Ultrahigh purity 316l stainless steel and method for producing the same

Abstract

The application provides an ultrahigh-purity 316L stainless steel and a preparation method thereof, and belongs to the technical field of metallurgy. The application provides a VIM (first FeO controlled aluminum, then vacuum C deoxidation pretreatment, and finally Mg+Ca composite treatment deep de-O and deep de-S) + VAR duplex smelting process, realizes efficient removal of controlled aluminum, deep de-O, deep de-S, deep de-H and inclusions of the ultrahigh-purity 316L stainless steel, and thus meets the urgent needs of semiconductor equipment for ultrahigh-purity stainless steel materials.

Description

Technical Field

[0001] This invention relates to the field of metallurgical technology, and in particular to an ultra-high purity 316L stainless steel and its preparation method. Background Technology

[0002] Ultra-high purity 316L stainless steel possesses excellent corrosion resistance and electropolishing properties, making it a key material for high-purity gas transmission pipelines, joints, filters, atomic layer deposition valves, and other components in semiconductor equipment. With the continuous advancement of semiconductor technology, higher requirements are placed on the purity of this steel, requiring strict control of impurity element content: O≤0.001%, S≤0.0015%, H≤0.0002%, Al≤0.01%, to prevent the formation of large amounts of carbides and large inclusions (Al2O3, MnS, etc.). Simultaneously, the harmless treatment of inclusions must be achieved. All these control measures aim to ensure the material has good corrosion resistance and polishing properties to prevent extremely small particles or impurities from detaching from the material and contaminating high-purity gases. However, under these stringent limitations, finding suitable deoxidation, desulfurization, and inclusion modification methods has become a challenge.

[0003] Al generally possesses strong deoxidizing capabilities, but it easily leads to Al enrichment in molten steel, forming a large number of harmful Al2O3 inclusions. Therefore, Al deoxidation is not suitable for ultra-high purity 316L stainless steel, and Al control treatment is required when the Al content in the raw material is too high. Vacuum carbon (C) deoxidation has strong deoxidizing capabilities and does not produce non-metallic inclusions, but this method usually requires the addition of a large amount of carbon to achieve good deoxidation results. Its use is generally discouraged for ultra-high purity stainless steel with low carbon content requirements. Mg and Ca treatments can not only effectively remove oxygen and sulfur, but also significantly reduce the number and size of inclusions, thereby significantly improving the purity of the steel. This is mainly attributed to the strong affinity of Mg and Ca for oxygen and sulfur, effectively modifying inclusions. Ca exhibits a stronger sulfur-removing effect than Mg and can modify harmful Al2O3 into liquid calcium aluminate, promoting the flotation and removal of inclusions. While both Mg and Ca treatments alone have good capabilities for purifying molten steel and modifying inclusions, each treatment has some drawbacks. Under low sulfur content conditions, Mg treatment has a weak desulfurization effect. Simple Ca treatment may lead to incomplete modification of Al2O3 inclusions, and excessive Ca can promote the formation of high-melting-point and large-sized inclusions. Currently, Mg+Ca composite treatment is mainly applied to aluminum-killed steel, and this method is rarely used in non-aluminum-killed steel, especially ultra-high purity stainless steel. In addition, the vapor pressures of Mg and Ca are very high, and the reaction between Mg and Ca is very intense at steelmaking temperatures, making it difficult to precisely control during the smelting process.

[0004] Therefore, there is an urgent need to explore a technical solution for deep deoxidation, deep desulfurization, and efficient modification of inclusions in ultra-high purity stainless steel used in the semiconductor industry, so as to meet the requirements of ultra-high purity for materials used in semiconductor equipment. Summary of the Invention

[0005] The purpose of this invention is to provide an ultra-high purity 316L stainless steel and its preparation method. The method of this invention can achieve deep deoxidation, deep desulfurization and efficient modification of inclusions, and prepare 316L austenitic stainless steel with ultra-high purity, good microstructure and properties, which meets the urgent demand for ultra-high purity stainless steel materials in fields such as semiconductor equipment.

[0006] To achieve the above-mentioned objectives, the present invention provides the following technical solution:

[0007] This invention provides an ultra-high purity 316L stainless steel, comprising, by weight percentage: C≤0.02%; Cr 16.00%~18.00%; Ni 10.00%~14.00%; Mo 2.00%~3.00%; N≤0.004%; Si 0.30%~0.75%; Mn 0.50%~2.00%; P≤0.003%; S≤0.001%; Al≤0.004%; O≤0.0007%; H≤0.0001%; The remainder is Fe; The coarse inclusions in the ultra-high purity 316L stainless steel are grade 0, and the fine inclusions (A+B+C+D) are ≤ grade 0.5.

[0008] Preferably, the mechanical properties at room temperature are: yield strength ≥ 260 MPa, tensile strength ≥ 570 MPa, and elongation ≥ 70%.

[0009] Preferably, the volume content of α-ferrite is 0, and the volume content of δ-ferrite is ≤0.1%.

[0010] Preferably, the ultra-high purity 316L stainless steel has a grain size of 20~35 μm after hot rolling and annealing.

[0011] This invention provides a method for preparing the ultra-high purity 316L stainless steel described above, comprising the following steps: (1) Prepare the raw materials according to the target composition and perform vacuum induction melting to obtain molten steel; by mass percentage, the total Al introduced by the raw materials is ≤0.04%, the total O is ≤0.04%, the total S is ≤0.005%, and the total P is ≤0.003%; the material of the crucible used for vacuum induction melting is MgO, CaO or MgO-CaO mixed material; (2) Add FeO to the molten steel to perform refining and Al removal, and the refining and Al removal time is 2~3 min; When the initial O content of the molten steel is >0.015% and ≤0.04%, the amount of FeO added is determined according to Formula 1: w[%FeO]=4×(w[%Al]-0.004%)-4.5×(w[%O]-0.015%) Formula 1; if w[%FeO]≤0 is calculated according to Formula 1, then it is not necessary to add FeO to the molten steel.

[0012] When the initial oxygen content of the molten steel is ≤0.015%, the amount of FeO added is determined according to formula 2: w[%FeO]=4×(w[%Al]-0.004%)+4.5(0.015%-w[%O]) Formula 2; In Equations 1 and 2: w[%FeO] is the amount of FeO added, w[%Al] is the initial Al content in the molten steel, and w[%O] is the initial O content in the molten steel; (3) Add high-purity C to the de-Al steel liquid and perform vacuum C deoxidation pretreatment to control the O content of the steel liquid at 0.006%~0.008%;

[0013] The amount of high-purity C added is determined according to formula 3: w[%C]=0.7×(w[%O]-0.006%) Equation 3; In Formula 3: w[%C] is the amount of high-purity C added, and w[%O] is the initial O content in the molten steel;

[0014] The time for the vacuum C deoxidation pretreatment is determined according to Equation 4: Equation 4; In Equation 4: t is the vacuum C deoxidation pretreatment time, s; F is the free surface area of ​​the molten steel, m². 2 V is the volume of molten steel, in meters. 3 ;w[%O] C The oxygen content in steel after vacuum C deoxidation pretreatment is % (%). (4) After the vacuum C deoxidation pretreatment is completed, high-purity argon gas is introduced into the vacuum induction melting furnace, the temperature of the molten steel is controlled at 1470~1490℃, and Mg alloy is added for deoxidation and desulfurization. The deoxidation and desulfurization time is 2~3 min.

[0015] The amount of Mg added to the molten steel using Mg alloy is determined according to formula 5 or formula 6:

[0016] When the initial sulfur content in the molten steel is >0.003% and ≤0.005%, it is determined according to Equation 5: w[%Mg]=1.5×(w[%O] C Equation 5: -0.003%) / 15%+0.75×(w[%S]-0.003%) / 15%

[0017] When the initial sulfur content in the molten steel is ≤0.003%, it is determined according to Equation 6: w[%Mg]=1.5×(w[%O] C Equation 6: -0.003%) / 15% In equations 5 and 6: w[%Mg] represents the amount of Mg added to the molten steel by adding Mg alloy, and w[%O] represents the amount of Mg added. C The O content in the molten steel after vacuum C deoxidation pretreatment is w[%S], and the initial S content in the molten steel is w[%S]. (5) Add Ca alloy to the deoxidized and desulfurized steel liquid for deep deoxidation and desulfurization, and the deep deoxidation and desulfurization time is 2~3 min;

[0018] The amount of Ca added to molten steel via Ca alloying is determined according to formula 7 or formula 8:

[0019] When the initial sulfur content in the molten steel is >0.003% and ≤0.005%, it is determined according to Equation 7: w[%Ca]=(5×w[%O] C -0.35×w[%Mg]+1.7×w[%S]-0.011%) / 5% Equation 7;

[0020] When the initial sulfur content in the molten steel is ≤0.003%, it is determined according to Equation 8: w[%Ca]=(2.5×w[%O] C -0.25×w[%Mg]+1.25×w[%S]-0.0188%) / 5% Equation 8; In equations 7 and 8: w[%Ca] represents the amount of Ca added to the molten steel by adding Ca alloy, and w[%O] represents the amount of Ca added. C The value represents the O content in the molten steel after vacuum C deoxidation pretreatment, w[%S] represents the initial S content in the molten steel, and w[%Mg] represents the amount of Mg added to the molten steel by adding Mg alloy. (6) The temperature of the molten steel after deep deoxidation and desulfurization is controlled at 1510~1550℃. After being killed, it is cast to obtain ingots. (7) Forge the ingot into an electrode, controlling the filling ratio to be 0.6~0.8; (8) The electrode is placed in a vacuum arc remelting furnace for refining, with the vacuum degree controlled to be ≤0.1 Pa and the melting rate controlled to be (0.009~0.012)×D kg·min. -1 D is the diameter of the crystallizer, in mm; (9) Multi-stage current reduction and shrinkage are carried out during the refining capping period to obtain the high-purity 316L stainless steel.

[0021] Preferably, in step (1), the temperature of the vacuum induction melting is 1500~1550℃.

[0022] Preferably, in step (3), the vacuum degree of the vacuum C deoxygenation pretreatment is ≤10 Pa.

[0023] Preferably, in step (4), the Mg alloy is a Ni-Mg alloy or an Fe-Mg alloy, and the mass content of Mg in the Mg alloy is 10%~30%.

[0024] Preferably, in step (5), the Ca alloy is a Si-Ca alloy, a silicon-calcium cored wire, or a Ni-Ca alloy, and the mass content of Ca in the Ca alloy is 20%~40%.

[0025] Preferably, in step (9), the multi-stage current reduction compensation time is 90~100 min, and the compensation cycle is 8~10 times.

[0026] The technical principle of this invention is as follows: Ultra-high purity 316L stainless steel is mainly used in semiconductor equipment materials. Because the semiconductor product manufacturing process requires a high-purity environment, extremely high purity requirements are placed on semiconductor equipment materials. Besides strictly controlling ultra-low levels of O, S, and H, the Al content also needs to be controlled below 0.01% to ensure the material's purity, electrical properties, corrosion resistance, and sealing performance. Therefore, traditional smelting processes cannot meet these requirements. This invention proposes a dual smelting process combining vacuum induction melting (VIM) and vacuum arc remelting (VAR).

[0027] Specifically, in the vacuum induction melting (VIM) stage, due to the compositional requirements of ultra-high purity 316L austenitic stainless steel, conventional Al deoxidation cannot be used. To control the Al content in the steel, a certain amount of FeO needs to be added to the furnace for Al control, and magnesium or calcium crucibles are used to adsorb the de-Al products. During the smelting process, Mg and Ca elements are also often used to purify molten steel. Mg and Ca not only have strong thermodynamic affinity with O and S, but can also modify inclusions. Based on this, this invention uses a Mg+Ca composite treatment method for deoxidation and desulfurization in the vacuum induction melting stage. The Mg+Ca composite treatment has a synergistic effect. While reducing the O content in the steel, Mg can significantly increase the activity of Ca in the molten steel. Therefore, the deoxidation and desulfurization effect of Ca in the molten steel is improved, which helps to achieve deep deoxidation and desulfurization of steel. To improve the effect of Mg+Ca composite treatment and the utilization rate of Mg and Ca, combined with the steel composition requirements, a vacuum C deoxidation pretreatment process is added before the Mg+Ca composite treatment to reduce the initial oxygen content of the molten steel. Compared to using Mg or Ca alone, the above-mentioned multi-step composite treatment process not only achieves better deoxidation and desulfurization, but also reduces the amount of Mg and Ca added to the steel, thus controlling smelting costs.

[0028] Vacuum arc remelting (VAR) has a significant effect on purifying molten steel. After vacuum induction melting, the impurity element composition (O, S, H) of ingots is further reduced, and inclusions are purified. In addition, VAR is a rapid solidification method, thus greatly improving the uniformity of the steel and facilitating the acquisition of a uniform and fine solidification structure.

[0029] Based on the above technical principles, this invention provides a VIM (first adding FeO to control aluminum, then vacuum C deoxidation pretreatment, and finally Mg+Ca composite treatment for deep O and deep S removal) + VAR dual smelting process, which realizes controlled aluminum and efficient removal of O, S, H and inclusions in ultra-high purity 316L stainless steel, thereby meeting the urgent needs of semiconductor equipment for ultra-high purity stainless steel materials.

[0030] The beneficial effects of this invention include: This invention provides a method for preparing ultra-high purity 316L stainless steel using a dual smelting process of vacuum induction melting (VIM) + vacuum arc remelting (VAR). The prepared 316L stainless steel has extremely high purity: O≤0.0007%, S≤0.001%, Al≤0.004%, N≤0.004%, H≤0.0001%, with efficient modification and detoxification of inclusions. Coarse inclusions are grade 0, and fine inclusions (A+B+C+D types) are ≤0.5. It is noteworthy that the highest purity of existing ultra-high purity 316L stainless steel only reaches O≤0.0008%, Al≤0.005%, S≤0.003%, and H≤0.0002%. Currently, the 316L stainless steel with such high purity as that described in this invention does not exist.

[0031] The ultra-high purity 316L stainless steel prepared by the method provided in this invention, after hot rolling and annealing, has a grain size of 20~35 μm, an α-ferrite volume content of 0, and a δ-ferrite volume content of ≤0.1%.

[0032] The ultra-high purity 316L stainless steel prepared using the method provided by this invention has a yield strength ≥260 MPa, tensile strength ≥570 MPa, and elongation ≥70%.

[0033] The ultra-high purity 316L stainless steel prepared using the method provided by this invention can meet the requirements of ultra-high purity for stainless steel materials used in semiconductor equipment, and can thus be applied to key components of semiconductor equipment such as high-purity gas transmission pipelines, joints, filters, and atomic layer deposition valves. Detailed Implementation

[0034] This invention provides an ultra-high purity 316L stainless steel, comprising, by weight percentage: C≤0.02%; Cr 16.00%~18.00%; Ni 10.00%~14.00%; Mo 2.00%~3.00%; N≤0.004%; Si 0.30%~0.75%; Mn 0.50%~2.00%; P≤0.003%; S≤0.001%; Al≤0.004%; O≤0.0007%; H≤0.0001%; The remainder is Fe; The coarse inclusions in the ultra-high purity 316L stainless steel are grade 0, and the fine inclusions (A+B+C+D) are ≤ grade 0.5.

[0035] The ultra-high purity 316L stainless steel provided by the present invention, by weight percentage, comprises C ≤ 0.02%, preferably ≤ 0.018%. In the embodiments of the present invention, it is specifically 0.018%, 0.014%, 0.017%, or 0.015%.

[0036] The ultra-high purity 316L stainless steel provided by the present invention comprises 16.00%~18.00% Cr, preferably 16.4%~17.6%, and more preferably 16.8%~17.2% by mass percentage. In the embodiments of the present invention, it is specifically 16.45%, 17.52%, 16.74%, or 17.16%.

[0037] The ultra-high purity 316L stainless steel provided by the present invention comprises 10.00%~14.00% Ni, preferably 11.00%~13.00%, and more preferably 11.50%~12.50% by mass percentage. In the embodiments of the present invention, it is specifically 10.36%, 12.53%, 13.74%, or 11.46%.

[0038] The ultra-high purity 316L stainless steel provided by the present invention comprises 2.00%~3.00% Mo, preferably 2.20%~2.80%, and more preferably 2.40%~2.60% by mass percentage. In the embodiments of the present invention, it is specifically 2.82%, 2.33%, 2.14%, or 2.56%.

[0039] The ultra-high purity 316L stainless steel provided by this invention, by mass percentage, contains N ≤ 0.004%, preferably ≤ 0.003%. In embodiments of this invention, specifically 0.0022%, 0.0018%, 0.0013%, or 0.0006%. In this invention, C and N are not considered impurity elements; both can improve the strength of 316L stainless steel. However, 316L stainless steel has stringent requirements for the content of C and N. Therefore, this invention controls their content within the aforementioned ranges to meet the compositional requirements of 316L stainless steel.

[0040] The ultra-high purity 316L stainless steel provided by the present invention comprises 0.30%~0.75% Si, preferably 0.40%~0.70%, and more preferably 0.50%~0.60% by mass percentage. In the embodiments of the present invention, it is specifically 0.63%, 0.32%, 0.61%, or 0.42%.

[0041] The ultra-high purity 316L stainless steel provided by the present invention comprises 0.50%~2.00% Mn, preferably 0.80%~1.60%, and more preferably 1.00%~1.40% by mass percentage. In the embodiments of the present invention, it is specifically 0.48%, 1.01%, 1.53%, or 1.79%.

[0042] The ultra-high purity 316L stainless steel provided by this invention, by mass percentage, contains P ≤ 0.003%, preferably ≤ 0.0025%. In embodiments of this invention, specifically 0.0025%, 0.0015%, 0.0017%, or 0.0020%. In this invention, P is considered an impurity element, and the lower the content, the better.

[0043] The ultra-high purity 316L stainless steel provided by this invention, by mass percentage, contains S ≤ 0.001%, preferably ≤ 0.0008%. In embodiments of this invention, specifically 0.0008%, 0.0004%, 0.0007%, or 0.0006%. In this invention, S is considered an impurity element, and the lower the content, the better.

[0044] The ultra-high purity 316L stainless steel provided by this invention, by mass percentage, contains Al ≤ 0.004%, preferably ≤ 0.0036%. In embodiments of this invention, specifically 0.0036%, 0.0022%, 0.0032%, or 0.0028%. In this invention, Al is considered an impurity element, and the lower the content, the better.

[0045] The ultra-high purity 316L stainless steel provided by this invention, by mass percentage, contains ≤0.0007% O, preferably ≤0.0006%. In embodiments of this invention, specifically 0.0006%, 0.0005%, or 0.0004%. In this invention, O is considered an impurity element, and the lower the content, the better.

[0046] The ultra-high purity 316L stainless steel provided by this invention, based on mass percentage, contains H ≤ 0.0001%; ​​specifically, in the embodiments of this invention, it is 0.0001%. In this invention, H is considered an impurity element, and the lower the content, the better.

[0047] The ultra-high purity 316L stainless steel provided by this invention includes a Fe balance.

[0048] The ultra-high purity 316L stainless steel provided by this invention also includes a very small amount of Mg and Ca. In this invention, Mg and Ca are not impurity elements, so there is no need to limit the content of Mg and Ca in the steel. Furthermore, during the VAR process, a large amount of Mg and Ca will be burned off, so the content of Mg and Ca in the final steel composition will not be very high.

[0049] In this invention, the coarse inclusions of the ultra-high purity 316L stainless steel are grade 0, and the fine inclusions (A+B+C+D types) are ≤ grade 0.5. The preferred mechanical properties at room temperature are: yield strength ≥ 260 MPa, tensile strength ≥ 570 MPa, and elongation ≥ 70%. After hot rolling and annealing (using mature hot rolling and annealing processes for 316L), the grain size of the ultra-high purity 316L stainless steel is preferably 20~35 μm. The volume content of α-ferrite in the ultra-high purity 316L stainless steel is preferably 0, and the volume content of δ-ferrite is preferably ≤ 0.1%.

[0050] The ultra-high purity 316L stainless steel provided by this invention has high purity, which can meet the requirements of ultra-high purity for stainless steel materials used in semiconductor equipment. Therefore, it can be applied to key components of semiconductor equipment such as high-purity gas transmission pipelines, joints, filters, and atomic layer deposition valves.

[0051] The present invention provides a method for preparing ultra-high purity 316L stainless steel as described above, comprising the following steps: (1) preparing materials according to the target composition, and performing vacuum induction melting on the raw materials to obtain molten steel; (2) Add FeO to the molten steel for refining and Al removal; (3) Add high-purity C to the de-Al steel liquid and perform vacuum C deoxidation pretreatment to control the O content of the steel liquid at 0.006%~0.008%; (4) After the vacuum C deoxidation pretreatment is completed, high-purity argon gas is introduced into the vacuum induction melting furnace to control the temperature of the molten steel at 1470~1490℃, and Mg alloy is added for deoxidation and desulfurization. (5) Add Ca alloy to the deoxidized and desulfurized molten steel for deep deoxidation and desulfurization; (6) The temperature of the molten steel after deep deoxidation and desulfurization is controlled at 1510~1550℃. After being killed, it is cast to obtain ingots. (7) Forge the ingot into an electrode, controlling the filling ratio to be 0.6~0.8; (8) The electrode is placed in a vacuum arc remelting furnace for refining, with the vacuum degree controlled to be ≤0.1 Pa and the melting rate controlled to be (0.009~0.012)×D kg·min. -1 D is the diameter of the crystallizer, in mm;

[0052] (9) Multi-stage current reduction and shrinkage are carried out during the refining capping period to obtain the high-purity 316L stainless steel.

[0053] In this invention, unless otherwise specified, all raw materials and equipment used are commercially available products well known in the art.

[0054] This invention involves batching ingredients according to the target composition and then performing vacuum induction melting on the raw materials to obtain molten steel.

[0055] This invention does not have special requirements for the batching process, which is common knowledge in the field. In this invention, the total Al introduced by the raw materials is ≤0.04%, total O is ≤0.04%, total S is ≤0.005%, and total P is ≤0.003% by mass percentage. In this invention, the vacuum induction melting temperature is preferably 1500~1550℃, more preferably 1510~1540℃, and even more preferably 1520~1530℃. In this invention, the vacuum induction melting preferably includes: weighing the required experimental raw materials and filling them into the vacuum induction melting furnace according to the principle of pressing down and loosening up; after closing the furnace, turning on the vacuum pump, evacuating to <5 Pa, and then starting to supply power, gradually increasing the power until all the furnace charge is melted, to obtain molten steel.

[0056] In this invention, the crucible used for vacuum induction melting is preferably made of MgO, CaO, or a mixture of MgO and CaO; the mass ratio of MgO to CaO in the MgO-CaO mixture is preferably (1.0~2.5):1.

[0057] After obtaining the molten steel, the present invention adds FeO to the molten steel for refining and de-Al removal.

[0058] In this invention, the temperature of the molten steel is preferably 1500~1550℃, more preferably 1510~1540℃, and even more preferably 1520~1530℃.

[0059] In this invention, when the initial O content of the molten steel is >0.015% and ≤0.04%, the amount of FeO added is determined according to Formula 1: w[%FeO]=4×(w[%Al]-0.004%)-4.5×(w[%O]-0.015%) Formula 1; If w[%FeO]≤0 is calculated according to Equation 1, then it is not necessary to add FeO to the molten steel.

[0060] When the initial oxygen content of the molten steel is ≤0.015%, the amount of FeO added is determined according to formula 2: w[%FeO]=4×(w[%Al]-0.004%)+4.5(0.015%-w[%O]) Formula 2; In Equations 1 and 2: w[%FeO] is the amount of FeO added, w[%Al] is the initial Al content in the molten steel, and w[%O] is the initial O content in the molten steel.

[0061] For example, if w[%FeO]=5% and the mass of molten steel is 100kg, then the amount of FeO added is 100kg×5%=5kg.

[0062] In this invention, the FeO is preferably added in block or disc form.

[0063] In this invention, the refining and de-Al removal time is 2-3 minutes. After adding FeO to the molten steel, the Al in the molten steel is fully oxidized. The resulting Al₂O₃ inclusions react with the magnesium or calcium crucible of this invention and are directly adsorbed onto the crucible for removal.

[0064] After completing the refining and de-Al removal process, the present invention adds high-purity C to the de-Al removed molten steel for vacuum C deoxidation pretreatment, thereby controlling the O content of the molten steel at 0.006%~0.008%.

[0065] In this invention, the purity of the high-purity C is preferably ≥99.99%.

[0066] In this invention, the vacuum degree of the vacuum C deoxidation pretreatment is preferably ≤10 Pa.

[0067] In this invention, the amount of high-purity C added is determined according to Formula 3: w[%C]=0.7×(w[%O]-0.006%) Equation 3; In Formula 3: w[%C] is the amount of high-purity C added, and w[%O] is the initial O content in the molten steel;

[0068] The time for the vacuum C deoxidation pretreatment is determined according to Equation 4: Equation 4; In Equation 4: t is the vacuum C deoxidation pretreatment time, s; F is the free surface area of ​​the molten steel, m². 2 V is the volume of molten steel, in meters. 3 ;w[%O] C The oxygen content in steel after vacuum C deoxidation pretreatment is expressed as %.

[0069] This invention reduces the oxygen content in molten steel through vacuum C deoxidation pretreatment.

[0070] After completing the vacuum C deoxidation pretreatment, the present invention introduces high-purity argon into the vacuum induction melting furnace, controls the temperature of the molten steel at 1470~1490℃, and adds Mg alloy for deoxidation and desulfurization, the deoxidation and desulfurization time being 2~3 min.

[0071] In this invention, the purity of the high-purity argon gas is preferably ≥99.999%; the pressure of the high-purity argon gas introduced into the vacuum induction melting furnace is preferably 0.01~0.03 MPa, more preferably 0.015~0.025 MPa.

[0072] In this invention, the Mg alloy is preferably a Ni-Mg alloy or an Fe-Mg alloy, and the mass content of Mg in the Mg alloy is preferably 10% to 30%, more preferably 15% to 25%.

[0073] In this invention, the amount of Mg added to the molten steel via Mg alloy is determined according to formula 5 or formula 6:

[0074] When the initial sulfur content in the molten steel is >0.003% and ≤0.005%, it is determined according to Equation 5: w[%Mg]=1.5×(w[%O] C Equation 5: -0.003%) / 15%+0.75×(w[%S]-0.003%) / 15%

[0075] When the initial sulfur content in the molten steel is ≤0.003%, it is determined according to Equation 6: w[%Mg]=1.5×(w[%O] C Equation 6: -0.003%) / 15% In equations 5 and 6: w[%Mg] represents the amount of Mg added to the molten steel by adding Mg alloy, and w[%O] represents the amount of Mg added. C The value represents the O content in the molten steel after vacuum C deoxidation pretreatment, and w[%S] represents the initial S content in the molten steel.

[0076] This invention uses magnesium alloy for deoxidation and desulfurization, which can reduce the content of O and S in steel, while increasing the activity of Ca in the molten steel and improving the deoxidation and desulfurization effect of Ca in the molten steel.

[0077] After completing the deoxidation and desulfurization, the present invention adds Ca alloy to the deoxidized and desulfurized molten steel for deep deoxidation and desulfurization, and the deep deoxidation and desulfurization time is 2~3 minutes.

[0078] In this invention, the Ca alloy is preferably a Si-Ca alloy, a silicon-calcium cored wire, or a Ni-Ca alloy, and the mass content of Ca in the Ca alloy is preferably 20% to 40%, more preferably 25% to 35%.

[0079] In this invention, the amount of Ca added to the molten steel via Ca alloy is determined according to formula 7 or formula 8:

[0080] When the initial sulfur content in the molten steel is >0.003% and ≤0.005%, it is determined according to Equation 7: w[%Ca]=(5×w[%O] C -0.35×w[%Mg]+1.7×w[%S]-0.011%) / 5% Equation 7;

[0081] When the initial sulfur content in the molten steel is ≤0.003%, it is determined according to Equation 8: w[%Ca]=(2.5×w[%O] C -0.25×w[%Mg]+1.25×w[%S]-0.0188%) / 5% Equation 8; In equations 7 and 8: w[%Ca] represents the amount of Ca added to the molten steel by adding Ca alloy, and w[%O] represents the amount of Ca added. C w[%S] represents the O content in the molten steel after vacuum C deoxidation pretreatment, w[%Mg] represents the initial S content in the molten steel, and w[%Mg] represents the amount of Mg added to the molten steel by adding Mg alloy.

[0082] This invention achieves deep desulfurization and deep deoxidation of molten steel by adding Ca alloy.

[0083] This invention employs a Mg+Ca composite treatment method for deoxidation and desulfurization during the vacuum induction melting stage. The Mg+Ca composite treatment has a synergistic effect; while Mg reduces the O content in the steel, it significantly increases the activity of Ca in the molten steel. Therefore, the deoxidation and desulfurization effect of Ca in the molten steel is improved, contributing to deep deoxidation and desulfurization of the steel. To further enhance the effectiveness of the Mg+Ca composite treatment and the utilization rate of Mg and Ca, a vacuum C deoxidation pretreatment process is added before the Mg+Ca composite treatment, based on the steel composition requirements, to reduce the initial oxygen content of the molten steel. Compared to using Mg or Ca alone, this multi-step composite treatment process not only achieves better deoxidation and desulfurization but also reduces the amount of Mg and Ca added to the steel, controlling smelting costs.

[0084] After completing the deep deoxidation and desulfurization, the present invention controls the temperature of the molten steel after deep deoxidation and desulfurization at 1510~1550℃, and after being killed, it is cast to obtain an ingot.

[0085] In this invention, the sedation treatment time is preferably 2 to 3 minutes.

[0086] The process described above, which involves first adding FeO to control aluminum, then undergoing vacuum C deoxidation pretreatment, Mg+Ca composite treatment for deep O and deep S removal, and finally casting, is the vacuum induction melting (VIM) stage.

[0087] After obtaining the ingot, the present invention forges the ingot into an electrode, and controls the filling ratio to be 0.6~0.8, preferably 0.65~0.75.

[0088] After obtaining the electrode, the present invention places the electrode in a vacuum arc remelting furnace for refining, controlling the vacuum degree to ≤0.1 Pa and the melting rate to (0.009~0.012)×D kg·min. -1 D is the diameter of the crystallizer, in mm. This invention maintains an extremely high vacuum, enabling deep dehydrogenation.

[0089] Once the refining process reaches its capping stage, the present invention performs multi-stage current reduction and shrinkage during the capping stage to obtain the high-purity 316L stainless steel.

[0090] In this invention, the current compensation time is preferably 90-100 min, more preferably 92-98 min; the compensation cycle is preferably 8-10 times, specifically 8, 9 or 10 times.

[0091] In this invention, vacuum arc remelting (VAR) significantly purifies molten steel. After vacuum induction melting, the impurity element composition (O, S, H) of the ingot is further reduced, and inclusions are purified. Furthermore, VAR is a rapid solidification method, thus greatly improving the uniformity of the steel and facilitating the acquisition of a uniform and fine solidification structure.

[0092] The following detailed description of the ultra-high purity 316L stainless steel and its preparation method provided by the present invention, with reference to the embodiments, should not be construed as limiting the scope of protection of the present invention.

[0093] Examples 1-4

[0094] This invention relates to a method for preparing ultra-high purity 316L stainless steel using a VIM+VAR dual smelting process. The smelting equipment is a 100 kg vacuum induction furnace with a furnace charge of 85 kg. Four heats of experimental steel were designed, namely Examples 1 to 4, and their target compositions are shown in Table 1.

[0095] Table 1. Target composition (wt.%) of ultra-high purity 316L stainless steel

[0096] The main impurity components of the smelting raw materials used are shown in Table 2.

[0097] Table 2. Impurity composition of raw materials used in smelting (wt.%)

[0098] The vacuum induction melting (VIM) processes in Examples 1-4 are as follows: (1) Loading: Weigh the required experimental raw materials according to the target composition shown in Table 1, and fill the raw materials into the vacuum induction melting furnace according to the principle of pressing down and loosening up; (2) Smelting: After the furnace is closed, turn on the vacuum pump, evacuate to less than 5 Pa and then start powering on, gradually increase the power until all the furnace charge is melted, and control the smelting temperature at 1500~1550℃. (3) Al removal treatment: Add the blocky FeO into the molten steel and refine for 2 to 3 minutes; (4) Vacuum C deoxidation pretreatment: Add high-purity C blocks with a purity ≥ 99.99% to the molten steel for deoxidation pretreatment, control the vacuum degree ≤ 10 Pa, and the refining time is 22~28 min; (5) Mg deoxidation and desulfurization treatment: 0.01~0.03 MPa of high-purity argon gas with a purity of ≥99.999% is introduced, and the temperature is controlled at 1470~1490℃. Fe-Mg alloy (Mg content is 25%) is added to the molten steel for deoxidation and desulfurization. The treatment time is 2~3 min. (6) Ca deep deoxidation and desulfurization treatment: After the Mg treatment is completed, Ni-Ca alloy (Ca content is 35%) is added to the molten steel for deep deoxidation and deep desulfurization, and the treatment time is 2~3 min; (7) Casting: Control the temperature of the molten steel at 1510~1550℃, and after 2~3 min of calming treatment, pour the molten steel into the ingot mold. After the molten steel cools and solidifies, take out the ingot.

[0099] The specific VIM smelting parameters for each embodiment are shown in Table 3.

[0100] Table 3. Smelting parameters of 316L austenitic stainless steel in Examples 1-4

[0101] The FeO content, C content, Fe-Mg alloy (Mg) content, and Ni-Ca alloy (Ca) content added in the examples are shown in Table 4.

[0102] Table 4. Amount of each substance added during smelting (wt.%)

[0103] The vacuum arc remelting (VAR) process in Examples 1-4 is as follows: (1) Forge the VIM ingot into electrodes and control the filling ratio to 0.6~0.8; (2) The electrode is placed in a vacuum arc remelting furnace for refining, the vacuum degree is controlled to be ≤0.1 Pa, and the melting rate is controlled to be (0.009~0.012)×D kg·min. -1 ; (3) During the capping period of smelting, multi-stage current reduction and feeding are carried out, and the feeding time is controlled at 90~100 min, with a feeding cycle of 8~10 times.

[0104] The specific VAR smelting parameters for each embodiment are shown in Table 5.

[0105] Table 5. VAR smelting parameters of 316L austenitic stainless steel in Examples 1-4

[0106] The chemical composition of the ultra-high purity 316L stainless steel obtained by the above method is shown in Table 6. It can be seen that after the VIM+VAR dual-process treatment, the contents of O, S, Al, N, P, and H elements in the steel were well controlled, achieving ultra-high purity smelting of 316L stainless steel.

[0107] Furthermore, the purity of the high-purity 316L stainless steel obtained by this invention was compared with that of 316L stainless steel in the prior art. Specifically, it was compared with Comparative Example 1 steel prepared by the patent "An Ultra-High Purity 316L Stainless Steel and Its Preparation Method" (CN116623105A), Comparative Example 2 steel prepared by the patent "An Ultra-Pure Austenitic Stainless Steel and Its Preparation Method" (CN116445828A), and Comparative Example 3 steel prepared by the patent "A Preparation Method of Ultra-High Cleanliness Stainless Steel Seamless Tube for Integrated Circuit and IC Industry Manufacturing Equipment" (CN114941055A). The results are shown in Table 6. As can be seen from Table 6, this invention achieves higher purity smelting of 316L stainless steel.

[0108] Table 6 Chemical composition (wt.%) of the steels used in the examples and comparative examples

[0109] The inclusion ratings of the ultra-high purity 316L stainless steel prepared in the examples are shown in Table 7. It can be seen that the coarse inclusions in the examples are all grade 0, and the inclusions of grades A+B+C+D are all ≤0.5. The α-ferrite volume content of the ultra-high purity 316L austenitic stainless steel in the examples is all 0, and the δ-ferrite volume content is all ≤0.1%.

[0110] Table 7 Inclusion grades in ultra-high purity 316L stainless steel in the examples

[0111] The ultra-high purity 316L stainless steel prepared in Examples 1-4 was processed into plates and then annealed at temperatures ranging from 1050℃ to 1080℃ for a time calculated based on the plate cross-sectional thickness of 1.25-1.45 min / mm. Tensile specimens of the required dimensions were prepared according to ASTM-E8 standards and tensile tests were conducted at room temperature. The grain size and mechanical properties of the ultra-high purity 316L stainless steel in the examples are shown in Table 8.

[0112] Table 8 Grain size and mechanical properties of ultra-high purity 316L stainless steel in the examples

[0113] As shown in Table 8, the steels of Examples 1 to 4 have good microstructure and mechanical properties. Specifically, the grain size of the ultra-high purity 316L stainless steel is 20 to 35 μm, the yield strength is ≥260 MPa, the tensile strength is ≥570 MPa, and the elongation is ≥70%.

[0114] In summary, the VIM (first adding FeO to control aluminum, then vacuum carbon deoxidation pretreatment, and finally Mg-Ca composite deoxidation and desulfurization) + VAR dual process can achieve ultra-high purity control and good microstructure and performance control of 316L stainless steel.

[0115] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A type of ultra-high purity 316L stainless steel, characterized in that, In terms of mass percentage, it includes: C ≤0.02%； Cr 16.00%~18.00%； Ni 10.00%~14.00%; Mo 2.00%~3.00%; N ≤0.004%； Si 0.30%~0.75%; Mn 0.50%~2.00%; P ≤0.003%； S ≤0.001%； Al ≤0.004%； O ≤0.0007%； H ≤0.0001%； The remainder is Fe; The coarse inclusions in the ultra-high purity 316L stainless steel are grade 0, and the fine inclusions (A+B+C+D) are ≤ grade 0.

5. The preparation method of the ultra-high purity 316L stainless steel includes the following steps: (1) Prepare the raw materials according to the target composition and perform vacuum induction melting to obtain molten steel; by mass percentage, the total Al introduced by the raw materials is ≤0.04%, the total O is ≤0.04%, the total S is ≤0.005%, and the total P is ≤0.003%; the material of the crucible used for vacuum induction melting is MgO, CaO or MgO-CaO mixed material; (2) Add FeO to the molten steel to perform refining and Al removal, and the refining and Al removal time is 2~3 min; When the initial O content of the molten steel is >0.015% and ≤0.04%, the amount of FeO added is determined according to Formula 1: w[%FeO]=4×(w[%Al]-0.004%)-4.5×(w[%O]-0.015%) Formula 1; if w[%FeO]≤0 is calculated according to Formula 1, then it is not necessary to add FeO to the molten steel. When the initial oxygen content of the molten steel is ≤0.015%, the amount of FeO added is determined according to formula 2: w[%FeO]=4×(w[%Al]-0.004%)+4.5(0.015%-w[%O]) Formula 2; In Equations 1 and 2: w[%FeO] is the amount of FeO added, w[%Al] is the initial Al content in the molten steel, and w[%O] is the initial O content in the molten steel; (3) Add high-purity C to the de-Al steel liquid and perform vacuum C deoxidation pretreatment to control the O content of the steel liquid at 0.006%~0.008%; The amount of high-purity C added is determined according to formula 3: w[%C]=0.7×(w[%O]-0.006%) Equation 3; In Formula 3: w[%C] is the amount of high-purity C added, and w[%O] is the initial O content in the molten steel; The time for the vacuum C deoxidation pretreatment is determined according to Equation 4: Equation 4; In Equation 4: t Vacuum C deoxidation pretreatment time, in seconds; F Let m be the free surface area of ​​the molten steel. 2 ; V Let m be the volume of molten steel. 3 ;w[%O] C The oxygen content in steel after vacuum C deoxidation pretreatment is % (%). (4) After the vacuum C deoxidation pretreatment is completed, high-purity argon gas is introduced into the vacuum induction melting furnace, the temperature of the molten steel is controlled at 1470~1490℃, and Mg alloy is added for deoxidation and desulfurization. The deoxidation and desulfurization time is 2~3 min. The amount of Mg added to the molten steel using Mg alloy is determined according to formula 5 or formula 6: When the initial sulfur content in the molten steel is >0.003% and ≤0.005%, it is determined according to Equation 5: w[%Mg] = 1.5 × (w[%O] C - 0.003%) / 15% + 0.75 × (w[%S] - 0.003%) / 15% Equation 5; When the initial sulfur content in the molten steel is ≤0.003%, it is determined according to Equation 6: w[%Mg] = 1.5 × (w[%O] C - 0.003%) / 15% Formula 6; In equations 5 and 6: w[%Mg] represents the amount of Mg added to the molten steel by adding Mg alloy, and w[%O] represents the amount of Mg added. C The O content in the molten steel after vacuum C deoxidation pretreatment is w[%S], and the initial S content in the molten steel is w[%S]. (5) Add Ca alloy to the deoxidized and desulfurized molten steel for deep deoxidation and desulfurization, the time of which is 2~3 min; The amount of Ca added to molten steel via Ca alloying is determined according to formula 7 or formula 8: When the initial sulfur content in the molten steel is >0.003% and ≤0.005%, it is determined according to Equation 7: w[%Ca]=(5×w[%O] C -0.35×w[%Mg]+1.7×w[%S]-0.011%) / 5% Formula 7; When the initial sulfur content in the molten steel is ≤0.003%, it is determined according to Equation 8: w[%Ca]=(2.5×w[%O] C -0.25×w[%Mg]+1.25×w[%S]-0.0188%) / 5% Formula 8; In equations 7 and 8: w[%Ca] represents the amount of Ca added to the molten steel by adding Ca alloy, and w[%O] represents the amount of Ca added. C The value represents the O content in the molten steel after vacuum C deoxidation pretreatment, w[%S] represents the initial S content in the molten steel, and w[%Mg] represents the amount of Mg added to the molten steel by adding Mg alloy. (6) The temperature of the molten steel after deep deoxidation and desulfurization is controlled at 1510~1550℃. After being killed, it is cast to obtain ingots. (7) Forge the ingot into an electrode, controlling the filling ratio to be 0.6~0.8; (8) The electrode is placed in a vacuum arc remelting furnace for refining, with the vacuum degree controlled to be ≤0.1 Pa and the melting rate controlled to be (0.009~0.012)×D kg·min. -1 D is the diameter of the crystallizer, in mm; (9) Multi-stage current reduction and shrinkage are carried out during the refining capping period to obtain the ultra-high purity 316L stainless steel.

2. The ultra-high purity 316L stainless steel according to claim 1, characterized in that, The mechanical properties at room temperature are: yield strength ≥260 MPa, tensile strength ≥570 MPa, and elongation ≥70%.

3. The ultra-high purity 316L stainless steel according to claim 1, characterized in that, The volume content of α-ferrite is 0, and the volume content of δ-ferrite is ≤0.1%.

4. The ultra-high purity 316L stainless steel according to claim 1, characterized in that, The ultra-high purity 316L stainless steel, after hot rolling and annealing, has a grain size of 20~35 μm.

5. A method for preparing ultra-high purity 316L stainless steel according to any one of claims 1 to 4, comprising the following steps: (1) Prepare the raw materials according to the target composition and perform vacuum induction melting to obtain molten steel; by mass percentage, the total Al introduced by the raw materials is ≤0.04%, the total O is ≤0.04%, the total S is ≤0.005%, and the total P is ≤0.003%; the material of the crucible used for vacuum induction melting is MgO, CaO or MgO-CaO mixed material; (2) Add FeO to the molten steel to perform refining and Al removal, and the refining and Al removal time is 2~3 min; When the initial O content of the molten steel is >0.015% and ≤0.04%, the amount of FeO added is determined according to Formula 1: w[%FeO]=4×(w[%Al]-0.004%)-4.5×(w[%O]-0.015%) Formula 1; if w[%FeO]≤0 is calculated according to Formula 1, then it is not necessary to add FeO to the molten steel. When the initial oxygen content of the molten steel is ≤0.015%, the amount of FeO added is determined according to formula 2: w[%FeO]=4×(w[%Al]-0.004%)+4.5(0.015%-w[%O]) Formula 2; In Equations 1 and 2: w[%FeO] is the amount of FeO added, w[%Al] is the initial Al content in the molten steel, and w[%O] is the initial O content in the molten steel; (3) Add high-purity C to the de-Al steel liquid and perform vacuum C deoxidation pretreatment to control the O content of the steel liquid at 0.006%~0.008%; The amount of high-purity C added is determined according to formula 3: w[%C]=0.7×(w[%O]-0.006%) Equation 3; In Formula 3: w[%C] is the amount of high-purity C added, and w[%O] is the initial O content in the molten steel; The time for the vacuum C deoxidation pretreatment is determined according to Equation 4: Equation 4; In Equation 4: t Vacuum C deoxidation pretreatment time, in seconds; F Let m be the free surface area of ​​the molten steel. 2 ; V Let m be the volume of molten steel. 3 ;w[%O] C The oxygen content in steel after vacuum C deoxidation pretreatment is % (%). (4) After the vacuum C deoxidation pretreatment is completed, high-purity argon gas is introduced into the vacuum induction melting furnace, the temperature of the molten steel is controlled at 1470~1490℃, and Mg alloy is added for deoxidation and desulfurization. The deoxidation and desulfurization time is 2~3 min. The amount of Mg added to the molten steel using Mg alloy is determined according to formula 5 or formula 6: When the initial sulfur content in the molten steel is >0.003% and ≤0.005%, it is determined according to Equation 5: w[%Mg]=1.5×(w[%O] C -0.003%) / 15%+0.75×(w[%S]-0.003%) / 15% Equation 5; When the initial sulfur content in the molten steel is ≤0.003%, it is determined according to Equation 6: w[%Mg]=1.5×(w[%O] C -0.003%) / 15% Formula 6; In equations 5 and 6: w[%Mg] represents the amount of Mg added to the molten steel by adding Mg alloy, and w[%O] represents the amount of Mg added. C The O content in the molten steel after vacuum C deoxidation pretreatment is w[%S], and the initial S content in the molten steel is w[%S]. (5) Add Ca alloy to the deoxidized and desulfurized molten steel for deep deoxidation and desulfurization, the time of which is 2~3 min; The amount of Ca added to molten steel via Ca alloying is determined according to formula 7 or formula 8: When the initial sulfur content in the molten steel is >0.003% and ≤0.005%, it is determined according to Equation 7: w[%Ca]=(5×w[%O] C -0.35×w[%Mg]+1.7×w[%S]-0.011%) / 5% Formula 7; When the initial sulfur content in the molten steel is ≤0.003%, it is determined according to Equation 8: w[%Ca]=(2.5×w[%O] C -0.25×w[%Mg]+1.25×w[%S]-0.0188%) / 5% Equation 8; In equations 7 and 8: w[%Ca] represents the amount of Ca added to the molten steel by adding Ca alloy, and w[%O] represents the amount of Ca added. C The value represents the O content in the molten steel after vacuum C deoxidation pretreatment, w[%S] represents the initial S content in the molten steel, and w[%Mg] represents the amount of Mg added to the molten steel by adding Mg alloy. (6) The temperature of the molten steel after deep deoxidation and desulfurization is controlled at 1510~1550℃. After being killed, it is cast to obtain ingots. (7) Forge the ingot into an electrode, controlling the filling ratio to be 0.6~0.8; (8) The electrode is placed in a vacuum arc remelting furnace for refining, with the vacuum degree controlled to be ≤0.1 Pa and the melting rate controlled to be (0.009~0.012)×D kg·min. -1 D is the diameter of the crystallizer, in mm; (9) Multi-stage current reduction and shrinkage are carried out during the refining capping period to obtain the ultra-high purity 316L stainless steel.

6. The preparation method according to claim 5, characterized in that, In step (1), the temperature of the vacuum induction melting is 1500~1550℃.

7. The preparation method according to claim 5, characterized in that, In step (3), the vacuum degree of the vacuum C deoxygenation pretreatment is ≤10 Pa.

8. The preparation method according to claim 5, characterized in that, In step (4), the Mg alloy is a Ni-Mg alloy or an Fe-Mg alloy, and the mass content of Mg in the Mg alloy is 10%~30%.

9. The preparation method according to claim 5, characterized in that, In step (5), the Ca alloy is a Si-Ca alloy, a silicon-calcium cored wire, or a Ni-Ca alloy, and the mass content of Ca in the Ca alloy is 20%~40%.

10. The preparation method according to claim 5, characterized in that, In step (9), the multi-stage current reduction compensation time is 90~100 min, and the compensation cycle is 8~10 times.

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