A low-cost ferritic stainless steel and a manufacturing method thereof

Abstract

The present invention relates to a low-cost ferritic stainless steel and a manufacturing method thereof, belonging to the field of metallic materials. The chemical composition thereof by mass percentage includes: C ≤ 0.009%, Si: 0.19 - 0.49%, S ≤ 0.0014%, P ≤ 0.009%, Ni: 1.1 - 1.9%, Cr: 19.1 - 19.9%, Cu: 0.19 - 0.49%, N ≤ 0.008%, 70×10<supgt;-4< / supgt>% ≤ Mg + Zr ≤ 290×10<supgt;-4< / supgt>%, and the contents of Zr and Mg satisfy Zr / Mg = 3 - 6, with the balance being Fe and inevitable impurities. The present invention uses hot metal rich in chromium and nickel smelted from laterite nickel ore as the smelting raw material, saving precious metals Ni and Mo. The obtained low-cost ferritic stainless steel does not contain high-cost alloy elements such as Nb, and good mechanical properties are obtained by using low-cost elements such as silicon, copper, and chromium. At the same time, zirconium and magnesium elements are compounded and added, obtaining good corrosion resistance while improving the welding performance.

Description

Technical Field

[0001] The present invention relates to the field of metal materials, and particularly to a low-cost ferritic stainless steel and a manufacturing method thereof. Background Art

[0002] In the entire temperature range from high temperature to normal temperature, the ferritic stainless steel is mainly a stable α-phase, containing 11-30% Cr and C below 0.1%. Even when quenched, it is impossible to harden the steel. In addition, the ferritic stainless steel also contains stabilizing elements such as Al, Si, Mo, Ti, Nb, etc. According to the chromium content in the steel, such stainless steels can be further divided into low-chromium type (chromium content 10.5-15%), medium-chromium type (chromium content 16-22%) and high-chromium type (chromium content 23-32%). The output of ferritic stainless steel ranks second only to chromium-nickel austenitic stainless steel. In the total output of stainless steel in the world, the output of ferritic stainless steel generally accounts for about 30%. Ferritic stainless steel has a wide range of applications. Under thin-section conditions, it has excellent comprehensive properties. Since it does not contain nickel or only contains a small amount of nickel in individual grades, the cost and price are relatively low, and it is the most important nickel-saving stainless steel among stainless steels.

[0003] The ferritic stainless steel has a low cold working hardening tendency. For large cold deformations, intermediate annealing or treatment is generally not required, and it is easy to cold bend, punch, expand pipes, curl edges, spin, cold forge and cut. The ferritic stainless steel has excellent overall corrosion resistance and various local corrosion resistances, especially excellent chloride stress corrosion resistance. It has a high thermal conductivity, about 135% of that of austenitic stainless steel, and is very suitable for applications with heat exchange. It has a small thermal expansion coefficient, about 60% of that of austenitic stainless steel, and is very suitable for use conditions with thermal expansion and contraction and thermal cycling. For example, typical applications such as the partition plates of heat transfer tubes in evaporators and heat exchangers with water as the medium. In the service environment of water medium, how to improve the local corrosion caused by water medium has become an important technical, engineering and application problem that needs to be urgently solved in this regard.

[0004] In addition, conventional ferritic stainless steels have other disadvantages and deficiencies. Generally speaking, conventional ferritic stainless steels have the following disadvantages and deficiencies: The room temperature toughness of ferritic stainless steel is low, it is more sensitive to intergranular corrosion, and the cold formability needs to be further improved. When welded, these disadvantages are more obvious, the performance deteriorates further, seriously affecting the application of traditional ferritic stainless steel in welding applications. How to improve toughness and weldability has become an important application performance that urgently needs to be solved for ferritic stainless steel. In view of this, the present invention provides a low-cost ferritic stainless steel and a manufacturing method thereof. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to provide a low-cost ferritic stainless steel and a manufacturing method thereof. The purpose is to improve the toughness, welding performance and intergranular corrosion performance of ferritic stainless steel.

[0006] To solve the above technical problems, the technical solution provided in the first aspect is as follows: A low-cost ferritic stainless steel, which comprises the following chemical components by mass percentage: C ≤ 0.009%, Si: 0.19 - 0.49%, S ≤ 0.0014%, P ≤ 0.009%, Ni: 1.1 - 1.9%, Cr: 19.1 - 19.9%, Cu: 0.19 - 0.49%, N ≤ 0.008%, 70×10 -4 % ≤ Mg + Zr ≤ 290×10 -4 %, and the contents of Zr and Mg satisfy Zr / Mg = 3 - 6, and the balance is Fe and inevitable impurities.

[0007] The functions of the above components in the microalloyed ferritic stainless steel are as follows:

[0008] Cr: The higher the content of Cr element in ferritic stainless steel, the easier it is to be passivated, which will make the surface of ferritic stainless steel not easily oxidized, thus having better corrosion resistance. At the same time, the abilities of resisting pitting corrosion, crevice corrosion and intergranular corrosion will also be improved; meanwhile, when the mass fraction of Cr is higher, the formation rate of brittle phases in ferritic stainless steel will be faster. In addition, the formation and precipitation rates of α' and σ phases are also related to the mass fraction of Cr. The higher the Cr mass fraction, the faster the precipitation rate, and the precipitated phases will reduce the toughness of the steel and significantly increase the brittle transition temperature.

[0009] C, N: In ferritic stainless steel, in addition to strengthening the stainless steel, the effects of C and N on other properties of the stainless steel are almost all harmful. For example, high brittle transition temperature, large notch sensitivity, and decreased corrosion resistance after welding are all related to C and N in the steel. However, there is currently no way to completely avoid the presence of C and N in ferritic stainless steel. Since both C and N are elements that strongly form and stabilize austenite and expand the γ region in the Fe-Cr alloy, therefore, the important influence of C and N on the structure of ferritic stainless steel is to shift the α + γ two-phase region towards a higher Cr content direction, so that ferritic stainless steel with higher C and N contents may have a ferritic + martensite (austenite) duplex structure. In addition, because the solubilities of C and N in ferrite are very low, carbonitrides will precipitate during the subsequent cooling process after high-temperature heating, and they have a very important impact on the properties of ferritic stainless steel.

[0010] Cu: It is a very weak austenite-forming element. The addition of a small amount generally does not have a significant impact on the microstructure of ferritic stainless steel. Its main function is to improve the corrosion resistance and cold-forming formability of ferritic stainless steel.

[0011] Ni: Similar to C and N, Ni can also shift the α+γ two-phase region of the Fe-Cr alloy towards a higher Cr concentration. Ni can significantly improve the room-temperature mechanical properties of ferritic stainless steels, especially strength and toughness, and lower the brittle transition temperature of the steel. Adding Ni to ferritic stainless steels can further improve the general corrosion resistance, pitting corrosion resistance, and crevice corrosion resistance of the steel in certain media, but it is harmful to the chloride stress corrosion resistance of ferritic stainless steels. To obtain a single pure ferritic structure in Ni-containing ferritic stainless steels, as the Ni content increases, not only the Cr content in the steel needs to be increased, but sometimes ferrite-forming elements such as Mo, Ti, and Nb also need to be added.

[0012] S: Sulfur is regarded as a harmful impurity in austenitic stainless steels. The harmful effects of sulfur are mainly as follows: It reduces the hot plasticity of austenitic stainless steels and affects the hot workability of the steel, which is related to the precipitation of MnS or (Fe,Mn)S along the grain boundaries at high temperatures; sulfur also reduces the corrosion resistance of austenitic stainless steels. MnS is easily soluble in acidic chloride solutions and often becomes a corrosion source, leading to a significant reduction in pitting corrosion resistance and crevice corrosion resistance.

[0013] P: Phosphorus is generally regarded as a harmful impurity in austenitic stainless steels. The harmful effects of phosphorus are mainly: It significantly reduces the corrosion resistance of chromium-nickel austenitic stainless steels in nitric acid at various concentrations in the solution-annealed and sensitized states; it significantly enhances the intergranular corrosion sensitivity of chromium-nickel austenitic stainless steels in concentrated nitric acid and nitric acid containing Cr 5+ and reduces the corrosion resistance under these service conditions.

[0014] Zr: Zirconium is a stabilizing element with a strong affinity for C and N. It is also a strong deoxidizing element and a composite oxygen-sulfide forming element. Adding a small amount of zirconium has the effects of degassing, purifying, and refining the grains, which is beneficial to improving the low-temperature properties of stainless steels, improving the stamping performance, and significantly increasing the hardenability of the steel when dissolved in austenite. In austenitic steels, it can prevent intergranular corrosion of the steel by oxidation media. Due to the carbon fixation and precipitation hardening effects, it can improve the high-temperature properties of heat-resistant steels, such as creep strength, etc.

[0015] Mg: Magnesium is a strong deoxidizing element and a composite oxygen-sulfide forming element. Magnesium can reduce the number, size, and make the distribution of inclusions in the steel uniform and improve their morphology. Trace amounts of magnesium can improve the size and distribution of carbides in stainless steels. The carbide particles are fine and uniform, and the formed MgO inclusions have the effect of pinning the austenite grain boundaries, which has a good control effect on the grain size.

[0016] In summary, the present invention uses hot metal rich in chromium and nickel smelted from laterite nickel ore as the smelting raw material, saving precious metals Ni and Mo. The low-cost ferritic stainless steel prepared does not contain high-cost alloy elements such as Nb, and good mechanical properties are obtained by using low-cost elements such as silicon, copper, and chromium. At the same time, zirconium and magnesium elements are compound-added, obtaining good corrosion resistance while improving the welding performance.

[0017] The beneficial effects of the present invention are as follows:

[0018] (1) In addition to ultra-low carbon and nitrogen, the stainless steel of the present invention contains more nickel (1.1 - 1.9 wt%), and has a ferritic structure in a suitable heat treatment state. Therefore, in addition to the properties of the above-mentioned low-chromium ferritic stainless steel, it also has good strength and toughness, and excellent formability, weldability, corrosion resistance, and wear resistance. Due to the addition of Zr and Mg, there is no intergranular corrosion sensitivity after welding. Therefore, it can be used for applications such as corrosion resistance and wear resistance, replacing low-carbon steel and corrosion-resistant low-alloy steel with or without coatings, and having a low life cycle cost. It can also be applied to vehicle bodies such as ore trucks, tank cars, and coal trucks, the chassis, frames, and outer panels of large sedans, trucks, and buses, telephone distribution boxes in municipal facilities, switch devices, and lighting poles and transmitting antennas for sidewalks, stairs, and entrances of large buildings.

[0019] (2) The present invention uses hot metal rich in chromium and nickel smelted from laterite nickel ore as the smelting raw material, saving precious metals Ni and Mo; the low-cost ferritic stainless steel prepared does not contain high-cost alloy elements such as Nb, and good mechanical properties are obtained by using low-cost elements such as silicon, copper, and chromium; zirconium and magnesium elements are compound-added, obtaining good corrosion resistance while improving the welding performance, especially seawater corrosion resistance.

[0020] (3) The microalloyed ferritic stainless steel of the present invention has strong tissue stability, good high-temperature thermoplasticity and high-temperature oxidation resistance, and more excellent comprehensive mechanical properties.

[0021] On the basis of the above technical solutions, the present invention can be further improved as follows.

[0022] Further, it includes the following chemical components by mass percentage: C ≤ 0.006%, Si: 0.29 - 0.49%, S ≤ 0.0010%, P ≤ 0.005%, Ni: 1.5 - 1.9%, Cr: 19.5 - 19.9%, Cu: 0.29 - 0.49%, N ≤ 0.006%, 165×10 -4 % ≤ Mg + Zr ≤ 290×10 -4 %, and the contents of Zr and Mg satisfy Zr / Mg = 3 - 6, and the balance is Fe and inevitable impurities.

[0023] Further, it includes the following chemical components by mass percentage: C ≤ 0.008%, Si: 0.19 - 0.29%, S ≤ 0.0013%, P ≤ 0.008%, Ni: 1.1 - 1.5%, Cr: 19.1 - 19.5%, Cu: 0.19 - 0.29%, N ≤ 0.007%, 70×10 -4 % ≤ Mg + Zr ≤ 200×10 -4 %, and the contents of Zr and Mg satisfy Zr / Mg = 3 - 6, with the balance being Fe and unavoidable impurities.

[0024] Further, it includes the following chemical components by mass percentage: C: 0.008%, Si: 0.29%, S ≤ 0.0013%, P: 0.008%, Ni: 1.5%, Cr: 19.5%, Cu: 0.29%, N: 0.007%, Mg: 0.0050%, Zr: 0.0150%, with the balance being Fe and unavoidable impurities.

[0025] The second aspect of the present invention provides a method for preparing a low-cost ferritic stainless steel, comprising the following steps:

[0026] Step 1, take the following raw materials according to the weight ratio: 35 - 40% of ultra-low P industrial pure iron, 10 - 15% of high-carbon ferrochrome, and 50 - 60% of hot metal smelted from laterite nickel ore; first add the ultra-low P industrial pure iron and the high-carbon ferrochrome to an intermediate frequency furnace for melting, and then add the hot metal smelted from laterite nickel ore to the intermediate frequency furnace to obtain the raw material for smelting low-cost ferritic stainless steel;

[0027] Step 2: Refine, continuously cast, roll, and heat-treat the raw material for smelting low-cost ferritic stainless steel obtained in Step 1 to obtain the low-cost ferritic stainless steel.

[0028] Further, in Step 1, take the following raw materials according to the weight ratio: 34 - 40% of ultra-low P industrial pure iron, 10 - 14% of high-carbon ferrochrome, and 52 - 60% of hot metal smelted from laterite nickel ore.

[0029] Further, in Step 1, the weight contents of chemical components P and C in the ultra-low P industrial pure iron are respectively: P ≤ 0.0035%, C ≤ 0.01%; the weight contents of chemical components Cr and C in the high-carbon ferrochrome are respectively: Cr ≥ 60%, C ≤ 9.5%.

[0030] Further, in Step 1, the weight contents of chemical components Cr and P in the hot metal smelted from laterite nickel ore are respectively: Cr ≥ 60%, P ≤ 0.03%.

[0031] Further, in Step 2, the specific steps of subjecting the raw materials for smelting low-cost ferritic stainless steel obtained in Step 1 to refining, continuous casting, rolling, and heat treatment are as follows:

[0032] Step 2-1, refining: subjecting the raw materials for smelting low-cost ferritic stainless steel to secondary refining in an LF furnace, and performing alloy fine-tuning and microalloying treatment according to the chemical components in the low-cost ferritic stainless steel to obtain microalloyed stainless steel;

[0033] Step 2-2, continuous casting: subjecting the microalloyed stainless steel to continuous casting under the conditions that the tundish temperature is 1465-1480°C and the working drawing speed is 1.05-1.20 m / min to obtain a continuous casting billet;

[0034] Step 2-3, rolling: rolling the continuous casting billet at a temperature of 1150-1250°C to obtain a hot-rolled black skin coil;

[0035] Step 2-4, heat treatment: heating the hot-rolled black skin coil to a temperature of 950-1050°C, holding for 60-120 min, and cooling to room temperature to obtain low-cost ferritic stainless steel.

[0036] Further, in Step 2-1, the refining time is 45-90 min, and the tapping temperature ≥ 1530°C. Description of the Drawings

[0037] Figure 1 is a morphology diagram of inclusions in the ferritic stainless steel of the conventional technology of the present invention;

[0038] Figure 2 is a morphology diagram of inclusions in the ferritic stainless steel prepared by the present invention;

[0039] Figure 3 is an optical micrograph of the substrate of the present invention;

[0040] Figure 4 is an optical micrograph of the heat-affected zone of the welding of the present invention;

[0041] Figure 5 is a physical photograph of the present invention after 30 days of corrosion in a sodium chloride solution (50 ± 5 g / L). Detailed Embodiments

[0042] The principles and features of the present invention are described below. The examples given are only for explaining the present invention and are not intended to limit the scope of the present invention.

[0043] Example 1

[0044] This embodiment relates to a low-cost ferritic stainless steel, which comprises the following chemical components by mass percentage: 0.006 wt% C, 0.19 wt% Si, 0.0010 wt% S, 0.005 wt% P, 1.1 wt% Ni, 19.1 wt% Cr, 0.19 wt% Cu, 0.006 wt% N, 0.0030 wt% Mg, 0.0135 wt% Zr, and the balance is Fe and inevitable impurities.

[0045] This embodiment relates to a preparation method of a low-cost ferritic stainless steel, comprising the following steps:

[0046] Step 1: Charge materials in the following manner: Use ultra-low P industrial pure iron as the smelting base material, with a mass ratio of 40%; charge high-carbon ferrochrome, with a mass ratio of 10%; then add molten iron smelted from laterite nickel ore, with a mass ratio of 50%. Among them, P in the ultra-low P industrial pure iron ≤ 0.0035 wt%, C ≤ 0.005 wt%. Cr in the high-carbon ferrochrome ≥ 60 wt%, C ≤ 6.5 wt%. Cr in the molten iron smelted from laterite nickel ore ≥ 60 wt%, P ≤ 0.01 wt%. Use an intermediate frequency furnace to melt the ultra-low P industrial pure iron (mass ratio 40%) and high-carbon ferrochrome (mass ratio 10%), and then add the molten iron smelted from laterite nickel ore (mass ratio 50%) to obtain the raw material for smelting low-cost ferritic stainless steel. After smelting, P in the stainless steel ≤ 0.014 wt%;

[0047] Step 2: Subject the raw material for smelting low-cost ferritic stainless steel obtained in Step 1 to refining, continuous casting, rolling, and heat treatment in sequence to obtain a low-cost ferritic stainless steel.

[0048] Preferably, in Step 2, the raw material for smelting low-cost ferritic stainless steel is subjected to refining, continuous casting, rolling, and heat treatment in sequence, including the following specific steps:

[0049] Step 2-1, LF refining outside the furnace: Before tapping, slag flowing and slag skimming are carried out to clean the furnace slag as much as possible; tapping with white slag, and a small amount of lime and fluorite can be used to adjust the reducibility and fluidity of the furnace slag; the tapping temperature ≥ 1530 °C; first, energize to melt the top slag, and then carry out alloy fine-tuning and microalloying to obtain a chemical composition by weight percentage including: 0.006 wt% C, 0.19 wt% Si, 0.0010 wt% S, 0.005 wt% P, 1.1 wt% Ni, 19.1 wt% Cr, 0.19 wt% Cu, 0.006 wt% N, N, 0.0030 wt% Mg, 0.0135 wt% Zr, and the balance is Fe and inevitable impurities; the smelting time of the LF refining furnace is 45 min.

[0050] Step 2-2, continuous casting: tundish temperature is 1465°C; working casting speed is 1.05 m / min; water flow rate on the wide face of the mold is 175 m 3 / h, and the water flow rate on the narrow face is 20 m 3 / h, and the temperature difference between the inlet and outlet water is 5°C; the mold uses non-sinusoidal vibration, the vibration frequency is 140 CPM, and the amplitude is 3.0 mm; the mold powder is a ferritic stainless steel mold powder.

[0051] Step 2-3, rolling: the continuously cast billet is first heated and then hot-rolled to obtain a hot-rolled black skin coil; the temperature of the hot rolling is 1150°C.

[0052] Step 2-4, heat treatment: heating and holding at 780°C, and then air cooling or furnace cooling.

[0053] Example 2

[0054] This example relates to a low-cost ferritic stainless steel, which includes the following chemical components by mass percentage: 0.008 wt% C, 0.29 wt% Si, 0.0013 wt% S, 0.008 wt% P, 1.5 wt% Ni, 19.5 wt% Cr, 0.29 wt% Cu, 0.007 wt% N, 0.0050 wt% Mg, 0.0150 wt% Zr, and the balance is Fe and unavoidable impurities.

[0055] This example relates to a preparation method of a low-cost ferritic stainless steel, which includes the following steps:

[0056] Step 1: Charge materials in the following way: Use ultra-low P industrial pure iron as the smelting base material, with a mass ratio of 34%; charge high-carbon ferrochrome, with a mass ratio of 14%; then add molten iron smelted from laterite nickel ore, with a mass ratio of 52%. Among them, P in the industrial pure iron ≤ 0.005 wt%, C ≤ 0.01 wt%. Cr in the high-carbon ferrochrome ≥ 60 wt%, C ≤ 9.5 wt%. Cr in the molten iron smelted from laterite nickel ore ≥ 60 wt%, P ≤ 0.03 wt%. Use an intermediate frequency furnace to melt ultra-low P industrial pure iron (mass ratio 34%) and high-carbon ferrochrome (mass ratio 14%), and then add molten iron smelted from laterite nickel ore (mass ratio 52%) to obtain the raw material for smelting low-cost ferritic stainless steel. After smelting, P in the stainless steel ≤ 0.015 wt%;

[0057] Step 2: Refine, continuously cast, roll, and heat-treat the raw material for smelting low-cost ferritic stainless steel obtained in Step 1 to obtain a low-cost ferritic stainless steel.

[0058] Preferably, in Step 2, the raw material for smelting low-cost ferritic stainless steel is successively refined, continuously cast, rolled, and heat-treated, including the following specific steps:

[0059] Step 2-1, LF secondary refining: Before tapping, slag flowing and slag skimming are carried out to clean the slag as much as possible; tapping with white slag, and a small amount of lime and fluorite can be used to adjust the reducibility and fluidity of the slag; the tapping temperature ≥ 1530 °C; first, the top slag is melted by electrifying, and then alloy fine-tuning and microalloying are carried out to obtain a chemical composition by weight percentage including: 0.008 wt% C, 0.29 wt% Si, 0.0013 wt% S, 0.008 wt% P, 1.5 wt% Ni, 19.5 wt% Cr, 0.29 wt% Cu, 0.007 wt% N, 0.0050 wt% Mg, 0.0150 wt% Zr, and the balance is Fe and unavoidable impurities; the smelting time of the LF refining furnace is 60 min.

[0060] Step 2-2, continuous casting: The tundish temperature is 1470 °C; the working drawing speed is 1.10 m / min; the water flow rate on the wide face of the mold is 185 m 3 / h, and the water flow rate on the narrow face is 22 m 3 / h, and the temperature difference between the inlet and outlet water is 6 °C; the mold adopts non-sinusoidal vibration, the vibration frequency is 140 CPM, and the amplitude is 3.0 mm; the mold powder is a ferritic stainless steel mold powder.

[0061] Step 2-3, rolling: The continuous casting billet is first heated and then hot-rolled to obtain a hot-rolled black skin coil; the temperature of the hot rolling is 1200 °C.

[0062] Step 2-4, heat treatment: Heating and holding at 800 °C, and then air cooling or furnace cooling.

[0063] Example 3

[0064] This example relates to a low-cost ferritic stainless steel, which includes the following chemical components by mass percentage: 0.009 wt% C, 0.49 wt% Si, 0.0014 wt% S, 0.009 wt% P, 1.9 wt% Ni, 19.9 wt% Cr, 0.49 wt% Cu, 0.008 wt% N, 0.0020 wt% Mg, 0.0120 wt% Zr, and the balance is Fe and unavoidable impurities.

[0065] This example relates to a preparation method of a low-cost ferritic stainless steel, including the following steps:

[0066] Step 1: Charge materials in the following manner: Use ultra-low P industrial pure iron as the smelting base material, with a mass ratio of 35%; add high-carbon ferrochrome, with a mass ratio of 15%; then add molten iron smelted from laterite nickel ore, with a mass ratio of 60%. Among them, P in the industrial pure iron is ≤0.005wt%, and C is ≤0.01wt%. Cr in the high-carbon ferrochrome is ≥60wt%, and C is ≤9.5wt%. Cr in the molten iron smelted from laterite nickel ore is ≥60wt%, and P is ≤0.03wt%. Use an intermediate frequency furnace to melt the ultra-low P industrial pure iron (mass ratio 35%) and high-carbon ferrochrome (mass ratio 15%), and then add the molten iron smelted from laterite nickel ore (mass ratio 60%) to obtain the raw material for smelting low-cost ferritic stainless steel. After smelting, P in the stainless steel is ≤0.012wt%;

[0067] Step 2: Subject the raw material for smelting low-cost ferritic stainless steel obtained in Step 1 to refining, continuous casting, rolling, and heat treatment in sequence to obtain low-cost ferritic stainless steel.

[0068] Preferably, the raw material for smelting low-cost ferritic stainless steel in Step 2 is subjected to refining, continuous casting, rolling, and heat treatment in sequence, including the following specific steps:

[0069] Step 2-1, LF refining outside the furnace: Before tapping, slag flowing and slag skimming are carried out to clean the furnace slag as much as possible; tapping with white slag, and a small amount of lime and fluorite can be used to adjust the reducibility and fluidity of the furnace slag; the tapping temperature is ≥1530°C; first, energize to melt the top slag, and then carry out alloy fine-tuning and microalloying to obtain its chemical composition by weight percentage including: 0.01wt% C, 0.49wt% Si, 0.0015wt% S, 0.010wt% P, 1.9wt% Ni, 19.9wt% Cr, 0.49wt% Cu, 0.008wt% N, 0.0020wt% Mg, 0.0120wt% Zr, and the balance is Fe and unavoidable impurities; the smelting time in the LF refining furnace is 90 minutes.

[0070] Step 2-2, continuous casting: The tundish temperature is 1480°C; the working drawing speed is 1.20 m / min; the water flow rate on the wide face of the mold is 195 m 3 / h, and the water flow rate on the narrow face is 25 m 3 / h, and the temperature difference between the inlet and outlet water is 7°C; the mold uses non-sinusoidal vibration, the vibration frequency is 140 CPM, and the amplitude is 3.0 mm; the mold powder is ferritic stainless steel mold powder.

[0071] Step 2-3, rolling: The continuous casting billet is first heated and then hot-rolled to obtain a hot-rolled black skin coil; the temperature of the hot rolling is 1250°C.

[0072] Step 2-4, heat treatment: Heat at 850°C, hold for heat preservation, and then air-cool or furnace-cool.

[0073] Experimental Example

[0074] The following tests were carried out on the inclusions characteristics, weldability and corrosion resistance of the low-cost ferritic stainless steel prepared in Example 1, and the results are as follows:

[0075] Ultra-pure (ultra-low carbon and nitrogen element content) ferritic stainless steel is a kind of stainless steel with low cost and good comprehensive performance. Compared with austenitic stainless steel, its physical properties are close to those of low-carbon steel, such as ferromagnetic, good thermal and electrical conductivity, small thermal expansion coefficient, etc. Ferritic stainless steel with a thin section (thickness ≤ 2mm) has excellent comprehensive properties, such as mechanical properties, formability, weldability, and corrosion resistance, etc., and its application fields are becoming wider and wider. Medium-chromium content ultra-pure ferritic stainless steel greatly improves its weldability and corrosion resistance by reducing the carbon and nitrogen content and adding micro-alloying elements such as niobium and titanium, and can replace AISI 304L austenitic stainless steel in many fields.

[0076] However, since this type of ferritic stainless steel is a single-phase ferritic stainless steel, it has strong thermal sensitivity. During the welding process, the grains are coarsened by heat, resulting in deterioration of properties, reduction of plasticity and toughness, and increase of brittleness. Since there is no phase change during the heating and cooling process, the structure and properties cannot be improved by heat treatment; at the same time, alloying elements are easily burned out during the welding process, affecting the corrosion resistance. Therefore, the mechanical properties and corrosion resistance of the welded joint have always been the technical bottleneck of this type of stainless steel. Among them, the most important alloying elements in chromium-containing ferritic stainless steel are elements such as titanium, niobium and molybdenum. The common second-phase particles are carbonitrides of niobium and titanium, and the precipitation behavior characteristics of these particles have an important impact on the structure and properties of ferritic stainless steel.

[0077] (1) Inclusion characteristics and their formation mechanism

[0078] The morphology of inclusions in the test samples was observed by SEM, and the results are as Figure 1 and 2 . From Figure 1 , it can be seen that the inclusions in conventional ferritic stainless steel are strip-shaped or irregular in shape, and the inclusion size is about 2 - 5μm. From Figure 2 , it can be seen that the inclusions in the ferritic stainless steel prepared in Example 1 of the present invention are spherical or equiaxed in shape, and the inclusion size is about 1 - 3μm. Fine and spherical inclusions play an important positive role in improving the plasticity and toughness of stainless steel.

[0079] According to the basic principles of metallurgical thermodynamics, Zr and Mg are both strong oxide-forming elements. Using Zr and Mg for composite deoxidation is beneficial to removing the free oxygen content in the molten steel. The density of ZrO2 is 5.68g / cm 3 , which is greater than the density of Al2O3 (3.97g / cm 3), especially the density of ZrO2 and molten steel (7.15 g / cm 3 ) is closer. Therefore, once a stable oxide is formed at high temperature, ZrO2 can float uniformly in the molten steel while Al2O3 will collide and aggregate on the surface of the molten steel to become a component of the steel slag. The part of Al2O3 that fails to float will remain in the steel as large cluster-shaped inclusions. The electrical conductivity of the oxide is the key factor for its movement in the molten steel. According to the existing research results, the driving force for the movement of Al2O3 in the molten steel is greater than that of ZrO2. During the electroslag refining process, ZrO2 particles tend to repel each other and are difficult to agglomerate, while Al2O3 is easy to collide with each other to form large particles and float to the surface of the molten steel, where it is absorbed by the covering agent on the surface of the molten steel. Therefore, compared with conventional Al and Si deoxidation, composite deoxidation with Zr and Mg can form fine and dispersed composite oxides, among which Figure 2 the experimental results have proved this point.

[0080] In addition, MnS and ZrO2 have extremely similar lattice constants, and the specific data are shown in Table 1. Due to the good lattice matching relationship between MnS and ZrO2, this will reduce the interfacial energy between the two. The lower interfacial energy will result in better adhesion between the grains of different interfaces. This further proves the reason why strip-shaped and string-shaped sulfides are not formed in the test samples. This is because MnS tends to form on the pre-formed ZrO2 particles, and the sulfides are thus refined, spheroidized, and dispersed. This is beneficial to improving the plasticity and toughness of Zr and Mg composite deoxidized ferritic stainless steel.

[0081] Table 1 Lattice constants of MnS and ZrO2

[0082]

[0083] (2) Microstructure morphology of the base material and the welded joint

[0084] The grain size is a key index for evaluating the comprehensive properties of ferritic stainless steel. The smaller the grain size, the better the plasticity and toughness of the stainless steel macroscopically, and the less likely it is to fail due to brittleness. Therefore, in the test results, the morphology and grain size of the microstructure of the welded joint are analyzed as an important evaluation index. Figure 3 Figure [ID number] is the optical micrograph of the base material of the test sample. It can be seen that the grain size is relatively small, and the average grain size is about between 20 - 30 μm. Figure 4It is the optical microstructure of the weld of the test sample. Since autogenous melting welding without filler wire is adopted, the weld is the as-welded structure formed by the solidification of the molten pool after the melting of the base metal. The heat affected zone has the same composition, microstructure and crystal structure as the weld metal, so there is no obvious fusion line. The carbonitrides of Zr are second-phase particles that are very stable at high temperatures and have a melting point much higher than that of stainless steel. The high-temperature stability of the oxides of Mg is also very excellent. When the stainless steel melts under the action of arc heat, due to the fast heating rate, these particles have no time to decompose and act as nucleating agents during the subsequent crystallization process, which can promote the crystallization of the liquid molten pool and also play a role in preventing grain boundary migration and refining the grains of the weld metal.

[0085] (3) Influence of microalloying elements on the corrosion resistance of welded joints

[0086] To study the influence of microalloying elements and microstructure in super-ferritic stainless steel on the corrosion resistance of welded joints, the welded joints were prepared into salt spray corrosion specimens and placed in a corrosion testing machine filled with neutral salt spray. The salt solution was made by dissolving sodium chloride in distilled water or deionized water, and its concentration was 50±5 g / L. After 30 days, the specimens were taken out to observe the corrosion situation in the welded joint area. The results showed that the surface of the specimens was smooth and no rust was generated, as Figure 5 shown. Composite microalloying improved the corrosion resistance of the welded joints.

[0087] In the description of this specification, the description with reference to terms such as "one embodiment", "some embodiments", "example", "specific example", or "some examples" means that the specific features, structures, materials, or characteristics described in connection with the embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the schematic representations of the above terms do not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials, or characteristics described can be combined in a suitable manner in any one or more embodiments or examples. In addition, without contradiction, those skilled in the art can combine and combine the different embodiments or examples described in this specification and the features of different embodiments or examples.

[0088] Although the embodiments of the present invention have been shown and described above, it can be understood that the above embodiments are exemplary and should not be construed as limiting the present invention. Those of ordinary skill in the art can make changes, modifications, substitutions, and variations to the above embodiments within the scope of the present invention.

Claims

1. A low-cost ferritic stainless steel, characterized in that, It includes the following chemical components by mass percentage: C ≤ 0.009%, Si: 0.19 - 0.49%, S ≤ 0.0014%, P ≤ 0.009%, Ni: 1.1 - 1.9%, Cr: 19.1 - 19.9%, Cu: 0.19 - 0.49%, N ≤ 0.008%, 70×10 -4 % ≤ Mg + Zr ≤ 290×10 -4 %, and the contents of Zr and Mg satisfy Zr / Mg = 3 - 6, and the balance is Fe and inevitable impurities.

2. The low-cost ferritic stainless steel according to claim 1, characterized in that, It includes the following chemical components by mass percentage: C ≤ 0.006%, Si: 0.29 - 0.49%, S ≤ 0.0010%, P ≤ 0.005%, Ni: 1.5 - 1.9%, Cr: 19.5 - 19.9%, Cu: 0.29 - 0.49%, N ≤ 0.006%, 165×10 -4 % ≤ Mg + Zr ≤ 290×10 -4 %, and the contents of Zr and Mg satisfy Zr / Mg = 3 - 6, and the balance is Fe and unavoidable impurities.

3. The low-cost ferritic stainless steel according to claim 1, wherein It includes the following chemical components by mass percentage: C ≤ 0.008%, Si: 0.19 - 0.29%, S ≤ 0.0013%, P ≤ 0.008%, Ni: 1.1 - 1.5%, Cr: 19.1 - 19.5%, Cu: 0.19 - 0.29%, N ≤ 0.007%, 70×10 -4 % ≤ Mg + Zr ≤ 200×10 -4 %, and the contents of Zr and Mg satisfy Zr / Mg = 3 - 6, with the balance being Fe and inevitable impurities.

4. The low-cost ferritic stainless steel according to claim 1, wherein It includes the following chemical components by mass percentage: C: 0.008%, Si: 0.29%, S ≤ 0.0013%, P: 0.008%, Ni: 1.5%, Cr: 19.5%, Cu: 0.29%, N: 0.007%, Mg: 0.0050%, Zr: 0.0150%, and the balance is Fe and unavoidable impurities.

5. A method for preparing a low-cost ferritic stainless steel according to any one of claims 1 to 4, characterized in that, It includes the following steps: Step 1: Take the following raw materials according to the weight ratio: ultra-low P industrial pure iron 35 - 40%, high-carbon ferrochrome 10 - 15%, hot metal smelted from laterite nickel ore 50 - 60%; first add the ultra-low P industrial pure iron and the high-carbon ferrochrome into an intermediate frequency furnace for melting, and then add the hot metal smelted from laterite nickel ore into the intermediate frequency furnace to obtain the raw material for smelting low-cost ferritic stainless steel. Step 2: Subject the raw material for smelting low-cost ferritic stainless steel obtained in Step 1 to refining, continuous casting, rolling, and heat treatment in sequence to obtain low-cost ferritic stainless steel.

6. The preparation method of a low-cost ferritic stainless steel according to claim 5, characterized in that, In Step 1, take the following raw materials according to the weight ratio: ultra-low P industrial pure iron 34 - 40%, high-carbon ferrochrome 10 - 14%, hot metal smelted from laterite nickel ore 52 - 60%.

7. The preparation method of a low-cost ferritic stainless steel according to claim 5 or 6, characterized in that In Step 1, the weight contents of chemical components P and C in the ultra-low P industrial pure iron are respectively: P ≤ 0.0035%, C ≤ 0.01%; the weight contents of chemical components Cr and C in the high-carbon ferrochrome are respectively: Cr ≥ 60%, C ≤ 9.5%.

8. The preparation method of a low-cost ferritic stainless steel according to claim 5 or 6, characterized in that, In Step 1, the weight contents of chemical components Cr and P in the hot metal smelted from laterite nickel ore are respectively: Cr ≥ 60%, P ≤ 0.03%.

9. The preparation method of a low-cost ferritic stainless steel according to claim 5 or 6, characterized in that, In Step 2, the specific steps of subjecting the raw material for smelting low-cost ferritic stainless steel obtained in Step 1 to refining, continuous casting, rolling, and heat treatment in sequence are as follows: Step 2-1, refining: Subject the raw material for smelting low-cost ferritic stainless steel to secondary refining in an LF furnace, and perform alloy fine-tuning and microalloying treatment according to the chemical components in the low-cost ferritic stainless steel to obtain microalloyed stainless steel. Step 2-2, continuous casting: Continuously cast the microalloyed stainless steel under the conditions that the tundish temperature is 1465 - 1480°C and the working drawing speed is 1.05 - 1.20 m / min to obtain a continuous casting billet. Step 2-3, rolling: Roll the continuous casting billet at a temperature of 1150 - 1250°C to obtain a hot-rolled black skin coil. Step 2-4, heat treatment: Heat the hot-rolled black skin coil to a temperature of 950 - 1050°C, hold for 60 - 120 min, and cool to room temperature to obtain low-cost ferritic stainless steel.

10. The preparation method of a low-cost ferritic stainless steel according to claim 9, characterized in that, In Step 2-1, the refining time is 45 - 90 min, and the tapping temperature ≥ 1530°C.

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