Free-cutting ferritic stainless steel, its preparation method and application

Abstract

The present invention provides a free-cutting ferritic stainless steel and its preparation method and application, relating to the field of metallurgical technology. For the free-cutting ferritic stainless steel, the chemical composition of the conventional ferritic stainless steel is optimized. By adding tin element, the cutting force during the processing of the ferritic stainless steel and the surface roughness after cutting are greatly reduced, making the cutting performance of the ferritic stainless steel excellent. At the same time, during the corrosion process of the ferritic stainless steel of the present invention, Sn and Cr form SnO2 and Cr2O3 passive films on the surface of the stainless steel, and the Sn 2+ generated during the corrosion process can adhere to the surface of the stainless steel, preventing the destruction of erosive ions such as Cl ‑ , effectively improving the corrosion resistance of the ferritic stainless steel. At the same time, by adding a small amount of Ni and Mn elements and combining specific forging conditions, the segregation of tin at the grain boundaries is reduced, thereby reducing the influence of the solidification segregation of tin in the steel on the mechanical properties and corrosion resistance of the stainless steel, etc.

Description

Technical Field

[0001] The present invention relates to the technical field of metallurgy, and particularly relates to a free-cutting ferritic stainless steel, a preparation method thereof, and an application thereof. Background Art

[0002] Stainless steel materials have been widely used due to their excellent corrosion resistance, high temperature resistance and other characteristics. The main alloying components of stainless steel are chromium, nickel, molybdenum, etc. Among them, nickel and molybdenum are expensive, and China is a country lacking nickel, short of molybdenum and poor in chromium. Thus, the problems of raw material supply and price are becoming increasingly prominent. At the same time, due to the characteristics of high toughness, high thermal strength and poor thermal conductivity of stainless steel, its cutting performance is poor, the service life of the cutting tool is low, and the processing cost is high, which limits the application of stainless steel materials. In this context, it is particularly important to develop high-quality stainless steel products with good cutting performance and chromium resource conservation.

[0003] At present, the mainstream free-cutting steels are mainly sulfur-based and lead-based free-cutting steels. Sulfur-based free-cutting steels mainly rely on manganese sulfide free-cutting phases to interrupt the matrix continuity, reduce the cutting force while promoting chip removal, and improve the cutting performance of stainless steel. However, sulfur will reduce the corrosion resistance of stainless steel and damage its hot workability and mechanical properties. Lead-based stainless steel can significantly reduce the cutting performance of stainless steel, but it has been prohibited from being added due to the high toxicity of lead and environmental pollution. Tin, as a homologous element of lead, has a similar mechanism of action to lead during cutting and can become a molten state during cutting to improve the cutting performance. Tin is non-toxic, and tin can promote the cathodic reaction of hydrogen evolution during corrosion, improving the self-corrosion potential and pitting potential of stainless steel. The recovery rate of tin is high during smelting, but the solidification segregation ratio of tin in steel is much higher than that of other elements, and it will segregate at the grain boundaries in large quantities, thus damaging the mechanical properties and corrosion resistance of stainless steel.

[0004] In view of this, the present invention is specifically proposed. Summary of the Invention

[0005] The purpose of the present invention is to provide a free-cutting ferritic stainless steel and a preparation method thereof to solve the above technical problems existing in the prior art.

[0006] In order to achieve the above purpose, the present invention provides the following technical solutions:

[0007] The first object of the present invention is to provide a free-cutting ferritic stainless steel, which comprises chemical components with the following mass fractions: Sn: 0.1% - 3.0%, Cr: 13% - 15%, C: 0.020% - 0.040%, Si: 2.0% - 2.5%, Mo: 0.15% - 0.40%, Nb: 0.03% - 0.12%, Cu: 0.10 - 0.25%, Mn: 0.20% - 0.35%, Ni: 0.10% - 0.25%, O: 0.0030% - 0.0090%, N: 0.0030% - 0.0090%, P: 0.0040% - 0.012%, S: 0.0025% - 0.010%, and the remaining components are iron and inevitable impurities.

[0008] Further, on the basis of the above technical solution of the present invention, the free-cutting ferritic stainless steel comprises chemical components with the following mass fractions: Sn: 0.1% - 2.7%, Cr: 13% - 14.8%, C: 0.025% - 0.038%, Si: 2.0% - 2.3%, Mo: 0.15% - 0.35%, Nb: 0.04% - 0.12%, Cu: 0.10 - 0.20%, Mn: 0.20% - 0.35%, Ni: 0.10% - 0.23%, O: 0.0032% - 0.0090%, N: 0.0042% - 0.0090%, P: 0.0040% - 0.012%, S: 0.0025% - 0.010%, and the remaining components are iron and inevitable impurities.

[0009] Further, on the basis of the above technical solution of the present invention, the free-cutting ferritic stainless steel comprises chemical components with the following mass fractions: Sn: 0.47%, Cr: 14.61%, C: 0.031%, Si: 2.04%, Mo: 0.35%, Nb: 0.12%, Cu: 0.15%, Mn: 0.30%, Ni: 0.23%, O: 0.0038%, N: 0.0065%, P: 0.0093%, S: 0.0044%, and the remaining components are iron and inevitable impurities.

[0010] Further, on the basis of the above technical solution of the present invention, the cutting force of the free-cutting ferritic stainless steel is 540 - 650 N;

[0011] and / or, the surface roughness of the free-cutting ferritic stainless steel after cutting is 6.80 - 7.70 μm.

[0012] Further, on the basis of the above technical solution of the present invention, the pitting potential of the free-cutting ferritic stainless steel is 70 - 240 mV;

[0013] and / or, the corrosion rate of the free-cutting ferritic stainless steel is 1.30 - 2.43 mg / (cm 2 *h).

[0014] The second object of the present invention is to provide a method for preparing the above-mentioned free-cutting ferritic stainless steel, comprising the following steps:

[0015] (a) Smelt the raw materials according to the set chemical composition of the free-cutting ferritic stainless steel and cast billets;

[0016] (b) Heat the billets obtained in step (a), then perform hot forging, and perform water cooling after the hot forging is completed to obtain forged bars; wherein, the initial forging temperature during hot forging is 950 - 1150 °C, and the final forging temperature is 900 - 950 °C;

[0017] (c) Anneal the forged bars obtained in step (b) to obtain free-cutting ferritic stainless steel.

[0018] Further, on the basis of the above technical solution of the present invention, in step (b), the heating temperature is 950 - 1150 °C, and the holding time is 2 - 3 h.

[0019] Further, on the basis of the above technical solution of the present invention, in step (b), the forging pressure during forging is 1000 - 3000 kg;

[0020] and / or, the forging deformation amount is 30 - 40%.

[0021] and / or, the diameter of the forged bar is 30 - 35 mm.

[0022] Further, on the basis of the above technical solution of the present invention, in step (c), the heating temperature during annealing treatment is 760 - 900 °C, and the holding time is 0.25 - 2 h.

[0023] The third object of the present invention is to provide the application of the above-mentioned free-cutting ferritic stainless steel or the free-cutting ferritic stainless steel prepared by using the above-mentioned preparation method of the free-cutting ferritic stainless steel in the fields of electrical equipment, utensils and building materials, etc.

[0024] Compared with the prior art, the technical solution of the present invention has at least the following technical effects:

[0025] (1) The present invention provides a free-cutting ferritic stainless steel, which optimizes the chemical composition of conventional ferritic stainless steels. By adding Sn element, the cutting force during the processing of ferritic stainless steel and the surface roughness after cutting are greatly reduced, making the ferritic stainless steel have excellent cutting performance. At the same time, a small amount of Ni and Mn elements are added and combined with specific forging conditions, reducing the segregation of Sn at grain boundaries, thereby reducing the influence of the solidification segregation of Sn in the steel on the mechanical properties of the stainless steel, etc. Moreover, during the corrosion process of the ferritic stainless steel of the present invention, Sn and Cr form SnO2 and Cr2O3 passivation films on the surface of the stainless steel, effectively hindering the corrosion of the substrate. As the corrosion progresses, the content of Sn ions, which effectively inhibits the anodic reaction, also continuously increases, further reducing the dissolution rate of the stainless steel substrate and effectively improving the corrosion resistance of the ferritic stainless steel. In addition, the content of Cr element and the addition of precious metal Mo element in the chemical composition of the free-cutting ferritic stainless steel of the present invention are also reduced, significantly reducing the production cost of the stainless steel while ensuring good corrosion resistance. 2+ The content of Sn ions, which effectively inhibits the anodic reaction, also continuously increases, further reducing the dissolution rate of the stainless steel substrate and effectively improving the corrosion resistance of the ferritic stainless steel. In addition, the content of Cr element and the addition of precious metal Mo element in the chemical composition of the free-cutting ferritic stainless steel of the present invention are also reduced, significantly reducing the production cost of the stainless steel while ensuring good corrosion resistance.

[0026] (2) The present invention provides a preparation method of the above free-cutting ferritic stainless steel. The billet with the above specific chemical composition is forged under specific forging conditions. These forging conditions enable the ferritic stainless steel to be forged in the austenite and ferritic dual-phase region. After water cooling and rapid cooling, austenite inhibits the growth of ferritic grains and at the same time inhibits the offset of Sn solute atoms to grain boundaries, reducing the adverse effects of the grain boundary segregation of Sn on the mechanical properties and corrosion resistance of the stainless steel. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] The specification drawings forming a part of the present invention are used to provide a further understanding of the present invention. The schematic embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute an improper limitation of the present invention. Among them:

[0028] Figure 1 is the phase diagram of the free-cutting ferritic stainless steel provided in Example 4 of the present invention;

[0029] Figure 2 is the scanning electron microscope analysis diagram of the precipitate SiO2 in Example 4 of the present invention;

[0030] Figure 3 is the influence of the tin content on the cutting force and surface roughness in each example and comparative example of the present invention;

[0031] Figure 4 is the polarization curve diagram of the free-cutting ferritic stainless steel in each example and comparative example of the present invention;

[0032] Figure 5 is the corrosion rate diagram of the free-cutting ferritic stainless steel in each example and comparative example of the present invention;

[0033] Figure 6 These are the stress-strain diagrams of the free-cutting ferritic stainless steels of the embodiments and comparative examples of the present invention. Detailed implementation manners

[0034] To make the objectives, technical solutions and advantages of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Those skilled in the art should understand that the described embodiments are only for helping to understand the present invention and should not be regarded as specific limitations on the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the scope of protection of the present invention. The process parameters without specific conditions noted in the following embodiments are usually in accordance with conventional conditions.

[0035] In the ranges disclosed in the present invention, the endpoints and any values are not limited to the exact ranges or values. These ranges or values should be understood to include values close to these ranges or values. For numerical ranges, between the endpoint values of each range, between the endpoint values of each range and individual point values, and between individual point values, they can be combined with each other to obtain one or more new numerical ranges, and these numerical ranges should be regarded as specifically disclosed in the present invention.

[0036] According to the first aspect of the present invention, a free-cutting ferritic stainless steel is provided, which includes chemical components with the following mass fractions: Sn: 0.1% to 3.0%, Cr: 13% to 15%, C: 0.020% to 0.040%, Si: 2.0% to 2.5%, Mo: 0.15% to 0.40%, Nb: 0.03% to 0.12%, Cu: 0.10 to 0.25%, Mn: 0.20% to 0.35%, Ni: 0.10% to 0.25%, O: 0.0030% to 0.0090%, N: 0.0030% to 0.0090%, P: 0.0040% to 0.012%, S: 0.0025% to 0.010%, and the remaining components are iron and inevitable impurities.

[0037] The free-cutting ferritic stainless steel provided by the present invention uses Sn element as the main free-cutting element. During the cutting process, the molten metal brittleness of Sn causes stress concentration sources, making the matrix easy to cut off and facilitating chip evacuation, reducing the cutting force and the surface roughness after cutting, and improving the cutting performance. The content of Sn element cannot be too low. If the Sn content is too low, the cutting performance will be too poor and the corrosion resistance of the ferritic stainless steel will not be strong enough. If the Sn content is too high, the plasticity will drop sharply, and intergranular corrosion is likely to occur, seriously damaging the mechanical and corrosion resistance properties. Typical but non-limiting mass fractions are 0.1%, 0.16%, 0.2%, 0.3%, 0.32%, 0.4%, 0.47%, 0.5%, 0.6%, 0.8%, 0.91%, 1.0%, 1.2%, 1.4%, 1.5%, 1.6%, 1.8%, 2.0%, 2.2%, 2.4%, 2.6%, 2.8% or 3.0% etc. and the numerical range between any two points.

[0038] Cr is the most important reason for the stainless steel to obtain good corrosion resistance. Because Cr can form an infinite solid solution with Fe, Cr will increase the electrode potential of the solid solution and form a dense Cr2O3 protective film on the surface. Cr is also the main ferrite-forming element. Cr will promote the passivation of the steel and maintain a stable passive state. The higher the content of Cr, the stronger the corrosion resistance of the stainless steel. Typical but non-limiting mass fractions of Cr are 13%, 13.2%, 13.5%, 13.8%, 14%, 14.2%, 14.5%, 14.8% or 15% etc. and the numerical range between any two points.

[0039] C is a harmful element in stainless steel. Generally, the corrosion resistance of stainless steel will decrease with the increase of C content, but its strength will increase with the increase of C content. C can cause intergranular corrosion of stainless steel. C will enrich at the grain boundaries during the heat treatment process, combine with Cr element to form the common M23C6 phase, causing intergranular Cr depletion, and with the increase of C content, the toughness of the ferritic stainless steel will decrease. Typical but non-limiting mass fractions of C are 0.020%, 0.022%, 0.025%, 0.028%, 0.030%, 0.032%, 0.033%, 0.035%, 0.036%, 0.038% or 0.040% etc. and the numerical range between any two points.

[0040] Si is a ferrite-forming element, which has a good deoxidizing effect on molten steel. Si can increase the hardness and strength of ferrite and austenite, and its effect is stronger than that of elements such as Mn, Ni, and Cr. It can significantly improve the elastic limit, yield strength, and yield ratio of steel, and enhance the fatigue performance. In stainless steel, Si cooperates with elements such as Mo and Cr to improve the corrosion resistance and high-temperature oxidation resistance. The typical but non-limiting mass fraction of Si is 2.0%, 2.1%, 2.2%, 2.3%, 2.4%, or 2.5%, etc., and the numerical range between any two points.

[0041] Mo can improve the passivation ability of stainless steel. In various corrosive media, a Mo-containing passivation film will be formed, expanding the range of its passivation media. Research shows that Mo can also improve the oxidation resistance of ferritic stainless steel by inhibiting the diffusion of oxygen and increasing the reaction activation energy. The most important role of Mo in ferritic stainless steel is to enhance its pitting corrosion resistance. Mo can promote the enrichment of Cr on the passivation film of ferritic stainless steel, thereby enhancing the stability and re-passivation ability of the stainless steel passivation film. The typical but non-limiting mass fraction of Mo is 0.15%, 0.16%, 0.2%, 0.25%, 0.3%, 0.32%, 0.35%, or 0.4%, etc., and the numerical range between any two points.

[0042] Nb can prevent intergranular corrosion, improve the strength and plasticity of stainless steel, and contribute to the surface finish of stainless steel. At the same time, Nb has a strong binding force with C and N, and can form Nb(C,N) phase with them, better controlling the harmful C and N contents in stainless steel, thereby improving the various properties of stainless steel. The typical but non-limiting mass fraction of Nb is 0.03%, 0.05%, 0.06%, 0.07%, 0.08%, 0.09%, 0.10%, or 0.12%, etc., and the numerical range between any two points.

[0043] Cu makes an extraordinary contribution to the corrosion resistance of stainless steel in non-oxidizing acids, can reduce the sensitivity to pitting corrosion, and at the same time improve the plasticity and cold working performance. The typical but non-limiting mass fraction of Cu is 0.10%, 0.12%, 0.15%, 0.16%, 0.2%, 0.22%, or 0.25%, etc., and the numerical range between any two points.

[0044] This free-cutting ferritic stainless steel also contains certain amounts of Mn and Ni elements. The typical but non-limiting mass fraction of Mn is 0.2%, 0.25%, 0.28%, 0.3%, 0.32%, or 0.35%, etc., and the numerical range between any two points. The typical but non-limiting mass fraction of Ni is 0.10%, 0.12%, 0.15%, 0.16%, 0.2%, 0.22%, or 0.25%, etc., and the numerical range between any two points.

[0045] Typical but non-limiting mass fractions are 0.003%, 0.004%, 0.005%, 0.006%, 0.007%, 0.008%, or 0.009%, etc., and the numerical range between any two points. Typical but non-limiting mass fractions for N are 0.003%, 0.004%, 0.005%, 0.006%, 0.007%, 0.008%, or 0.009%, etc., and the numerical range between any two points; typical but non-limiting mass fractions for P are 0.004%, 0.005%, 0.006%, 0.007%, 0.008%, 0.009%, 0.010%, or 0.012%, etc., and the numerical range between any two points. Typical but non-limiting mass fractions for S are 0.0025%, 0.003%, 0.004%, 0.005%, 0.006%, 0.007%, 0.008%, 0.009%, or 0.010%, etc., and the numerical range between any two points.

[0046] The present invention optimizes the chemical composition of conventional ferritic stainless steel. By adding the Sn element, the cutting force during the processing of ferritic stainless steel is greatly reduced. There is no built-up edge phenomenon during the processing, the cutting is in a short and curly C shape, the surface after processing is smooth, the roughness is small, and the cutting performance is excellent; at the same time, a small amount of Ni and Mn elements are added and combined with specific forging conditions to reduce the segregation of Sn at the grain boundaries, thereby reducing the influence of the solidification segregation of Sn in the steel on the mechanical properties of the stainless steel and the like. Moreover, during the corrosion process of the ferritic stainless steel of the present invention, Sn and Cr form SnO2 and Cr2O3 passivation films on the surface of the stainless steel, effectively hindering the corrosion of the substrate. As the corrosion progresses, the Sn 2+ ion content also continuously increases, further reducing the dissolution rate of the stainless steel substrate and effectively improving the corrosion resistance of the ferritic stainless steel. In addition, the content of the Cr element and the addition of the precious metal Mo element are reduced in the chemical composition of the free-cutting ferritic stainless steel of the present invention, while ensuring good corrosion resistance, significantly reducing the production cost of the stainless steel.

[0047] As an alternative embodiment of the present invention, the free-cutting ferritic stainless steel comprises the following chemical components by mass fraction: Sn: 0.1% - 2.7%, Cr: 13% - 14.8%, C: 0.025% - 0.038%, Si: 2.0% - 2.3%, Mo: 0.15% - 0.35%, Nb: 0.04% - 0.12%, Cu: 0.10 - 0.20%, Mn: 0.20% - 0.35%, Ni: 0.10% - 0.23%, O: 0.0032% - 0.0090%, N: 0.0042% - 0.0090%, P: 0.0040% - 0.012%, S: 0.0025% - 0.010%, and the balance is iron and unavoidable impurities.

[0048] As a preferred embodiment of the present invention, the free-cutting ferritic stainless steel comprises the following chemical components by mass fraction: Sn: 0.47%, Cr: 14.61%, C: 0.031%, Si: 2.04%, Mo: 0.35%, Nb: 0.12%, Cu: 0.15%, Mn: 0.30%, Ni: 0.23%, O: 0.0038%, N: 0.0065%, P: 0.0093%, S: 0.0044%, and the balance is iron and unavoidable impurities.

[0049] By further limiting the specific components of the free-cutting ferritic stainless steel, the ferritic stainless steel has more excellent cutting performance and corrosion resistance. As a preferred embodiment of the present invention, the cutting force of the free-cutting ferritic stainless steel is 540 - 650 N, and typical but non-limiting cutting forces are 540 N, 550 N, 560 N, 570 N, 580 N, 590 N, 600 N, 610 N, 620 N, 630 N, 640 N or 650 N, etc., and the numerical range between any two points.

[0050] As an alternative embodiment of the present invention, the surface roughness of the free-cutting ferritic stainless steel after cutting is 6.80 - 7.70 μm, and typical but non-limiting surface roughnesses after cutting are 6.80 μm, 6.90 μm, 7.00 μm, 7.10 μm, 7.20 μm, 7.40 μm, 7.50 μm, 7.60 μm or 7.70 μm, etc., and the numerical range between any two points.

[0051] As an alternative embodiment of the present invention, the pitting potential of the free-cutting ferritic stainless steel is 70 - 240 mV. Typical but non-limiting pitting potentials are 70 mV, 80 mV, 90 mV, 70 mV, 100 mV, 110 mV, 120 mV, 140 mV, 150 mV, 160 mV, 180 mV, 200 mV, 220 mV or 240 mV, etc., and the numerical range between any two points.

[0052] As an alternative embodiment of the present invention, the corrosion rate of the free-cutting ferritic stainless steel is 1.30 - 2.45 mg / (cm 2 *h). Typical but non-limiting corrosion rates are 1.30 mg / (cm 2 *h), 1.50 mg / (cm 2 *h), 1.60 mg / (cm 2 *h), 1.80 mg / (cm 2 *h), 2.00 mg / (cm 2 *h), 2.10 mg / (cm 2 *h), 2.20 mg / (cm 2 *h), 2.30 mg / (cm 2 *h) or 2.45 mg / (cm 2 *h), etc., as well as the numerical ranges between any two points.

[0053] According to the second aspect of the present invention, there is also provided a method for preparing the above-mentioned free-cutting ferritic stainless steel, comprising the following steps:

[0054] (a) Smelting the raw materials according to the set chemical composition of the free-cutting ferritic stainless steel and casting the billets;

[0055] (b) Heating the billets obtained in step (a), then performing hot forging, and performing water cooling after the hot forging is completed to obtain forged bars; wherein, the starting forging temperature during hot forging is 950 - 1150 °C, and the final forging temperature is 900 - 950 °C;

[0056] (c) Annealing the forged bars obtained in step (b) to obtain the free-cutting ferritic stainless steel.

[0057] Specifically, in step (a), the raw materials used can be conventional smelting raw materials in the art, and the specific smelting and casting methods are also conventional methods in the art, and are not specifically limited herein.

[0058] In the ferritic stainless steel of the present invention, Sn element is used as the main free-cutting element. However, the solidification segregation ratio of Sn in steel is much higher than that of other elements, and it will segregate at grain boundaries in large amounts, thus damaging the mechanical properties of the stainless steel. Therefore, a small amount of Ni element and Mn element are specifically added to the ferritic stainless steel of the present invention. Ni and Mn are austenite-forming elements. Under the action of elements such as Ni and Mn, the ferritic stainless steel containing Sn will have a ferrite-austenite two-phase region at 910-1230 °C. The ferritic stainless steel is forged in the austenite and ferrite two-phase region. After water-cooling and rapid cooling, the austenite inhibits the growth of ferrite grains and at the same time inhibits the migration of Sn solute atoms to grain boundaries. Sn is a typical adsorbed atom, which will be more adsorbed around the precipitation phase and dissolved in the matrix of ferrite, reducing the segregation of Sn at grain boundaries and reducing the adverse effects of Sn grain boundary segregation on the mechanical properties and corrosion resistance of stainless steel.

[0059] That is, the forging conditions in step (b) are set such that the ferritic stainless steel containing Sn has a ferrite-austenite two-phase region during forging. Therefore, typical but non-limiting starting forging temperatures during hot forging are 950 °C, 960 °C, 970 °C, 980 °C, 990 °C, 1000 °C, 1020 °C, 1040 °C, 1050 °C, 1060 °C, 1080 °C, 1100 °C, 1120 °C, 1140 °C or 1150 °C, etc., and the numerical range between any two points, and typical but non-limiting final forging temperatures are 900 °C, 910 °C, 920 °C, 930 °C, 940 °C or 950 °C, etc., and the numerical range between any two points. When the starting forging temperature exceeds the above range of values, the ferritic stainless steel containing Sn will not be able to undergo hot deformation in the two-phase region, and the austenite and ferrite cannot deform together, resulting in non-uniform microstructure and properties of the stainless steel during cooling, and even cracking and other problems in severe cases. When the final forging temperature exceeds the above range, harmful phases such as carbonitrides are likely to form, and defects such as hot cracks are also likely to occur.

[0060] As an optional implementation manner of the present invention, in step (b), the heating temperature is 950-1150 °C, and the holding time is 2-3 h.

[0061] Typical but non-limiting heating temperatures are 950°C, 960°C, 970°C, 980°C, 990°C, 1000°C, 1020°C, 1040°C, 1050°C, 1060°C, 1080°C, 1100°C, 1120°C, 1140°C, 1150°C, etc., and the numerical ranges between any two points; typical but non-limiting holding times are 2 h, 2.5 h, 3 h, etc., and the numerical ranges between any two points. When the forging temperature is too high (exceeding 1150°C) or the holding time is too long (exceeding 3 h), the ferrite grains will grow significantly, resulting in a decrease in the mechanical properties and corrosion resistance of the stainless steel. When the forging temperature is too low (below 950°C) or the holding time is too short (below 2 h), the deformation resistance of the stainless steel is too large, and defects such as cracks will occur during the forging process, unable to achieve the required forging deformation amount, and damaging the tissue properties of the stainless steel.

[0062] As an alternative embodiment of the present invention, in step (b), the forging pressure during forging is 1000 - 3000 kg. Typical but non-limiting forging pressures are 1000 kg, 1200 kg, 1500 kg, 1800 kg, 2000 kg, 2200 kg, 2500 kg, 2800 kg, 3000 kg, etc., and the numerical ranges between any two points.

[0063] As an alternative embodiment of the present invention, the forging deformation amount is 30 - 40%, preferably 35 - 40%. Typical but non-limiting forging deformation amounts are 30%, 32%, 34%, 35%, 36%, 38%, 40%, etc., and the numerical ranges between any two points. By controlling an appropriate forging deformation amount, the plasticity of the stainless steel can be improved, and better mechanical properties can be obtained.

[0064] As an alternative embodiment of the present invention, the diameter of the forging bar is 30 - 35 mm (such as 30 mm, 32 mm, 34 mm, 35 mm, etc.).

[0065] As an alternative embodiment of the present invention, in step (c), the heating temperature during annealing treatment is 760°C - 900°C, and the holding time is 0.25 - 2 h. Typical but non-limiting annealing treatment temperatures are 760°C, 780°C, 800°C, 820°C, 850°C, 860°C, 880°C, 900°C, etc., and the numerical ranges between any two points; typical but non-limiting holding times are 0.25 h, 0.5 h, 0.8 h, 1.0 h, 1.5 h, 1.8 h, 2.0 h, etc., and the numerical ranges between any two points.

[0066] By further defining the temperature and time of the annealing treatment, elements such as Sn are better dissolved in the ferrite matrix or adsorbed around the second phase, reducing grain boundary segregation and the precipitation of harmful phases.

[0067] According to the third aspect of the present invention, there is also provided an application of the above free-cutting ferritic stainless steel or the free-cutting ferritic stainless steel prepared by the preparation method of the above free-cutting ferritic stainless steel in the fields of electrical equipment, utensils, building materials, etc.

[0068] In view of the advantages of the above free-cutting ferritic stainless steel, it has good applications in various fields, such as electrical equipment, utensils, building materials, etc.

[0069] The present invention will be further described in detail below with reference to specific examples and comparative examples.

[0070] Example 1

[0071] This example provides a free-cutting ferritic stainless steel, including the following chemical components by mass fraction: Sn: 0.11%, Cr: 13.79%, C: 0.033%, Si: 2.16%, Mo: 0.21%, Nb: 0.05%, Cu: 0.17%, Mn: 0.22%, Ni: 0.21%, O: 0.0032%, N: 0.0051%, P: 0.0079%, S: 0.0064%, and the remaining components are iron and inevitable impurities.

[0072] The preparation method of the free-cutting ferritic stainless steel in this example includes the following steps:

[0073] (1) Smelt and cast billets according to the chemical components of the free-cutting ferritic stainless steel set in this example.

[0074] (2) After cooling the billet to room temperature, reheat it to 1050 °C, hold for 2.5 hours, then perform hot forging. The starting forging temperature is 1050 °C, the final forging temperature is 950 °C, the forging pressure is 1500 kg, the forging deformation is 40%, and water cooling is carried out after forging to obtain a bar with a diameter of 35 mm.

[0075] (3) Heat the forged bar to 850 °C and hold for 0.5 hours for annealing treatment to obtain the free-cutting ferritic stainless steel.

[0076] Example 2

[0077] This example provides a free-cutting ferritic stainless steel, including the following chemical components by mass fraction: Sn: 0.16%, Cr: 13.06%, C: 0.037%, Si: 2.13%, Mo: 0.16%, Nb: 0.05%, Cu: 0.09%, Mn: 0.25%, Ni: 0.17%, O: 0.0092%, N: 0.0057%, P: 0.0043%, S: 0.0025%, and the remaining components are iron and inevitable impurities.

[0078] The preparation method of the free-cutting ferritic stainless steel in this embodiment comprises the following steps:

[0079] (1) Smelt the raw materials according to the chemical composition of the free-cutting ferritic stainless steel set in this embodiment and cast billets;

[0080] (2) After cooling the billets to room temperature, reheat them to 1150 °C, hold for 3 hours, then carry out hot forging. The starting forging temperature is 1150 °C, the final forging temperature is 950 °C, the forging pressure is 1000 kg, the forging deformation is 40%, and water cooling is carried out after forging to obtain bars with a diameter of 35 mm.

[0081] (3) Heat the forged bars to 870 °C and hold for 2 hours for annealing treatment to obtain the free-cutting ferritic stainless steel.

[0082] Example 3

[0083] This embodiment provides a free-cutting ferritic stainless steel, including the following chemical components by mass fraction: Sn: 0.32%, Cr: 14.59%, C: 0.027%, Si: 2.26%, Mo: 0.15%, Nb: 0.04%, Cu: 0.15%, Mn: 0.35%, Ni: 0.22%, O: 0.0083%, N: 0.0061%, P: 0.0059%, S: 0.0095%, and the remaining components are iron and inevitable impurities.

[0084] The preparation method of the free-cutting ferritic stainless steel in this embodiment comprises the following steps:

[0085] (1) Smelt the raw materials according to the chemical composition of the free-cutting ferritic stainless steel set in this embodiment and cast billets;

[0086] (2) After cooling the billets to room temperature, reheat them to 1100 °C, hold for 3 hours, then carry out hot forging. The starting forging temperature is 1100 °C, the final forging temperature is 950 °C, the forging pressure is 1000 kg, the forging deformation is 40%, and water cooling is carried out after forging to obtain bars with a diameter of 30 mm.

[0087] (3) Heat the forged bars to 820 °C and hold for 1 hour for annealing treatment to obtain the free-cutting ferritic stainless steel.

[0088] Example 4

[0089] This embodiment provides a free - machining ferritic stainless steel, including chemical components with the following mass fractions: Sn: 0.47%, Cr: 14.61%, C: 0.031%, Si: 2.04%, Mo: 0.35%, Nb: 0.12%, Cu: 0.15%, Mn: 0.30%, Ni: 0.23%, O: 0.0038%, N: 0.0065%, P: 0.0093%, S: 0.0044%, and the remaining components are iron and inevitable impurities.

[0090] The preparation method of the free - machining ferritic stainless steel in this embodiment includes the following steps:

[0091] (1) Smelt and cast billets from raw materials according to the chemical components of the free - machining ferritic stainless steel set in this embodiment;

[0092] (2) After cooling the billets to room temperature, reheat them to 1050 °C, hold for 2 hours, then perform hot forging. The starting forging temperature is 1050 °C, the final forging temperature is 950 °C, the forging pressure is 1500 kg, the forging deformation is 40%, and water - cool after forging to obtain bars with a diameter of 30 mm.

[0093] (3) Heat the forged bars to 820 °C, hold for 1 hour for annealing treatment to obtain the free - machining ferritic stainless steel.

[0094] Example 5

[0095] This embodiment provides a free - machining ferritic stainless steel, including chemical components with the following mass fractions: Sn: 0.91%, Cr: 13.57%, C: 0.040%, Si: 2.19%, Mo: 0.29%, Nb: 0.09%, Cu: 0.08%, Mn: 0.20%, Ni: 0.14%, O: 0.0063%, N: 0.0087%, P: 0.0093%, S: 0.0087%, and the remaining components are iron and inevitable impurities.

[0096] The preparation method of the free - machining ferritic stainless steel in this embodiment includes the following steps:

[0097] (1) Smelt and cast billets from raw materials according to the chemical components of the free - machining ferritic stainless steel set in this embodiment;

[0098] (2) After cooling the billets to room temperature, reheat them to 1000 °C, hold for 2.5 hours, then perform hot forging. The starting forging temperature is 1000 °C, the final forging temperature is 900 °C, the forging pressure is 2000 kg, the forging deformation is 40%, and water - cool after forging to obtain bars with a diameter of 35 mm.

[0099] (3) Heat the forged bar to 900 °C and hold for 1 hour for annealing treatment to obtain free - machining ferritic stainless steel.

[0100] Example 6

[0101] This example provides a free - machining ferritic stainless steel, including the following chemical components by mass fraction: Sn: 2.68%, Cr: 14.08%, C: 0.031%, Si: 2.11%, Mo: 0.30%, Nb: 0.07%, Cu: 0.19%, Mn: 0.31%, Ni: 0.11%, O: 0.0045%, N: 0.0078%, P: 0.0116%, S: 0.0067%, and the remaining components are iron and inevitable impurities.

[0102] The preparation method of the free - machining ferritic stainless steel in this example includes the following steps:

[0103] (1) Smelt the raw materials according to the chemical components of the free - machining ferritic stainless steel set in this example and cast billets;

[0104] (2) After cooling the billet to room temperature, re - heat it to 950 °C, hold for 2.5 hours, then perform hot forging. The initial forging temperature is 950 °C, the final forging temperature is 900 °C, the forging pressure is 3000 kg, the forging deformation is 40%, and water - cool after forging to obtain a bar with a diameter of 35 mm.

[0105] (3) Heat the forged bar to 820 °C and hold for 1.5 hours for annealing treatment to obtain free - machining ferritic stainless steel.

[0106] Comparative Example 1

[0107] This comparative example provides a ferritic stainless steel, including the following chemical components by mass fraction: Cr: 13.27%, C: 0.022%, Si: 2.10%, Mo: 0.19%, Nb: 0.04%, Cu: 0.10%, Mn: 0.27%, Ni: 0.19%, O: 0.0077%, N: 0.0042%, P: 0.0050%, S: 0.0037%, and the remaining components are iron and inevitable impurities.

[0108] The preparation method of the free - machining ferritic stainless steel in this comparative example includes the following steps:

[0109] (1) Smelt the raw materials according to the chemical components of the free - machining ferritic stainless steel set in this comparative example and cast billets;

[0110] (2) After cooling the billet to room temperature, it is reheated to 1000 °C, held for 2 hours, and then hot forged. The initial forging temperature is 1000 °C, the final forging temperature is 900 °C, the forging pressure is 2000 kg, the forging deformation is 40%, and it is water-cooled after forging to obtain a bar with a diameter of 30 mm.

[0111] (3) The forged bar is heated to 760 °C and held for 0.5 hours for annealing treatment to obtain a free-cutting ferritic stainless steel.

[0112] Comparative Example 2

[0113] This comparative example provides a ferritic stainless steel. Except that Sn is not contained in the chemical composition of the ferritic stainless steel, the remaining chemical compositions are basically the same as those in Example 4.

[0114] The preparation method of the ferritic stainless steel in this comparative example is the same as that in Example 4 and will not be elaborated here.

[0115] Comparative Example 3

[0116] This comparative example provides a free-cutting ferritic stainless steel. Except that Mn and Ni are not contained in the chemical composition of the ferritic stainless steel, the remaining chemical compositions are basically the same as those in Example 4.

[0117] The preparation method of the ferritic stainless steel in this comparative example is the same as that in Example 4 and will not be elaborated here.

[0118] Comparative Example 4

[0119] This comparative example provides a free-cutting ferritic stainless steel. Except that Mn is not contained in the chemical composition of the ferritic stainless steel, the remaining chemical compositions are basically the same as those in Example 4.

[0120] The preparation method of the ferritic stainless steel in this comparative example is the same as that in Example 4 and will not be elaborated here.

[0121] Comparative Example 5

[0122] This comparative example provides a free-cutting ferritic stainless steel. Except that Ni is not contained in the chemical composition of the ferritic stainless steel, the remaining chemical compositions are basically the same as those in Example 4.

[0123] The preparation method of the ferritic stainless steel in this comparative example is the same as that in Example 4 and will not be elaborated here.

[0124] Comparative Example 6

[0125] This comparative example provides a free-cutting ferritic stainless steel, and its chemical composition is the same as that in Example 4.

[0126] In the preparation method of the free-cutting ferritic stainless steel of this comparative example, except that the heating temperature in step (b) is adjusted from 1100°C to 1200°C and the initial forging temperature is adjusted from 1100°C to 1200°C, the other steps are the same as those in Example 4.

[0127] Comparative Example 7

[0128] This comparative example provides a free-cutting ferritic stainless steel, and its chemical composition is the same as that in Example 4.

[0129] In the preparation method of the free-cutting ferritic stainless steel of this comparative example, except that the final forging temperature in step (b) is adjusted from 950°C to 850°C, the other steps are the same as those in Example 4.

[0130] Comparative Example 8

[0131] This comparative example provides a free-cutting ferritic stainless steel, and its chemical composition is the same as that in Example 4.

[0132] In the preparation method of the free-cutting ferritic stainless steel of this comparative example, except that the water cooling after hot forging in step (b) is adjusted to air cooling, the other steps are the same as those in Example 4.

[0133] In order to further verify the technical effects of the above-mentioned examples and comparative examples, the following experimental examples are specially set up.

[0134] Experimental Example 1

[0135] Taking the free-cutting ferritic stainless steel provided in Example 4 as an example, its phase diagram was measured, specifically as Figure 1 . From Figure 1 It can be seen that there is a duplex region of austenite and ferrite at 910°C - 1120°C. After forging in the duplex region, the austenite phase will hinder the growth of the ferrite phase and hinder the segregation of tin to the grain boundary.

[0136] At the same time, the free-cutting ferritic stainless steel provided in Example 4 was detected by a field emission scanning electron microscope, Figure 2 which is the line scan result of the second phase SiO2 of the free-cutting ferritic stainless steel in Example 4. It can be clearly seen from the figure that the content of Sn increases significantly at the edge of SiO2. In addition to being solid-solved in the ferritic matrix, Sn atoms will also be enriched at the edges of precipitates such as SiO2.

[0137] Experimental Example 2

[0138] The cutting performance, pitting potential and pitting corrosion resistance of the ferritic stainless steels provided in each example and comparative example were detected, and the specific results are shown in Table 1 and Figures 3 - 6 as follows.

[0139] The indicators for measuring the cutting performance of materials mainly include cutting force, cutting heat, cutting surface roughness, tool wear, and chip control, etc. The cutting performance of the present invention is measured by testing the cutting force and cutting surface roughness. The cutting equipment for the cutting test is a CA6140 lathe. A dry turning test is carried out using a YM2 hard alloy indexable turning tool. The geometric angles of the tool are: rake angle 87 degrees, clearance angle 87 degrees, and edge angle 87 degrees. A Kistler sensor is used to measure the real-time cutting force, and the surface roughness is measured by a surface roughness meter. The cutting parameters are cutting speed 36 mm / min, feed rate 0.2 mm / r, and cutting depth 1 mm.

[0140] For the pitting potential detection of the present invention, a specimen of 10.1 mm×10.1 mm is cut from the stainless steel and wet ground successively on 400#, 600#, 800#, 1200#, 1500#, and 2000# sandpapers using deionized water. An electrochemical workstation with a three-electrode system is used for experiments in a 3.5 wt% NaCl solution. The working electrode is polarized to -1.5 V / SCE and maintained for 120 s for pre-cathodic polarization treatment, and then the anodic polarization curve is measured to obtain the pitting potential of the stainless steel.

[0141] For the pitting resistance performance test of the present invention, according to the standard GB / T 17897, a specimen with dimensions of 30 mm×20 mm×5 mm is cut from the stainless steel using wire cutting and wet ground successively on 400#, 600#, 800#, 1200#, 1500#, and 2000# sandpapers using deionized water. After drying with alcohol, the weight and surface area are measured at room temperature; the corrosion solution: 6% FeCl3 solution, prepared with deionized water, and continuously soaked in a water bath at 22°C±1°C for 72 hours; after soaking, according to the standard GB / T 16545, the corrosion products are cleaned with 20% nitric acid, and after drying with alcohol, the mass of the specimen after corrosion is measured to obtain the corrosion rate of the stainless steel.

[0142] The mechanical property test of the present invention is carried out in accordance with GB / T228.1-2021. A rectangular tensile specimen with a parallel section length of 36 mm and a single-sided clamping section length of 30 mm is cut from the specimen along the direction perpendicular to the forging direction, and the parallel section is polished smooth using 600# sandpaper. During the experiment, the tensile rate is selected as 1 mm / min, the extensometer size is 25 mm, and each specimen experiment is carried out three times to plot the stress-strain curve.

[0143] Table 1

[0144]

[0145] As can be seen from the data in Table 1, in Examples 1-6, with the continuous increase of the content of tin (the main variable element), the lubricating effect of tin during cutting becomes more obvious, the cutting force continuously decreases, and the surface roughness first decreases and then increases. This is because too much Sn is carried away with the chips in the molten state, resulting in an increase in surface roughness. The pitting potential and corrosion rate also first improve and then deteriorate with the increase of tin content. This is because during the corrosion process, Sn atoms in the stainless steel will lose electrons as an anode and become Sn 2+ , while Sn 2+ has strong adsorption and will adsorb on the surface of the stainless steel to prevent the attack of erosive ions such as Cl - , enhancing the corrosion resistance of the stainless steel. At the same time, SnO2 in the passive film will be corroded into Sn 4+ ions and enter the solution. The H 4+ produced by the hydrolysis of Sn + will further exacerbate the corrosion. The combined action of the corrosion inhibition behavior of this Sn 2+ and the corrosion promotion behavior of Sn 4+ ions makes the corrosion resistance of the stainless steel improve within a certain range after adding Sn. After adding tin, both the strength and plasticity are improved to a certain extent, but too much tin will lead to a decrease in plasticity. Considering the comprehensive cutting, corrosion, and mechanical properties, adding 0.47% Sn shows the best performance.

[0146] In the comparative examples, no tin was added in Comparative Example 1 and Comparative Example 2. Compared with the tin-containing ferritic stainless steel, the cutting force and surface roughness of the ferritic stainless steel without tin addition are significantly increased. At the same time, the pitting potential decreases and the corrosion rate increases. In the comparative examples, whether reducing the addition of Mn and / or Ni in Comparative Examples 3-5, or changing the forging temperature in Comparative Examples 6 or 7 will cause changes in the ferritic-austenitic two-phase region, making the austenite unable to restrict the growth of the ferritic and the segregation of tin. The grain boundary segregation of tin is the reason for the performance degradation in the comparative examples. In Comparative Example 8, the cooling method used after forging is air cooling, and air cooling causes excessive grain growth, and more segregation of Sn and M 23 C6 will occur, thus affecting the performance of the stainless steel.

[0147] The improvement of the cutting performance of ferritic stainless steel is mainly due to the molten metal brittleness effect of low-melting-point tin and the stress concentration source caused by the segregation of tin, which effectively reduces the cutting force during cutting, the chips are easy to break, the surface roughness is reduced, and the surface is smooth. With the increase of tin content, the cutting performance gradually improves, as Figure 3 shown. Combining Figure 4 and Figure 5, according to the measurement of the anodic polarization curve and corrosion rate, it can be seen that within a certain range, as the tin content increases, the pitting potential of ferritic stainless steel rises and the corrosion rate decreases. However, when the tin content exceeds 0.47%, with the excessive tin content, the grain boundary segregation intensifies and the corrosion performance deteriorates. When the tin content is 0.47%, the pitting potential of the tin-containing ferritic stainless steel is the highest, which is 240 mV, and the corrosion rate is the lowest, which is 1.38 mg / (cm 2 *h). The addition of tin can effectively improve the strength of stainless steel and improve the plasticity of stainless steel within a certain range. The segregation of tin will cause a sharp drop in the plasticity of stainless steel, and the results are as Figure 6 shown.

[0148] The above are only the preferred embodiments of the present invention and are not used to limit the present invention. For those skilled in the art, the present invention can have various changes and modifications. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A free-cutting ferritic stainless steel, characterized in that: Including the following chemical compositions by mass fraction: Sn: 0.1%-3.0%, Cr: 13%-15%, C: 0.020%-0.040%, Si: 2.0%-2.5%, Mo: 0.15%-0.40%, Nb: 0.03%-0.12%, Cu: 0.10%-0.25%, Mn: 0.20%-0.35%, Ni: 0.10%-0.25%, O: 0.0030%-0.0090%, N: 0.0030%-0.0090%, P: 0.0040%-0.012%, S: 0.0025%-0.010%, and the remaining components are iron and unavoidable impurities; The method for preparing the free-cutting ferritic stainless steel comprises the following steps: (a) smelting raw materials according to the set chemical composition of free-cutting ferritic stainless steel and casting ingots; (b) heating the ingot obtained in step (a), and then hot forging, and water cooling after the hot forging to obtain a forged bar; wherein the initial forging temperature during the hot forging is 950-1150° C., and the final forging temperature is 900-950° C.; (c) annealing the forged bar obtained in step (b) to obtain free-cutting ferritic stainless steel.

2. The free-cutting ferritic stainless steel according to claim 1, characterized in that: The invention comprises the following chemical compositions in mass fraction: Sn: 0.1%-2.7%, Cr: 13%-14.8%, C: 0.025%-0.038%, Si: 2.0%-2.3%, Mo: 0.15%-0.35%, Nb: 0.04%-0.12%, Cu: 0.10%-0.20%, Mn: 0.20%-0.35%, Ni: 0.10%-0.23%, O: 0.0032%-0.0090%, N: 0.0042%-0.0090%, P: 0.0040%-0.012%, S: 0.0025%-0.010%, and the rest are iron and unavoidable impurities.

3. The free-cutting ferritic stainless steel according to claim 1, characterized in that: The chemical composition includes the following mass fractions: Sn: 0.47%, Cr: 14.61%, C: 0.031%, Si: 2.04%, Mo: 0.35%, Nb: 0.12%, Cu: 0.15%, Mn: 0.30%, Ni: 0.23%, O: 0.0038%, N: 0.0065%, P: 0.0093%, S: 0.0044%, and the rest are iron and inevitable impurities.

4. The free-cutting ferritic stainless steel according to any one of claims 1 to 3, characterized in that: The cutting force of the free-cutting ferritic stainless steel is 540-650N; And / or, the surface roughness of the free-cutting ferritic stainless steel after cutting is 6.80-7.70 μm.

5. The free-cutting ferritic stainless steel according to any one of claims 1 to 3, characterized in that: The pitting potential of the free-cutting ferritic stainless steel is 70 to 240 mV; And / or, the corrosion rate of the free-cutting ferrite stainless steel is 1.30-2.43 mg / (cm 2 *h).

6. The method for preparing free-cutting ferritic stainless steel according to any one of claims 1 to 5, characterized in that: The following steps are involved: (a) smelting raw materials according to the set chemical composition of free-cutting ferritic stainless steel and casting ingots; (b) heating the ingot obtained in step (a), and then hot forging, and water cooling after the hot forging to obtain a forged bar; wherein the initial forging temperature during the hot forging is 950-1150° C., and the final forging temperature is 900-950° C.; (c) annealing the forged bar obtained in step (b) to obtain free-cutting ferritic stainless steel.

7. The method for preparing free-cutting ferritic stainless steel according to claim 6, characterized in that: In step (b), the heating temperature is 950-1150° C. and the insulation time is 2-3 hours.

8. The method for preparing free-cutting ferritic stainless steel according to claim 6, characterized in that: In step (b), the forging pressure during forging is 1000 to 3000 kg; and / or, the forging deformation is 30 to 40%; And / or, the diameter of the forged bar is 30 to 35 mm.

9. The method for preparing free-cutting ferritic stainless steel according to any one of claims 6 to 8, characterized in that: In step (c), the heating temperature during annealing treatment is 760-900° C., and the holding time is 0.25-2 h.

10. Application of the free-cutting ferritic stainless steel according to any one of claims 1 to 5 or the free-cutting ferritic stainless steel prepared by the method for preparing the free-cutting ferritic stainless steel according to any one of claims 6 to 9 in the fields of electrical equipment, utensils and building materials.

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