Economical free-cutting high-formability austenitic stainless steel

Abstract

The invention relates to the technical field of preparation of special austenitic stainless steel, in particular to economical free-cutting high-formability austenitic stainless steel and a preparation method thereof. The economical free-cutting high-formability austenitic stainless steel is prepared from the following components: 5.5 to 6.8 percent of Ni, 1.5 to 3.5 percent of Mn, 16.5 to 18.5 percent of Cr, 0.15 to 0.20 percent of S, 0.8 to 1.5 percent of Cu, 0.01 to 0.03 percent of RE, 0.12 to 0.25 percent of Ca, 0.002 to 0.005 percent of Mg, less than or equal to 0.08 percent of C, less than or equal to 1.0 percent of Si, less than or equal to 0.05 percent of N and the balance of iron and inevitable impurities. According to the invention, the nickel content is reduced to 5.5-6.8%, the material cost is reduced by 20-25%, the cutting force is obviously reduced, the service life of a cutter can be effectively prolonged, and the cutter has excellent mechanical properties and corrosion resistance.

Description

Technical Field

[0001] The present invention relates to the technical field of the preparation of special austenitic stainless steel, and in particular to an economical free-cutting and high-formability austenitic stainless steel. Background Art

[0002] Free-cutting stainless steel has excellent machinability and corrosion resistance, and is widely used in industries such as precision machining, fastener industry, automotive industry, precision instrument industry, aerospace, and household appliances.

[0003] At present, free-cutting stainless steel is mainly formed by adding free-cutting elements (S, P, Pb, Bi, etc.) to stainless steel series steel grades. Among them, sulfur-based free-cutting stainless steel and lead-based free-cutting stainless steel are widely used free-cutting stainless steel grades. Lead-based free-cutting steel has good cutting performance, but lead seriously pollutes the environment and has gradually been phased out by lead-free environmentally friendly free-cutting stainless steel.

[0004] For sulfur-based free-cutting steel to obtain excellent cutting performance, a relatively high sulfur content needs to be added. For example, the Special Steel Branch of Baosteel Co., Ltd. uses an electroslag furnace process to add ferrosulfur to produce sulfur-containing stainless steel, with the sulfur content controlled at 0.06 - 0.10%. Although the cutting performance is improved, the sulfur content control is unstable and the sulfur content is too low, resulting in mediocre cutting performance and unable to well meet the needs of the consumer market, with a narrow application field. Increasing the sulfur content in stainless steel can improve its cutting performance, but it will significantly reduce the mechanical properties of stainless steel, deteriorate the corrosion resistance, and sulfur segregation leads to hot rolling cracking and easy occurrence of hot working crack peeling defects, with a high product defect rate. It is not only difficult to achieve large-scale stable production, but also has a high production cost and a low market industrial value.

[0005] In summary, the existing free-cutting stainless steel has problems such as high nickel cost, poor cutting performance with low sulfur, and deteriorated formability of high-sulfur steel. There is an urgent need for an economical free-cutting and high-formability austenitic stainless steel. Summary of the Invention

[0006] In order to solve the problems of high nickel cost, poor cutting performance with low sulfur, and deteriorated formability of high-sulfur steel existing in the existing stainless steel, the present invention provides an economical free-cutting and high-formability austenitic stainless steel and its preparation method. The stainless steel in the present invention reduces nickel and increases sulfur while stabilizing the austenite structure, improving its cutting performance, formability, and corrosion resistance, and solving the contradiction problems of cutting performance and hot rolling cracking, cold formability, and material corrosion resistance.

[0007] An economical free-cutting and high-formability austenitic stainless steel provided by the present invention is achieved through the following technical solutions:

[0008] An economical free-cutting and high-formability austenitic stainless steel consists of the following components: 5.5% - 6.8% Ni, 1.5 - 3.5% Mn, 16.5 - 18.5% Cr, 0.15% - 0.20% S, 0.8% - 1.5% Cu, 0.01% - 0.03% RE, 0.002 - 0.004% Ca, 0.002 - 0.005% Mg, P ≤ 0.05%, C ≤ 0.08%, Si ≤ 1.0%, N ≤ 0.05%, and the balance is iron and unavoidable impurities.

[0009] Preferably, the RE element is cerium Ce and lanthanum La, and the mass ratio of cerium Ce to lanthanum La is 1:(0.5 - 2).

[0010] Preferably, the economical free-cutting and high-formability austenitic stainless steel consists of the following components: 6.0% - 6.5% Ni, 2.0 - 3.0% Mn, 16.8 - 17.5% Cr, 0.15% - 0.20% S, 1.0% - 1.5% Cu, 0.005% - 0.015% Ce, 0.005% - 0.015% La, 0.002 - 0.004% Ca, 0.003 - 0.005% Mg, P ≤ 0.05%, C ≤ 0.08%, Si ≤ 1.0%, N ≤ 0.05%, and the balance is iron and unavoidable impurities, and Ca / S = 0.8 - 1.2.

[0011] In the present invention, the nickel content is reduced to 5.5 - 6.8%, which can reduce the cost of the produced stainless steel while synergistically stabilizing austenite with sulfur, copper, and rare earth elements, thereby ensuring its good corrosion resistance and mechanical properties.

[0012] In the present invention, the sulfur content is 0.15% - 0.20%, forming spherical sulfides in the stainless steel matrix, which can improve the cutting performance of the stainless steel. By controlling the sulfur content, on the premise of ensuring the corrosion resistance and mechanical properties of the stainless steel, the cutting performance of the stainless steel can be effectively improved, and the service life of the cutting tool can be effectively extended.

[0013] In the present invention, the addition of an appropriate amount of copper element results in nano-precipitation strengthening in the stainless steel matrix, improving the problem of insufficient strength of low nickel, and thereby ensuring its good mechanical properties and corrosion resistance.

[0014] In the present invention, the addition of an appropriate amount of rare earth elements can purify the grain boundaries, improve the corrosion resistance, endow the stainless steel with good corrosion resistance, and improve the problem of hot working crack peeling defects.

[0015] Through the optimization of the stainless steel composition design and preparation process, in the specific formula, the coordinated combination of low nickel + high sulfur + copper / rare earth is used to control the sulfide morphology (Ca-Mg composite treatment), and the dynamic grain boundary regulation (three-stage rolling + gradient annealing) in the preparation process is combined, so that the prepared austenitic stainless steel achieves the following technical effects: ① The nickel content is reduced to 5.5-6.8% (30% lower than that of 304), and the material cost is reduced by 20-25%; ② The sulfur content is increased to 0.15-0.20%. Compared with 304 stainless steel, the cutting force of the austenitic stainless steel in the present invention is reduced by 45%, and the tool life is extended by more than 3 times; ③ The forming performance is excellent, the limiting drawing ratio LDR reaches 2.35, and the salt spray corrosion resistance is rust-free for 1200 hours, with excellent mechanical properties and corrosion resistance.

[0016] The preparation method of an economical and easy-to-cut high-formability austenitic stainless steel provided by the present invention is achieved through the following technical solutions:

[0017] A preparation method of an economical and easy-to-cut high-formability austenitic stainless steel includes the following steps:

[0018] Step 1, electric furnace smelting: Heat the scrap steel to 1600-1650 °C, melt it into molten iron, then add low-phosphorus molten iron and nickel plates, maintain melting at 1600-1650 °C for 5-15 min, P≤0.05%, control the Ni content to 5.5%-6.8%, then add manganese ingots and copper ingots, maintain melting at 1600-1650 °C for 5-15 min, control the Mn content to 1.5-3.5%, control the Cu content to 0.8%-1.5%, and the carbon content: C≤0.08%;

[0019] Step 2, AOD refining: Adjust the temperature to 1580-1600 °C, blow in an Ar / O2 mixed gas, and the volume ratio of O2 to Ar in the Ar / O2 mixed gas is (3-5):(5-7), decarburize to C≤0.03%, and then add ferrosulfur alloy at least three times at intervals, and control the total sulfur content to 0.15-0.20%;

[0020] Step 3, calcium-magnesium composite treatment in LF furnace: Adjust the temperature to 1550-1580 °C, feed in a calcium-silicon alloy wire, and calculate the addition amount of the calcium-silicon alloy wire according to the ratio of Ca / S of 0.8-1.2, then add a magnesium wire, control the magnesium content to 0.002-0.005%, feed in a rare earth calcium-silicon alloy cored wire, control the RE content to 0.01%-0.03%, control the calcium content to 0.002-0.004%, and finally blow argon from the bottom of the ladle, with the argon flow rate of 20-30 L / min and the soft blowing argon time ≥10 min;

[0021] Step 4, continuous casting: Adjust the molten steel temperature to 1550 - 1560 °C, casting speed: 0.8 - 1.2 m / min, mold oscillation: amplitude 4 - 6 mm, frequency 120 - 150 times / min, secondary cooling water volume: 0.8 - 1.2 L / kg of molten steel, electromagnetic stirring: frequency 3 - 5 Hz, current 200 - 300 A;

[0022] Step 5, three-stage hot rolling: Pass through high-temperature large-deformation rough rolling, low-temperature finish rolling, and laminar cooling in sequence to obtain rough products;

[0023] Step 6, perform gradient annealing on the rough products to obtain economic free-cutting high-formability austenitic stainless steel finished products.

[0024] The preparation method of the present invention is relatively simple and mature, with a high yield rate, and is convenient for large-scale manufacturing.

[0025] Preferably, in step 2, ferrosulfur alloy is added in three intervals, with an interval time of 8 - 10 minutes. For the first addition, 40 wt% ferrosulfur alloy is added, and the C content is controlled at 0.15 - 0.20%. For the second addition, 30 wt% ferrosulfur alloy is added, and the C content is controlled at 0.05 - 0.08%. For the third addition, 30 wt% ferrosulfur alloy is added, and the C content is controlled at ≤0.03%. Chromium ingots and nickel blocks are supplemented and added, and the composition is finely adjusted so that the Ni content is controlled at 5.5% - 6.8%, and the Cr content is controlled at 16.5% - 18.5%.

[0026] The functions of adding ferrosulfur alloy in three intervals in the preparation method of the present invention are as follows: ① Prevent sulfur segregation (traditional single addition of sulfur easily leads to sulfur enrichment in local areas, forming coarse MnS inclusions, and the aspect ratio of the formed MnS inclusions > 5, causing hot rolling cracks); ② Improve sulfur recovery rate; ③ Make sulfur evenly distributed (adding sulfur in batches can make sulfur evenly distributed. Combining with subsequent calcium and magnesium treatment, fine spherical sulfides are formed, with an aspect ratio < 2).

[0027] Preferably, in step 3, the feeding speed of calcium-silicon alloy wire is 3 - 5 m / min, the feeding speed of magnesium wire is 1 - 2 m / min, and the feeding speed of rare earth calcium-silicon alloy cored wire is 2 - 4 m / min.

[0028] The advantages of calcium and magnesium composite treatment in the LF furnace in the preparation method of the present invention are as follows:

[0029] (1) The core functions of calcium treatment are sulfide morphology control and inhibition of sulfur segregation. Sulfide morphology control: Calcium (Ca) combines with sulfur (S) to form CaS, and forms (Ca,Mn)S composite inclusions with MnS. Its technical effect: The sulfide changes from long strip shape (the aspect ratio of traditional MnS > 5) to spherical shape (the aspect ratio < 2), reducing stress concentration, that is, the average size of inclusions is refined from the traditional 5 - 10 μm to 1 - 3 μm. Inhibition of sulfur segregation: The strong desulfurization ability of calcium reduces the free sulfur content, inhibits the segregation of sulfur at grain boundaries, and avoids hot rolling cracking.

[0030] (2) The synergistic effects of magnesium treatment are composite sulfide modification and deoxidation purification. Composite sulfide modification: Magnesium (Mg) further reacts with CaS to form (Ca,Mg,Mn)S ternary composite inclusions. Its technical effect: The melting point of inclusions increases (from 1610 °C of traditional MnS to 1750 °C), and it does not deform during hot rolling. And the distribution of inclusions is more dispersed, and the number density increases to 2000 - 3000 per mm, while the number density of the traditional process is only 500 - 800 per mm. Deoxidation purification: Magnesium combines with free oxygen in the steel to form MgO, reducing oxide inclusions (oxygen content ≤ 30 ppm).

[0031] (3) Calcium-magnesium ratio design: Ca / S = 0.8 - 1.2. Insufficient calcium (Ca / S < 0.8): The sulfide is still mainly MnS, and the morphology control fails; Excessive calcium (Ca / S > 1.2): High-melting-point CaO - CaS composite inclusions are formed, deteriorating the cutting performance. And the addition amount of Mg is 0.002 - 0.005%, Insufficient magnesium: Unable to form a stable (Ca,Mg,Mn)S phase; Excessive magnesium (> 0.005%): Causes nitrogen increase in the molten steel (Mg reacts with N2 in the air).

[0032] Single calcium treatment can only partially modify sulfides, and the addition of magnesium further optimizes through the following mechanisms: Reducing sulfur activity: Mg combines with S to reduce the free sulfur content and reduce the tendency of sulfur segregation. Size refinement: The strong deoxidation ability of Mg inhibits the growth of inclusions. High-temperature stability: Ternary sulfides remain spherical at rolling temperature, avoiding elongation.

[0033] Preferably, the water volume in the secondary cooling zone in step four is cooled in 4 segments, including the full roll segment, the first segment of the segment, the second segment of the segment, and the third segment of the segment. The cooling water flow rate in the full roll segment is 0.4 - 0.6 L / kg of molten steel, the cooling water flow rate in the first segment of the segment is 0.35 - 0.45 L / kg of molten steel, the cooling water flow rate in the second segment of the segment is 0.25 - 0.35 L / kg of molten steel, and the cooling water flow rate in the third segment of the segment is 0.2 - 0.3 L / kg of molten steel.

[0034] The 4-stage cooling method in the present invention can control the temperature at the solidification front, reduce internal stress, with a cooling rate ≤ 100 °C / min, prevent cracks, and is beneficial to improving the processing yield. In the present invention, the superheat of the molten steel is 12 - 22 °C, which inhibits the growth of columnar crystals and reduces central segregation. In the present invention, the mold oscillation: amplitude 4 - 6 mm, frequency 120 - 150 times / min, can improve lubrication and prevent surface cracks.

[0035] Preferably, in step four, the electromagnetic stirring is divided into mold electromagnetic stirring M-EMS and final electromagnetic stirring F-EMS. In the mold electromagnetic stirring M-EMS, the frequency is 3 - 5 Hz and the current is 200 - 250 A; in the final electromagnetic stirring F-EMS, the frequency is 2 - 3 Hz and the current is 250 - 300 A.

[0036] By adopting the above technical solutions, equiaxed crystals can be refined, the uniformity of sulfide distribution can be improved, and thus the overall mechanical properties and corrosion resistance can be improved.

[0037] Preferably, the specific operation of the high-temperature large-deformation rough rolling in step five is as follows: The slab is heated to 1150 - 1200 °C, held for 2 - 4 h, rolled in 5 - 7 passes, with a total deformation of 50 - 60% and a single-pass deformation of 15 - 20%; the rolling temperature is controlled at 1100 - 1150 °C.

[0038] By adopting the above technical solutions, if the temperature < 1100 °C or the deformation < 50%, dynamic recrystallization is incomplete, and residual coarse grains lead to cracking during subsequent rolling. The large deformation makes the sulfides (Ca, Mg, Mn)S in the continuous casting billet further disperse, and the spacing shrinks from 50 μm to 20 - 30 μm, improving hot rolling cracking.

[0039] Preferably, the specific operation of the low-temperature finish rolling in step five is as follows: The finish rolling temperature is 900 - 950 °C, the total deformation is 30 - 40%, the single-pass deformation is 8 - 12%, and the final rolling thickness is 2 - 3 mm.

[0040] By adopting the above technical solutions, if the temperature > 950 °C: the grains grow excessively and the strength decreases; low-temperature rolling inhibits dynamic recrystallization, induces static recrystallization through strain accumulation, refines the grains, and lays a good foundation for the subsequent improvement of annealing mechanics.

[0041] Preferably, the specific operation of the laminar cooling in step five is as follows: After water cooling to 700 °C, it is input into the coiling equipment for coiling to obtain the rough steel coil.

[0042] Preferably, the specific operation of the gradient annealing in step six is as follows: Rapidly heat up to 1100 °C and hold for 5 min, then slowly cool to 950 °C at 5 - 40 °C / min and hold for 15 - 30 min, and then water cool to room temperature to complete the gradient annealing.

[0043] The function of rapidly heating to 1100 °C and holding for 5 min in the gradient annealing of the present invention is to achieve complete austenitization and eliminate work hardening; the function of slowly cooling to 950 °C at 5 - 40 °C / min and holding for 15 - 30 min is to promote the formation of annealing twins and regulate the grain boundary type; the function of water cooling to room temperature with a cooling rate ≥ 50 °C / s is to rapidly cool, fix the microstructure, and inhibit grain growth.

[0044] It should be noted that 950 °C is crucial because when the temperature > 1000 °C: the proportion of twin boundaries decreases and the grains coarsen; while when the temperature < 900 °C: the kinetics of twin formation is insufficient, the proportion of small-angle grain boundaries is low, and other substances are prone to precipitate. Copper (0.8 - 1.5%) forms nano-scale ε-Cu precipitation phases through gradient annealing, improving the tensile strength and taking into account both high strength and high formability. After gradient annealing, the LDR of the stainless steel can be increased to 2.35.

[0045] In summary, the present invention has the following advantages:

[0046] 1. In the present invention, the nickel content is reduced to 5.5 - 6.8%, the material cost is reduced by 20 - 25%, the cutting force is significantly reduced, the tool life can be effectively extended, and it has excellent mechanical properties and corrosion resistance.

[0047] 2. In the present invention, through sulfide control, spherical (Ca, Mg, Mn)S is formed, reducing hot rolling cracking, effectively improving the yield rate, and at the same time improving the cutting performance and corrosion resistance.

[0048] 3. After gradient annealing provided in the present invention, the LDR of the stainless steel can be increased to 2.35, which can effectively improve the limiting drawing ratio and forming performance of the stainless steel, further improve the yield rate, and reduce the overall production cost.

[0049] 4. The preparation method of the present invention is relatively simple and mature, with a high yield rate, and is convenient for large-scale manufacturing. BRIEF DESCRIPTION OF THE DRAWINGS

[0050] Figure 1 It is a TEM bright-field image of nano ε-Cu precipitation phases in the austenitic stainless steel of Example 1.

[0051] Figure 2 It is an SEM photograph of spherical sulfides in the austenitic stainless steel of Example 1. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0052] In order to further understand the creativity and technological progress of the present invention, the preferred implementation schemes of the present invention will be discussed in detail below in combination with examples and comparative examples.

[0053] Example: An economical free-cutting and high-formability austenitic stainless steel consists of the following components: 5.5% - 6.8% Ni, 1.5 - 3.5% Mn, 16.5 - 18.5% Cr, 0.15% - 0.20% S, 0.8% - 1.5% Cu, 0.01% - 0.04% RE, 0.002 - 0.004% Ca, 0.002 - 0.005% Mg, P ≤ 0.05%, C ≤ 0.08%, Si ≤ 1.0%, N ≤ 0.05%, and the balance is iron and inevitable impurities. The RE element is cerium Ce and lanthanum La, the mass ratio of cerium Ce to lanthanum La is 1:(0.5 - 2), and Ca / S = 0.8 - 1.2.

[0054] Preferably, the economical free-cutting and high-formability austenitic stainless steel consists of the following components: 6.0% - 6.5% Ni, 2.0 - 3.0% Mn, 16.8 - 17.5% Cr, 0.15% - 0.20% S, 1.0% - 1.5% Cu, 0.005% - 0.015% Ce, 0.005% - 0.015% La, 0.002 - 0.004% Ca, 0.003 - 0.005% Mg, P ≤ 0.05%, C ≤ 0.08%, Si ≤ 1.0%, N ≤ 0.05%, and the balance is iron and inevitable impurities.

[0055] A preparation method of an economical free-cutting and high-formability austenitic stainless steel, comprising the following steps:

[0056] Step 1, electric furnace smelting: Heat the scrap steel to 1600 - 1650 °C, melt it into molten iron, then add low-phosphorus molten iron and nickel plates, maintain the melting at 1600 - 1650 °C for 5 - 15 min, with P ≤ 0.05% and the Ni content controlled at 5.5% - 6.8%. Subsequently, add manganese ingots and copper ingots, maintain the melting at 1600 - 1650 °C for 5 - 15 min, with the Mn content controlled at 1.5 - 3.5% and the Cu content controlled at 0.8% - 1.5%, and the carbon content: C ≤ 0.08%.

[0057] Step 2, AOD refining: Adjust the temperature to 1580 - 1600 °C, blow in an Ar / O2 mixed gas, where the volume ratio of O2 to Ar in the Ar / O2 mixed gas is (3 - 5):(5 - 7), decarburize until C ≤ 0.03%, and then add ferrosulfur alloy at least three times at intervals, with the total sulfur content controlled at 0.15 - 0.20%.

[0058] Preferably, in step two, ferrosulfur alloy is added in three intervals with an interval time of 8 - 10 minutes. For the first addition, 40wt% ferrosulfur alloy is added, and the C content is controlled at 0.15 - 0.20%. For the second addition, 30wt% ferrosulfur alloy is added, and the C content is controlled at 0.05 - 0.08%. For the third addition, 30wt% ferrosulfur alloy is added, and the C content is controlled at ≤0.03%. Chrome ingots and nickel blocks are supplemented and added, and the composition is finely adjusted so that the Ni content is controlled at 5.5% - 6.8% and the Cr content is controlled at 16.5% - 18.5%.

[0059] Step three, calcium - magnesium composite treatment in the LF furnace: Adjust the temperature to 1550 - 1580°C, feed silicon - calcium alloy wire with a feeding speed of 3 - 5m / min. The addition amount of the silicon - calcium alloy wire is calculated according to the ratio of Ca / S of 0.8 - 1.2. Subsequently, add magnesium wire with a feeding speed of 1 - 2m / min, and the magnesium content is controlled at 0.002 - 0.005%. Then feed rare - earth silicon - calcium alloy cored wire with a feeding speed of 2 - 4m / min, the RE content is controlled at 0.01% -  0.03%, and the calcium content is controlled at 0.002 - 0.004%. Finally, blow argon from the bottom of the ladle with an argon flow rate of 20 - 30L / min, and the soft blowing argon time is ≥10min.

[0060] Step four, continuous casting: Adjust the molten steel temperature to 1550 - 1560°C, casting speed: 0.8 - 1.2m / min, mold oscillation: amplitude 4 - 6mm, frequency 120 - 150 times / min, water volume in the secondary cooling zone: 0.8 - 1.2L / kg of molten steel. The water volume in the secondary cooling zone is cooled in 4 sections, including the dummy bar section, the first segment of the segmental mold, the second segment of the segmental mold, and the third segment of the segmental mold. The cooling water flow rate in the dummy bar section is 0.4 - 0.6L / kg of molten steel, the cooling water flow rate in the first segment of the segmental mold is 0.35 - 0.45L / kg of molten steel, the cooling water flow rate in the second segment of the segmental mold is 0.25 - 0.35L / kg of molten steel, and the cooling water flow rate in the third segment of the segmental mold is 0.2 - 0.3L / kg of molten steel. Electromagnetic stirring: frequency 3 - 5Hz, current 200 - 300A. Specifically, electromagnetic stirring is divided into mold electromagnetic stirring M - EMS and final electromagnetic stirring F - EMS. In the mold electromagnetic stirring M - EMS, the frequency is 3 - 5Hz and the current is 200 - 250A; in the final electromagnetic stirring F - EMS, the frequency is 2 - 3Hz and the current is 250 - 3I00A.

[0061] Step five, three - stage hot rolling: successively pass through high - temperature large - deformation rough rolling, low - temperature finish rolling, and laminar cooling to obtain semi - finished products.

[0062] The specific operation of high - temperature large - deformation rough rolling is as follows: The slab is heated to 1150 - 1200°C, held for 2 - 4h, rolled through 5 - 7 passes, with a total deformation of 50 - 60% and a single - pass deformation of 15 - 20%; the rolling temperature is controlled at 1100 - 1150°C.

[0063] The specific operation of cold finishing rolling is as follows: the finishing rolling temperature is 900 - 950 °C, the total reduction is 30 - 40%, the reduction per pass is 8 - 12%, and the final rolling thickness is 2 - 3 mm;

[0064] The specific operation of laminar cooling is as follows: after water cooling to 700 °C, it is coiled in a coiling device to obtain a semi-finished steel coil;

[0065] Step six, gradient annealing of the semi-finished steel coil, the specific operation is as follows: quickly heat up to 1100 °C and hold for 5 min, then slowly cool to 950 °C at 5 - 40 °C / min and hold for 15 - 30 min, water cool to room temperature, and complete the gradient annealing to obtain the finished product of economic free-cutting and high formability austenitic stainless steel.

[0066] The raw material sources are shown in Table 1:

[0067] Table 1 is the raw material source table of economic free-cutting and high formability austenitic stainless steel

[0068]

[0069] Example 1: A preparation method of economic free-cutting and high formability austenitic stainless steel, the steps are as follows:

[0070] Step one, electric furnace smelting: heat up the scrap steel to 1650 °C, melt it into molten iron, then add low-phosphorus molten iron and nickel plates, maintain melting at 1650 °C for 10 min, P ≤ 0.05%, detect the Ni content as 5.51%, then add manganese ingots and copper ingots, maintain melting at 1650 °C for 10 min, detect Mn as 3.15%, detect the Cu content as 1.47%, and detect the carbon content as 0.072%;

[0071] Step two, AOD refining: adjust the temperature to 1590 °C, blow in an Ar / O2 mixed gas, the volume ratio of O2 to Ar in the Ar / O2 mixed gas is 4:6, after decarburization for 30 min, detect the carbon content as 0.027%, then add ferrosulfur alloy in three intervals with an interval of 10 minutes. The first time, add 40 wt% ferrosulfur alloy, after melting for 5 min, detect the carbon content as 0.187%, after an interval of 10 min, add 30 wt% ferrosulfur alloy for the second time, after melting for 5 min, detect the carbon content as 0.069%, after an interval of 10 min, add 30 wt% ferrosulfur alloy for the third time, after melting for 15 min, detect the carbon content as 0.016%, supplement and add chromium ingots and nickel blocks for fine adjustment of composition, after melting for 10 min, detect the Ni content as 5.54%, the Cr content as 17.46%, and the S content as 0.196%;

[0072] Step 3, Ca-Mg composite treatment in LF furnace: Adjust the temperature to 1565°C, feed in calcium-silicon alloy wire with a calcium content of 30 wt% at a feeding speed of 4 m / min. The addition amount of calcium-silicon alloy wire is calculated according to the ratio of Ca / S = 1.2:1. Subsequently, feed in magnesium wire with a purity of 99.95% at a feeding speed of 1 m / min. Detect that the magnesium content is 0.004%. Then feed in rare earth calcium-silicon alloy cored wire with calcium-silicon alloy as the carrier, Ce content of 18 wt%, La of 12 wt%, and calcium-silicon alloy content of 70 wt% at a feeding speed of 2.5 m / min. Detect that the Ce content is 0.015%, La content is 0.010%, and calcium content is 0.004%. Finally, blow argon from the bottom of the ladle at an argon flow rate of 25 L / min for a soft argon blowing time of 12 min;

[0073] Step 4, continuous casting: Adjust the molten steel temperature to 1550°C, casting speed: 1 m / min, mold oscillation: amplitude 5 mm, frequency 120 times / min. The water volume in the secondary cooling zone is cooled in 4 sections including the foot roll section, the first segment, the second segment, and the third segment. The cooling water flow rate in the foot roll section is 0.5 L / kg of molten steel, in the first segment is 0.4 L / kg of molten steel, in the second segment is 0.3 L / kg of molten steel, and in the third segment is 0.25 L / kg of molten steel. The electromagnetic stirring is divided into mold electromagnetic stirring M-EMS and final electromagnetic stirring F-EMS. In the mold electromagnetic stirring M-EMS, the frequency is 5 Hz and the current is 250 A; in the final electromagnetic stirring F-EMS, the frequency is 3 Hz and the current is 300 A;

[0074] Step 5, three-stage hot rolling: Pass through high-temperature large-deformation rough rolling, low-temperature finish rolling, and laminar cooling to obtain semi-finished products;

[0075] The specific operation of high-temperature large-deformation rough rolling is as follows: Heat the slab to 1180°C, hold for 150 min, roll through 6 passes with a total deformation of 60% and a single-pass deformation of 15%, and control the rolling temperature at 1140°C;

[0076] The specific operation of low-temperature finish rolling is as follows: The finish rolling temperature is 950°C, roll through 4 passes with a total deformation of 36% and a single-pass deformation of 9%, and the final rolling thickness is 2.5 ± 0.05 mm;

[0077] The specific operation of laminar cooling is as follows: Cool to 700°C by water cooling at a cooling rate of 50°C / s, and then transfer it to the coiling equipment for coiling. The coiling temperature in the coiling section is 645°C, and semi-finished steel coils can be obtained;

[0078] Step 6: Gradient annealing is performed on the semi-finished steel coil, and the specific operation is as follows: rapidly heat up to 1100 °C and hold for 5 min, then slowly cool to 950 °C at a furnace cooling rate of 10 °C / min and hold for 20 min, and then water-cool to room temperature to complete the gradient annealing to obtain the finished product of economical free-cutting high-formability austenitic stainless steel.

[0079] The difference between Example 2 and Example 1 lies in Step 1 of the preparation method of economical free-cutting high-formability austenitic stainless steel: electric furnace smelting: heat the scrap steel to 1650 °C, melt it into molten iron, then add low-phosphorus molten iron and nickel plates, maintain melting at 1650 °C for 10 min, with P ≤ 0.05%, detect the Ni content to be 6.14%, then add manganese ingots and copper ingots, maintain melting at 1650 °C for 10 min, detect Mn to be 2.21%, detect the Cu content to be 1.05%, and detect the carbon content to be 0.075%;

[0080] Step 2: AOD refining: adjust the temperature to 1590 °C, blow in an Ar / O2 mixed gas, the volume ratio of O2 to Ar in the Ar / O2 mixed gas is 4:6, after decarburization for 30 min, detect the carbon content to be 0.025%, then add ferrosulfur alloy in three intervals with an interval time of 10 minutes. The first time, add 40 wt% ferrosulfur alloy, detect the carbon content to be 0.185% after melting for 5 min, add 30 wt% ferrosulfur alloy for the second time after an interval of 10 min, detect the carbon content to be 0.067% after melting for 5 min, add 30 wt% ferrosulfur alloy for the third time after an interval of 10 min, detect the carbon content to be 0.015% after melting for 15 min, supplement and add chromium ingots and nickel blocks for fine composition adjustment, detect the Ni content to be 6.19%, the Cr content to be 17.08%, and the S content to be 0.176% after melting for 10 min;

[0081] Step 3: Calcium-magnesium composite treatment in the LF furnace: adjust the temperature to 1565 °C, feed in a calcium-silicon alloy wire with a calcium content of 30 wt% in the calcium-silicon alloy wire at a feeding speed of 4 m / min, and calculate the feeding amount of the calcium-silicon alloy wire according to the ratio of Ca / S = 1:1. Then add a magnesium wire with a purity of 99.95% at a feeding speed of 1 m / min, detect the magnesium content to be 0.004%, and then feed in a rare earth calcium-silicon alloy cored wire with calcium-silicon alloy as the carrier, with a cerium Ce content of 18 wt% and a lanthanum La content of 12 wt%, and a calcium-silicon alloy content of 70 wt%, at a feeding speed of the rare earth calcium-silicon alloy cored wire of 2.5 m / min, detect the Ce content to be 0.012%, the La content to be 0.008%, and the calcium content to be 0.003%. Finally, blow in argon from the bottom of the ladle at an argon flow rate of 25 L / min and soft blow argon for 12 min.

[0082] Example 3 is different from Example 1 in that in Step 1 of the preparation method of the economical free-cutting and high-formability austenitic stainless steel, electric furnace smelting: The scrap steel is heated to 1650 °C, melted into molten iron, and then low-phosphorus molten iron and nickel plates are added. It is maintained at 1650 °C and melted for 10 min, P≤0.05%, the Ni content is detected to be 6.78%, then manganese ingots and copper ingots are added, maintained at 1650 °C and melted for 10 min, Mn is detected to be 1.87%, the Cu content is detected to be 0.89%, and the carbon content is detected to be 0.081%;

[0083] Step 2, AOD refining: Adjust the temperature to 1590 °C, blow in an Ar / O2 mixed gas, and the volume ratio of O2 to Ar in the Ar / O2 mixed gas is 4:6. After decarburization for 35 min, the carbon content is detected to be 0.025%. Subsequently, ferrosulfur alloy is added in three intervals, with an interval of 10 minutes. For the first time, 40 wt% ferrosulfur alloy is added. After melting for 5 min, the carbon content is detected to be 0.185%. After an interval of 10 min, 30 wt% ferrosulfur alloy is added for the second time. After melting for 5 min, the carbon content is detected to be 0.069%. After an interval of 10 min, 30 wt% ferrosulfur alloy is added for the third time. After melting for 20 min, the carbon content is detected to be 0.016%. Supplementary addition of chromium ingots and nickel blocks is carried out for fine adjustment of the composition. After melting for 10 min, the Ni content is detected to be 6.78%, the Cr content is 16.67%, and the S content is 0.165%;

[0084] Step 3, LF furnace calcium-magnesium composite treatment: Adjust the temperature to 1565 °C, feed in a calcium-silicon alloy wire, the calcium content in the calcium-silicon alloy wire is 30 wt%, the feeding speed is 4 m / min, and the addition amount of the calcium-silicon alloy wire is calculated according to the ratio of Ca / S = 0.8:1. Subsequently, a magnesium wire with a purity of 99.95% is added, the feeding speed is 1 m / min, the magnesium content is detected to be 0.004%, and then a rare earth calcium-silicon alloy cored wire is fed. The rare earth calcium-silicon alloy cored wire uses calcium-silicon alloy as the carrier, the Ce content is 18 wt% and the La content is 12 wt%, and the calcium-silicon alloy content is 70%. The feeding speed of the rare earth calcium-silicon alloy cored wire is 2.5 m / min, the Ce content is detected to be 0.009%, the La content is 0.006%, the calcium content is 0.002%, and finally argon is blown in from the bottom of the ladle, the argon flow rate is 25 L / min, and the soft argon blowing time is 12 min.

[0085] Example 4 is different from Example 1 in that in Step 1 of the preparation method of the economical free-cutting and high-formability austenitic stainless steel, electric furnace smelting: The scrap steel is heated to 1650 °C, melted into molten iron, and then low-phosphorus molten iron and nickel plates are added. It is maintained at 1650 °C and melted for 10 min, P≤0.05%, the Ni content is detected to be 6.65%, then manganese ingots and copper ingots are added, maintained at 1650 °C and melted for 10 min, Mn is detected to be 2.48%, the Cu content is detected to be 1.36%, and the carbon content is detected to be 0.081%;

[0086] Step 2, AOD refining: Adjust the temperature to 1590 °C, blow in an Ar / O2 mixed gas, the volume ratio of O2 to Ar in the Ar / O2 mixed gas is 4:6. After decarburizing for 35 minutes, the carbon content is detected to be 0.026%. Subsequently, ferrosilicon alloy is added in three intervals with an interval of 10 minutes. For the first addition, 40 wt% ferrosilicon alloy is added. After melting for 5 minutes, the carbon content is detected to be 0.189%. After an interval of 10 minutes, 30 wt% ferrosilicon alloy is added for the second time. After melting for 5 minutes, the carbon content is detected to be 0.067%. After an interval of 10 minutes, 30 wt% ferrosilicon alloy is added for the third time. After melting for 20 minutes, the carbon content is detected to be 0.015%. Supplementary addition of chromium ingots and nickel blocks is carried out for fine adjustment of the composition. After melting for 10 minutes, the Ni content is detected to be 6.78%, the Cr content is 17.25%, and the S content is 0.188%.

[0087] Step 3, calcium-magnesium composite treatment in LF furnace: Adjust the temperature to 1565 °C, feed in a calcium-silicon alloy wire with a calcium content of 30 wt% at a feeding speed of 4 m / min. The addition amount of the calcium-silicon alloy wire is calculated according to the ratio of Ca / S = 1:1. Subsequently, a magnesium wire with a purity of 99.95% is added at a feeding speed of 1 m / min. The magnesium content is detected to be 0.004%. Then, a rare-earth calcium-silicon alloy cored wire is fed in. The rare-earth calcium-silicon alloy cored wire uses calcium-silicon alloy as the carrier, with a cerium Ce content of 18 wt% and a lanthanum La content of 12 wt%, and a calcium-silicon alloy content of 70 wt%. The feeding speed of the rare-earth calcium-silicon alloy cored wire is 2.5 m / min. The Ce content is detected to be 0.018%, the La content is 0.012%, and the calcium content is 0.003%. Finally, argon is blown in from the bottom of the ladle at an argon flow rate of 25 L / min for 12 minutes of soft argon blowing.

[0088] The difference between Example 5 and Example 1 lies in: In Step 1 of the preparation method of economic free-cutting high-formability austenitic stainless steel, electric furnace smelting: The scrap steel is heated to 1650 °C, melted into molten iron, and then low-phosphorus molten iron and nickel plates are added. Maintain melting at 1650 °C for 10 minutes, with P ≤ 0.05%. The Ni content is detected to be 5.51%. Subsequently, manganese ingots and copper ingots are added. Maintain melting at 1650 °C for 10 minutes. The Mn is detected to be 3.15%, the Cu content is detected to be 0.83%, and the carbon content is detected to be 0.081%.

[0089] Step 2, AOD refining: Adjust the temperature to 1590 °C, blow in an Ar / O2 mixed gas, where the volume ratio of O2 to Ar in the Ar / O2 mixed gas is 4:6. After decarburizing for 35 minutes, the carbon content is detected to be 0.026%. Subsequently, ferrosilicon alloy is added in three intervals with an interval time of 10 minutes. For the first time, 40 wt% ferrosilicon alloy is added. After melting for 5 minutes, the carbon content is detected to be 0.191%. After an interval of 10 minutes, 30 wt% ferrosilicon alloy is added for the second time. After melting for 5 minutes, the carbon content is detected to be 0.071%. After an interval of 10 minutes, 30 wt% ferrosilicon alloy is added for the third time. After melting for 20 minutes, the carbon content is detected to be 0.016%. Chrome ingots and nickel blocks are supplemented and added for fine composition adjustment. After melting for 10 minutes, the Ni content is detected to be 5.54%, the Cr content is 17.46%, and the S content is 0.195%.

[0090] Step 3, Ca-Mg composite treatment in LF furnace: Adjust the temperature to 1565 °C, feed in a calcium-silicon alloy wire with a calcium content of 30 wt% at a feeding speed of 4 m / min. The addition amount of the calcium-silicon alloy wire is calculated according to the ratio of Ca / S = 1.2:1. Subsequently, a magnesium wire with a purity of 99.95% is added at a feeding speed of 1 m / min. The magnesium content is detected to be 0.004%. Then, a rare earth calcium-silicon alloy cored wire is fed in. The rare earth calcium-silicon alloy cored wire uses calcium-silicon alloy as the carrier, with a cerium Ce content of 18 wt% and a lanthanum La content of 12 wt%, and a calcium-silicon alloy content of 70 wt%. The feeding speed of the rare earth calcium-silicon alloy cored wire is 2.5 m / min. The Ce content is detected to be 0.018%, the La content is 0.012%, and the calcium content is 0.004%. Finally, argon is blown in from the bottom of the ladle at an argon flow rate of 25 L / min for a soft argon blowing time of 12 minutes.

[0091] The difference between Example 6 and Example 1 lies in: In Step 1 of the preparation method of economic free-cutting high-formability austenitic stainless steel, electric furnace smelting: The scrap steel is heated to 1650 °C, melted into molten iron, and then low-phosphorus molten iron and nickel plates are added. It is maintained at 1650 °C and melted for 10 minutes, with P ≤ 0.05%. The Ni content is detected to be 5.51%. Subsequently, manganese ingots and copper ingots are added. It is maintained at 1650 °C and melted for 10 minutes. The Mn is detected to be 3.15%, the Cu content is detected to be 1.19%, and the carbon content is detected to be 0.081%.

[0092] Step 2, AOD refining: Adjust the temperature to 1590 °C, blow in an Ar / O2 mixed gas, the volume ratio of O2 to Ar in the Ar / O2 mixed gas is 4:6. After decarburization for 35 minutes, the carbon content is detected to be 0.026%. Subsequently, ferrosilicon alloy is added in three intervals with an interval time of 10 minutes. For the first time, 40 wt% ferrosilicon alloy is added. After melting for 5 minutes, the carbon content is detected to be 0.181%. After an interval of 10 minutes, 30 wt% ferrosilicon alloy is added for the second time. After melting for 5 minutes, the carbon content is detected to be 0.065%. After an interval of 10 minutes, 30 wt% ferrosilicon alloy is added for the third time. After melting for 20 minutes, the carbon content is detected to be 0.015%. Supplementary addition of chromium ingots and nickel blocks is carried out for fine adjustment of the composition. After melting for 10 minutes, the Ni content is detected to be 5.54%, the Cr content is 17.46%, and the S content is 0.196%.

[0093] Step 3, calcium-magnesium composite treatment in LF furnace: Adjust the temperature to 1565 °C, feed in a calcium-silicon alloy wire with a calcium content of 30 wt% at a feeding speed of 4 m / min. The addition amount of the calcium-silicon alloy wire is calculated and fed according to the ratio of Ca / S = 1.2:1. Subsequently, a magnesium wire with a purity of 99.95% is added at a feeding speed of 1 m / min. The magnesium content is detected to be 0.004%. Then, a rare-earth calcium-silicon alloy cored wire is fed. The rare-earth calcium-silicon alloy cored wire uses calcium-silicon alloy as the carrier, with a cerium Ce content of 18 wt% and a lanthanum La content of 12 wt%, and a calcium-silicon alloy content of 70 wt%. The feeding speed of the rare-earth calcium-silicon alloy cored wire is 2.5 m / min. The Ce content is detected to be 0.018%, the La content is 0.012%, and the calcium content is 0.004%. Finally, argon is blown in from the bottom of the ladle at an argon flow rate of 25 L / min for a soft argon blowing time of 12 minutes.

[0094] The difference between Example 7 and Example 1 lies in: Step 1 of the preparation method of economically easy-to-cut and high-formability austenitic stainless steel, electric furnace smelting: Heat the scrap steel to 1650 °C, melt it into molten iron, then add low-phosphorus molten iron and nickel plates, maintain melting at 1650 °C for 10 minutes, P ≤ 0.05%, detect the Ni content to be 5.51%. Subsequently, add manganese ingots and copper ingots, maintain melting at 1650 °C for 10 minutes, detect Mn to be 3.15%, detect the Cu content to be 1.47%, and detect the carbon content to be 0.074%.

[0095] Step 2, AOD refining: Adjust the temperature to 1590 °C, blow in an Ar / O2 mixed gas. The volume ratio of O2 to Ar in the Ar / O2 mixed gas is 4:6. After decarburization for 35 minutes, the carbon content is detected to be 0.025%. Subsequently, ferrosilicon alloy is added in three intervals with an interval of 10 minutes. For the first time, 40 wt% ferrosilicon alloy is added. After melting for 5 minutes, the carbon content is detected to be 0.183%. After an interval of 10 minutes, 30 wt% ferrosilicon alloy is added for the second time. After melting for 5 minutes, the carbon content is detected to be 0.066%. After an interval of 10 minutes, 30 wt% ferrosilicon alloy is added for the third time. After melting for 20 minutes, the carbon content is detected to be 0.015%. Supplementary addition of chromium ingots and nickel blocks is carried out for fine adjustment of the composition. After melting for 10 minutes, the Ni content is detected to be 5.54%, the Cr content is 17.46%, and the S content is 0.195%.

[0096] Step 3, Ca-Mg composite treatment in LF furnace: Adjust the temperature to 1565 °C, feed in a calcium-silicon alloy wire. The calcium content in the calcium-silicon alloy wire is 30 wt%. The feeding speed is 4 m / min. The feeding amount of the calcium-silicon alloy wire is calculated and fed according to the ratio of Ca / S = 1.2:1. Subsequently, a magnesium wire with a purity of 99.95% is added. The feeding speed is 1 m / min. The magnesium content is detected to be 0.004%. Then, a rare earth calcium-silicon alloy cored wire is fed. The rare earth calcium-silicon alloy cored wire uses calcium-silicon alloy as the carrier, the cerium Ce content is 18 wt%, the lanthanum La content is 12 wt%, and the calcium-silicon alloy content is 70 wt%. The feeding speed of the rare earth calcium-silicon alloy cored wire is 2.5 m / min until the Ce content is 0.006% and the La content is 0.004%. Subsequently, a calcium-silicon alloy wire is fed. The feeding speed is 2.5 m / min. The calcium content is adjusted to 0.004%. Finally, argon is blown in from the bottom of the ladle. The argon flow rate is 25 L / min, and the soft blowing time of argon is 12 minutes.

[0097] The difference between Example 8 and Example 1 lies in: In Step 1 of the preparation method of the economical free-cutting and high formability austenitic stainless steel, electric furnace smelting: Heat the scrap steel to 1650 °C. After melting into molten iron, low-phosphorus molten iron and nickel plates are added. Maintain melting at 1650 °C for 10 minutes, P ≤ 0.05%. The Ni content is detected to be 5.51%. Subsequently, manganese ingots and copper ingots are added. Maintain melting at 1650 °C for 10 minutes. The Mn is detected to be 3.15%, the Cu content is detected to be 1.47%, and the carbon content is detected to be 0.079%.

[0098] Step 2, AOD refining: Adjust the temperature to 1590 °C, blow in an Ar / O2 mixed gas. The volume ratio of O2 to Ar in the Ar / O2 mixed gas is 4:6. After decarburizing for 35 minutes, the carbon content is detected to be 0.026%. Subsequently, ferrosilicon alloy is added in three intervals with an interval time of 10 minutes. For the first time, 40 wt% ferrosilicon alloy is added. After melting for 5 minutes, the carbon content is detected to be 0.188%. After an interval of 10 minutes, 30 wt% ferrosilicon alloy is added for the second time. After melting for 5 minutes, the carbon content is detected to be 0.071%. After an interval of 10 minutes, 30 wt% ferrosilicon alloy is added for the third time. After melting for 20 minutes, the carbon content is detected to be 0.016%. Chromium ingots and nickel blocks are supplemented and added for fine composition adjustment. After melting for 10 minutes, the Ni content is detected to be 5.54%, the Cr content is 17.46%, and the S content is 0.196%.

[0099] Step 3, Ca-Mg composite treatment in LF furnace: Adjust the temperature to 1565 °C, feed in a calcium-silicon alloy wire. The calcium content in the calcium-silicon alloy wire is 30 wt%. The feeding speed is 4 m / min. The addition amount of the calcium-silicon alloy wire is calculated and fed according to the ratio of Ca / S = 1.2:1. Subsequently, a magnesium wire with a purity of 99.95% is added. The feeding speed is 1 m / min. The magnesium content is detected to be 0.004%. Then, a rare earth calcium-silicon alloy cored wire is fed. The calcium-silicon alloy is used as the carrier in the rare earth calcium-silicon alloy cored wire. The Ce content is 18 wt% and the La content is 12 wt%. The calcium-silicon alloy content is 70 wt%. The feeding speed of the rare earth calcium-silicon alloy cored wire is 2.5 m / min. The Ce content is detected to be 0.021%, the La content is 0.014%, and the calcium content is 0.004%. Finally, argon is blown in from the bottom of the ladle. The argon flow rate is 25 L / min, and the soft argon blowing time is 12 minutes.

[0100] The control group is 304 traditional J1 steel. The elemental composition of 304 traditional J1 steel is as follows: Ni: 8.5%, S: 0.04%, C: 0.031%, Si: 1.15%, Mn: 2.20%, P: 0.032%, Cr: 18.50%, Cu: 1.50%, N: 0.025%, and the balance is iron and inevitable impurities.

[0101] The difference between Comparative Example 1 and Example 1 lies in: Step 1 of the preparation method of economic and easy-to-cut high-formability austenitic stainless steel, electric furnace smelting: Heat the scrap steel to 1650 °C. After melting into molten iron, low-phosphorus molten iron and nickel plates are added. Maintain melting at 1650 °C for 10 minutes, with P ≤ 0.05%. The Ni content is detected to be 5.51%. Subsequently, manganese ingots are added. Maintain melting at 1650 °C for 10 minutes. The Mn is detected to be 3.15%, and the carbon content is detected to be 0.071%.

[0102] Step 2, AOD refining: Adjust the temperature to 1590 °C, blow in an Ar / O2 mixed gas, the volume ratio of O2 to Ar in the Ar / O2 mixed gas is 4:6. After decarburization for 30 min, the carbon content is detected to be 0.027%. Subsequently, ferrosilicon alloy is added in three intervals with an interval time of 10 minutes. For the first time, 40 wt% ferrosilicon alloy is added. After melting for 5 min, the carbon content is detected to be 0.182%. After an interval of 10 min, 30 wt% ferrosilicon alloy is added for the second time. After melting for 5 min, the carbon content is detected to be 0.065%. After an interval of 10 min, 30 wt% ferrosilicon alloy is added for the third time. After melting for 15 min, the carbon content is detected to be 0.015%. Chrome ingots and nickel blocks are supplemented and added for fine composition adjustment. After melting for 10 min, the Ni content is detected to be 5.54%, the Cr content is 17.46%, and the S content is 0.196%.

[0103] Step 3, Ca-Mg composite treatment in LF furnace: Adjust the temperature to 1565 °C, feed in a calcium-silicon alloy wire with a calcium content of 30 wt% at a feeding speed of 4 m / min. The addition amount of the calcium-silicon alloy wire is calculated and fed according to the ratio of Ca / S = 1.2:1. Subsequently, a magnesium wire with a purity of 99.95% is added at a feeding speed of 1 m / min. The magnesium content is detected to be 0.004%. The calcium-silicon alloy wire is fed again at a feeding speed of 2.5 m / min and a calcium content of 0.004%. Finally, argon is blown in from the bottom of the ladle at an argon flow rate of 25 L / min, and the soft argon blowing time is 12 min.

[0104] The difference between Comparative Example 2 and Example 1 lies in: Step 1, electric furnace smelting in the preparation method of economic free-cutting high formability austenitic stainless steel: The scrap steel is heated to 1650 °C and melted into molten iron, then low-phosphorus molten iron and nickel plates are added, and it is maintained at 1650 °C and melted for 10 min with P ≤ 0.05%. The Ni content is detected to be 5.51%. Subsequently, manganese ingots and copper ingots are added, and it is maintained at 1650 °C and melted for 10 min. The Mn is detected to be 3.15%, the Cu content is detected to be 1.47%, and the carbon content is detected to be 0.074%.

[0105] Step 2, AOD refining: Adjust the temperature to 1590 °C, blow in an Ar / O2 mixed gas, the volume ratio of O2 to Ar in the Ar / O2 mixed gas is 4:6. After decarburization for 30 minutes, the carbon content is detected to be 0.026%. Subsequently, ferrosilicon alloy is added in three intervals with an interval time of 10 minutes. For the first time, 40 wt% ferrosilicon alloy is added. After melting for 5 minutes, the carbon content is detected to be 0.189%. After an interval of 10 minutes, 30 wt% ferrosilicon alloy is added for the second time. After melting for 5 minutes, the carbon content is detected to be 0.071%. After an interval of 10 minutes, 30 wt% ferrosilicon alloy is added for the third time. After melting for 15 minutes, the carbon content is detected to be 0.016%. Chromium ingots and nickel blocks are supplemented and added for fine-tuning of the composition. After melting for 10 minutes, the Ni content is detected to be 5.54%, the Cr content is 17.46%, and the S content is 0.195%;

[0106] Step 3, LF furnace calcium-magnesium composite treatment: Adjust the temperature to 1565 °C, feed in a calcium-silicon alloy wire with a calcium content of 30 wt% at a feeding speed of 4 m / min. The feeding amount of the calcium-silicon alloy wire is calculated and fed according to the ratio of Ca / S = 1.2:1. Subsequently, a magnesium wire with a purity of 99.95% is added at a feeding speed of 1 m / min. The magnesium content is detected to be 0.004%. The calcium-silicon alloy wire is fed again at a feeding speed of 2.5 m / min and a calcium content of 0.004%. Finally, argon is blown in from the bottom of the ladle at an argon flow rate of 25 L / min, and the soft argon blowing time is 12 minutes.

[0107] The difference between Comparative Example 3 and Example 1 lies in: Step 1 of the preparation method of the economic free-cutting high-formability austenitic stainless steel, electric furnace smelting: Heat the scrap steel to 1650 °C. After melting into molten iron, low-phosphorus molten iron and nickel plates are added, and it is maintained at 1650 °C and melted for 10 minutes with P ≤ 0.05%. The Ni content is detected to be 5.51%. Subsequently, manganese ingots are added, and it is maintained at 1650 °C and melted for 10 minutes. The Mn is detected to be 3.15%, and the carbon content is detected to be 0.071%;

[0108] Step 2, AOD refining: Adjust the temperature to 1590 °C, blow in an Ar / O2 mixed gas, the volume ratio of O2 to Ar in the Ar / O2 mixed gas is 4:6. After decarburization for 30 minutes, the carbon content is detected to be 0.027%. Subsequently, ferrosilicon alloy is added in three intervals with an interval time of 10 minutes. For the first time, 40 wt% ferrosilicon alloy is added. After melting for 5 minutes, the carbon content is detected to be 0.185%. After an interval of 10 minutes, 30 wt% ferrosilicon alloy is added for the second time. After melting for 5 minutes, the carbon content is detected to be 0.067%. After an interval of 10 minutes, 30 wt% ferrosilicon alloy is added for the third time. After melting for 15 minutes, the carbon content is detected to be 0.015%. Chromium ingots and nickel blocks are supplemented and added for fine-tuning of the composition. After melting for 10 minutes, the Ni content is detected to be 5.54%, the Cr content is 17.46%, and the S content is 0.196%;

[0109] Step 3, Ca-Mg composite treatment in LF furnace: Adjust the temperature to 1565°C, feed in calcium-silicon alloy wire with a calcium content of 30 wt% in the calcium-silicon alloy wire at a feeding speed of 4 m / min. The addition amount of the calcium-silicon alloy wire is calculated and fed according to the ratio of Ca / S = 1.2:1. Subsequently, feed in magnesium wire with a purity of 99.95% at a feeding speed of 1 m / min. Detect that the magnesium content is 0.004%. Then feed in rare earth calcium-silicon alloy cored wire with calcium-silicon alloy as the carrier, Ce content of 18 wt% and La of 12 wt%, and calcium-silicon alloy content of 70 wt% at a feeding speed of 2.5 m / min. Detect that the Ce content is 0.015%, La content is 0.010%, and calcium content is 0.004%. Finally, blow in argon from the bottom of the ladle at an argon flow rate of 25 L / min for a soft argon blowing time of 12 min.

[0110] The difference between Comparative Example 4 and Example 1 lies in: Step 3 of the preparation method of economic free-cutting high formability austenitic stainless steel, Ca-Mg composite treatment in LF furnace: Adjust the temperature to 1565°C, feed in calcium-silicon alloy wire with a calcium content of 30 wt% in the calcium-silicon alloy wire at a feeding speed of 4 m / min. The addition amount of the calcium-silicon alloy wire is calculated and fed according to the ratio of Ca / S = 1.3:1. Subsequently, feed in magnesium wire with a purity of 99.95% at a feeding speed of 1 m / min. Detect that the magnesium content is 0.004%. Then feed in rare earth calcium-silicon alloy cored wire with calcium-silicon alloy as the carrier, Ce content of 18 wt% and La of 12 wt%, and calcium-silicon alloy content of 70 wt% at a feeding speed of 2.5 m / min. Detect that the Ce content is 0.015%, La content is 0.010%, and calcium content is 0.005%. Finally, blow in argon from the bottom of the ladle at an argon flow rate of 25 L / min for a soft argon blowing time of 12 min.

[0111] The difference between Comparative Example 5 and Example 1 is that: Step 1 of the preparation method of economical, free-cutting, and high-formability austenitic stainless steel, electric furnace smelting: heating the scrap steel to 1650°C, melting it into molten iron, adding low-phosphorus molten iron and nickel plate, maintaining the melting temperature at 1650°C for 10 minutes, P≤0.05%, detecting the Ni content to be 5.21%, then adding manganese ingots and copper ingots, maintaining the melting temperature at 1650°C for 10 minutes, detecting Mn to be 3.15%, detecting Cu content to be 1.47%, and detecting carbon content to be 0.071%; Step 2, AOD refining: adjusting the temperature The temperature was raised to 1590°C, and an Ar / O2 mixed gas was blown in, wherein the volume ratio of O2 to Ar in the Ar / O2 mixed gas was 4:6. After decarburization for 30 minutes, the carbon content was detected to be 0.025%. Subsequently, ferrosulfide was added three times at intervals of 10 minutes. The first addition of 40wt% ferrosulfide was performed, and the carbon content was detected to be 0.185% after melting for 5 minutes. After an interval of 10 minutes, 30wt% ferrosulfide was added for the second time, and the carbon content was detected to be 0.064% after melting for 5 minutes. After an interval of 10 minutes, 30wt% ferrosulfide was added for the third time. Sulfur-iron alloy, melted for 15 minutes and detected the carbon content to be 0.015%, supplemented with chromium ingots and nickel blocks for fine-tuning the composition, and after melting for 10 minutes, the Ni content was 5.29%, the Cr content was 17.46%, and the S content was 0.196%; Step 3, LF furnace calcium-magnesium composite treatment: adjust the temperature to 1565 ° C, feed silicon-calcium alloy wire, the calcium content in the silicon-calcium alloy wire is 30wt%, the feeding speed is 4m / min, the addition amount of silicon-calcium alloy wire is calculated according to the Ca / S=0.7:1 ratio, and then add magnesium with a purity of 99.95% wire, feeding speed 1m / min, magnesium content detected to be 0.004%, and then feeding rare earth silicon calcium alloy cored wire, rare earth silicon calcium alloy cored wire silicon calcium alloy as carrier, Ce content is 18wt% and La is 12wt%, silicon calcium alloy content is 70wt%, rare earth silicon calcium alloy cored wire feeding speed 2.5m / min, adjust Ce content to 0.015%, La content to 0.010%, Ca content to 0.002%, finally blow argon from the bottom of the ladle, argon flow rate 25L / min, soft blowing argon time 12min.

[0112] The difference between Comparative Example 6 and Example 1 is: Step 2 of the preparation method of economical, easy-to-cut and highly formable austenitic stainless steel, AOD refining: adjusting the temperature to 1590°C, blowing in an Ar / O2 mixed gas, the volume ratio of O2 to Ar in the Ar / O2 mixed gas is 4:6, and after decarburization for 30 minutes, the carbon content is detected to be 0.027%, and then the ferrosulfur alloy is added at one time, and the Ar / O2 mixed gas is continuously blown in. The carbon content is detected to be 0.027% after melt decarburization for 30 minutes, and chromium ingots and nickel blocks are added to fine-tune the composition. After melting for 10 minutes, the Ni content is detected to be 5.54%, the Cr content is 17.46%, and the S content is 0.196%.

[0113] Performance detection tests: 1. Mechanical properties are determined in accordance with ISO 6892-1:2021. 2. Salt spray resistance is determined in accordance with ISO9227:2017. 3. Cutting force is determined in accordance with ISO 3685:1993. 4. The limiting drawing ratio (LDR) is determined in accordance with GB / T15825.3-2008.

[0114] Table 2: Elemental composition table of austenitic stainless steels in Examples 1-8 and Comparative Examples 1-5

[0115]

[0116] Table 3: Performance test parameter table of austenitic stainless steels in Examples 1-8 and Comparative Examples 1-6

[0117]

[0118] Combined with Examples 1-6 and the control group and in combination with Tables 2-3, it can be seen that in the present invention, the nickel content is reduced to 5.5-6.8%, the material cost is reduced by 20-25%, its cutting force is significantly reduced, the tool life can be effectively extended, and it has excellent mechanical properties and corrosion resistance.

[0119] Combined with Example 1 and Comparative Examples 1-3 and in combination with Tables 2-3, it can be seen that the combination of low nickel + high sulfur + copper / rare earth can improve the cutting performance, mechanical properties and corrosion resistance of the prepared stainless steel.

[0120] Combined with Example 1 and Comparative Example 2 and in combination with Tables 2-3 and Figure 1 it can be seen that the addition of copper strengthens by nano-precipitation, improving the mechanical properties and corrosion resistance of the prepared stainless steel. And combined with Example 1, Examples 5-6 and Comparative Examples 1-3 and in combination with Tables 2-3, it can be seen that it is appropriate to control the copper addition amount to 0.8-1.5%. When the copper addition amount is less than 0.8%, the mechanical strength of the stainless steel will be low and the strengthening effect is not obvious; while when the copper addition amount is higher than 1.5%, the increase in the mechanical strength of the stainless steel is lower, and instead, the overall production cost is increased.

[0121] Combined with Example 1 and Comparative Examples 4-5 and in combination with Tables 2-3, it can be seen that when Ca / S = 0.8-1.2, the cutting performance, mechanical properties and corrosion resistance of the prepared stainless steel can be improved. Insufficient calcium (Ca / S < 0.8): The sulfide is still mainly MnS, the morphology control fails, and the mechanical properties and corrosion resistance decrease significantly; excessive calcium (Ca / S > 1.2): High-melting-point CaO-CaS composite inclusions are formed, significantly deteriorating the cutting performance.

[0122] Combined with Example 1 and Comparative Example 6 and in combination with Tables 2-3 and Figure 2It can be seen that the cutting performance, mechanical properties and corrosion resistance of the stainless steel prepared by adding ferrosulfur alloy at three intervals can reduce the incidence of hot rolling cracks and increase the chromium recovery rate, play a role in preventing sulfur segregation and increasing the sulfur recovery rate. The traditional one-time sulfur addition is likely to cause sulfur to accumulate in local areas, forming coarse MnS inclusions, and the aspect ratio of the formed MnS inclusions >5, which triggers hot rolling cracks; in addition, it also makes sulfur evenly distributed, forming fine spherical sulfides with an aspect ratio <2.

[0123] Combined with Example 1 and Examples 7-8 and combined with Tables 1-2, it can be seen that the cerium Ce and lanthanum La of rare earth elements improve the mechanical properties and corrosion resistance of the prepared stainless steel. The addition amount of rare earth element RE is preferably 0.1-0.4 wt%. When the addition amount of rare earth element RE is lower than 0.1 wt%, the improvement of the mechanical properties and corrosion resistance of the stainless steel is not obvious. When the addition amount of rare earth element RE is higher than 0.4%, the increase in the mechanical strength and corrosion resistance of the stainless steel is relatively low, but instead increases the overall production cost.

[0124] It should be noted that: this specific embodiment is only an explanatory illustration of the technical solution of the present invention, and it is not a limitation of the present invention. Those skilled in the art can make modifications without creative contributions to this embodiment according to needs after reading this specification, but as long as it is within the scope of the claims of the present invention, it is protected by the patent law.

Claims

1. An economical free-cutting and high formability austenitic stainless steel, characterized in that: It consists of the following components: 5.5% - 6.8% Ni, 1.5% - 3.5% Mn, 16.5% - 18.5% Cr, 0.15% - 0.20% S, 0.8% - 1.5% Cu, 0.01% - 0.04% RE, 0.002% - 0.004% Ca, 0.002% - 0.005% Mg, P ≤ 0.05%, C ≤ 0.08%, Si ≤ 1.0%, N ≤ 0.05%, and the balance is iron and inevitable impurities.

2. An economical free-cutting and high formability austenitic stainless steel according to claim 1, characterized in that: The RE element is cerium Ce and lanthanum La, and the mass ratio of cerium Ce to lanthanum La is 1:(0.5 - 2).

3. An economic free-cutting and high formability austenitic stainless steel according to claim 2, characterized in that: It consists of the following components: 6.0% - 6.5% Ni, 2.0 - 3.0% Mn, 16.8 - 17.5% Cr, 0.15% - 0.20% S, 1.0% - 1.5% Cu, 0.005% - 0.015% Ce, 0.005% - 0.015% La, 0.002 - 0.004% Ca, 0.003 - 0.005% Mg, P ≤ 0.05%, C ≤ 0.08%, Si ≤ 1.0%, N ≤ 0.05%, and the balance is iron and inevitable impurities.

4. A method for preparing an economical free-cutting and high formability austenitic stainless steel according to any one of claims 1 to 3, characterized in that: It includes the following steps: Step 1, electric furnace smelting: Heat the scrap steel to 1600 - 1650 °C, melt it into molten iron, then add low - phosphorus molten iron and nickel plates, maintain melting at 1600 - 1650 °C for 5 - 15 min, with P ≤ 0.05% and the Ni content controlled at 5.5% - 6.8%. Subsequently, add manganese ingots and copper ingots, maintain melting at 1600 - 1650 °C for 5 - 15 min, with the Mn content controlled at 1.5 - 3.5% and the Cu content controlled at 0.8% - 1.5%, and the carbon content: C ≤ 0.08%. Step 2, AOD refining: Adjust the temperature to 1580 - 1600 °C, blow in an Ar / O2 mixed gas, where the volume ratio of O2 to Ar in the Ar / O2 mixed gas is (3 - 5):(5 - 7), decarburize until C ≤ 0.03%, and then add ferrosulfur alloy at least three times at intervals, with the total sulfur content controlled at 0.15 - 0.20%. Step 3, LF furnace calcium - magnesium composite treatment: Adjust the temperature to 1550 - 1580 °C, feed in a calcium - silicon alloy wire, and the addition amount of the calcium - silicon alloy wire is calculated according to the Ca / S ratio of 0.8 - 1.

2. Subsequently, add a magnesium wire, with the magnesium content controlled at 0.002 - 0.005%. Then feed in a rare - earth calcium - silicon alloy cored wire, with the RE content controlled at 0.01% - 0.03% and the calcium content controlled at 0.002 - 0.004%. Finally, blow argon from the bottom of the ladle, with the argon flow rate of 20 - 30 L / min and the soft - blowing argon time ≥ 10 min. Step 4, continuous casting: Adjust the molten steel temperature to 1550 - 1560 °C, casting speed: 0.8 - 1.2 m / min, mold vibration: amplitude 4 - 6 mm, frequency 120 - 150 times / min, secondary cooling water volume: 0.8 - 1.2 L / kg of molten steel, electromagnetic stirring: frequency 3 - 5 Hz, current 200 - 300 A. Step 5, three - stage hot rolling: Pass through high - temperature large - deformation rough rolling, low - temperature finish rolling, and laminar cooling in sequence to obtain rough products. Step 6: Gradient annealing the crude product to obtain the finished product of economic free-cutting and high formability austenitic stainless steel.

5. The preparation method of an economical free-cutting and high formability austenitic stainless steel according to claim 4, characterized in that: In Step 2, ferrosulphur alloy is added in three intervals. The interval time is 8 - 10 minutes. For the first addition, 40wt% ferrosulphur alloy is added, and the C content is controlled at 0.15 - 0.20%. For the second addition, 30wt% ferrosulphur alloy is added, and the C content is controlled at 0.05 - 0.08%. For the third addition, 30wt% ferrosulphur alloy is added, and the C content is controlled at ≤0.03%. Chromium ingots and nickel blocks are supplemented and added, and the composition is fine-tuned so that the Ni content is controlled at 5.5% - 6.8% and the Cr content is controlled at 16.5% - 18.5%.

6. The preparation method of an economical free-cutting and high formability austenitic stainless steel according to claim 4, characterized in that: In Step 3, the feeding speed of calcium-silicon alloy wire is 3 - 5m / min, the feeding speed of magnesium wire is 1 - 2m / min, and the feeding speed of rare earth calcium-silicon alloy cored wire is 2 - 4m / min.

7. The preparation method of an economical free-cutting and high formability austenitic stainless steel according to claim 4, characterized in that: In Step 4, the water volume in the secondary cooling zone is cooled in 4 sections, including the dummy bar section, the first segment, the second segment, and the third segment. The cooling water flow rate in the dummy bar section is 0.4 - 0.6L / kg molten steel, the cooling water flow rate in the first segment is 0.35 - 0.45L / kg molten steel, the cooling water flow rate in the second segment is 0.25 - 0.35L / kg molten steel, and the cooling water flow rate in the third segment is 0.2 - 0.3L / kg molten steel.

8. The preparation method of an economical free-cutting and high-formability austenitic stainless steel according to claim 4, characterized in that: In Step 4, electromagnetic stirring is divided into mold electromagnetic stirring M-EMS and final electromagnetic stirring F-EMS. In the mold electromagnetic stirring M-EMS, the frequency is 3 - 5Hz and the current is 200 - 250A; in the final electromagnetic stirring F-EMS, the frequency is 2 - 3Hz and the current is 250 - 300A.

9. The preparation method of an economical free-cutting and high formability austenitic stainless steel according to claim 4, characterized in that: The specific operation of high-temperature large-deformation rough rolling in Step 5 is as follows: The slab is heated to 1150 - 1200°C, held for 2 - 4h, rolled in 5 - 7 passes, the total deformation is 50 - 60%, and the single-pass deformation is 15 - 20%; the rolling temperature is controlled at 1100 - 1150°C; the specific operation of low-temperature finish rolling in Step 5 is as follows: The finish rolling temperature is 900 - 950°C, the total deformation is 30 - 40%, the single-pass deformation is 8 - 12%, and the final rolling thickness is 2 - 3mm; the specific operation of intermediate cooling in Step 5 is as follows: After water cooling to 700°C, it is input into the coiling equipment for coiling to obtain the crude product steel coil.

10. The preparation method of an economical free-cutting and high formability austenitic stainless steel according to claim 4, characterized in that: The specific operation of gradient annealing in Step 6 is as follows: Rapidly heat up to 1100°C and hold for 5min, then slowly cool to 950°C at a rate of 5 - 40°C / min and hold for 15 - 30min, and then water cool to room temperature to complete the gradient annealing.

Images