the claims.

Claims

1. A type of easily machinable, highly hardening austenitic stainless steel, characterized in that, By weight percentage, the composition is as follows: C: 0.038%~0.12%, Si≤1.00%, Mn: 2.50%~4.50%, P≤0.045%, S≤0.030%, Cr: 16.00%~16.9%, Ni: 5.50%~6.28%, N: 0.04%~0.09%, Cu: 0.02%~0.04%, with the remainder being Fe and unavoidable impurities; The preparation method of the easily machinable high-hardness austenitic stainless steel includes the following steps: smelting step → continuous casting step → hot rolling step → hot annealing and pickling step → cold rolling step → cold annealing and pickling step → leveling step → slitting step. In the hot rolling process, the heating temperature of the slab before continuous casting is controlled between 1200 and 1300°C, and the heating time is 9.5 to 12.5 min / 10 mm; the rolling temperature of the hot continuous rolling process is controlled above 950°C; in the cold rolling process, the total deformation is 40 to 70%; in the cold annealing and pickling process, the annealing temperature is controlled between 1050 and 1150°C. Among them, the metal stacking fault energy γ of the austenitic stainless steel SFE 24~29 J / m 2 .

2. The easily machinable, highly hardening austenitic stainless steel according to claim 1, characterized in that, The composition by weight percentage is: C: 0.038%~0.07%, Si: 0.2%~0.5%, Mn: 3.0%~4.0%, P≤0.045%, S≤0.030%, Cr: 16.5%~16.9%, Ni: 6.0%~6.28%, N: 0.04%~0.09%, Cu: 0.02%~0.04%, with the remainder being Fe and unavoidable impurities.

3. A method for preparing easily machinable, highly hardening austenitic stainless steel, characterized in that, The process includes the following steps: smelting process → continuous casting process → hot rolling process → hot annealing and pickling process → cold rolling process → cold annealing and pickling process → leveling process → slitting process; wherein, the composition of the molten steel obtained in the smelting process is the same as that of the easy-to-process high-hardening austenitic stainless steel according to any one of claims 1 to 2. In the hot rolling process, the heating temperature of the slab before continuous casting is controlled between 1200 and 1300°C, and the heating time is 9.5 to 12.5 min / 10 mm; the rolling temperature of the hot continuous rolling process is controlled above 950°C; in the cold rolling process, the total deformation is 40 to 70%; in the cold annealing and pickling process, the annealing temperature is controlled between 1050 and 1150°C.

4. The method for preparing easily machinable, highly hardening austenitic stainless steel according to claim 3, characterized in that, In the hot annealing and pickling process, the annealing temperature is controlled between 1100 and 1200°C.

Images