Medium-carbon free-cutting steel bar and manufacturing method thereof

Abstract

The invention discloses a medium-carbon free-cutting steel bar and a manufacturing method thereof, and belongs to the field of free-cutting steel. The bar comprises the following chemical components in percentage by weight: 0.25% to 0.34% of C, 0.05% to 0.28% of Si, 1.65% to 2.80% of Mn, 0.05% to 0.08% of P, 0.15% to 0.24% of S, 0.0065% to 0.012% of T.O and the balance of Fe. The alloy also contains at least two of 0.01%-0.030% of Ti, 0.01%-0.019% of V and 0.01%-0.019% of Nb; 0.05%-0.25% of Se, and the balance Fe and inevitable impurities. The manufacturing method comprises the steps of converter smelting, LF refining, VD vacuum degassing, continuous casting, continuous casting billet heating, continuous casting billet rolling and cooling. The alloy steel has excellent cutting performance.

Description

Technical Field

[0001] The invention belongs to the field of free-cutting steel, and in particular relates to a medium-carbon free-cutting steel bar suitable for important mechanical structural parts and a manufacturing method thereof. Background Art

[0002] The rapid development of automated machining technology has put new demands on the machining performance of certain critical components in the field of mechanical structural steel. For example, machining requirements for motor vehicle drive shafts and precision gears are becoming increasingly stringent. Machining costs often constitute a significant portion of the manufacturing cost, sometimes exceeding 40% of the component manufacturing cost. Consequently, there is a strong desire to reduce these costs by improving the machinability of steel. Free-cutting steel offers superior machinability and surface finish compared to ordinary carbon steel, making it a popular choice among manufacturers of mechanical structural components. The primary reason for these excellent machinability and surface finish is the addition of free-cutting elements such as sulfur, lead, phosphorus, bismuth, tin, selenium, and calcium. my country's free-cutting steel industry is rapidly developing. Currently, new steel grades have been developed, including 40CrCaS and 53CaS for automotive engine connecting rods, 40VS and 35MnVS for automotive crankshafts, and Y5Cr21Mn9Ni4N for automotive exhaust valves. However, there is still a significant gap in output, quality, and production levels compared to leading free-cutting steel producers such as the United States and Japan. Domestically produced free-cutting steel still struggles to meet the demands of the domestic manufacturing industry, forcing the country to rely on imports for high-performance free-cutting steel. Therefore, the present invention provides a medium-carbon free-cutting steel bar suitable for critical mechanical structural parts and a method for manufacturing the same, thereby enhancing the cutting performance of these structural parts.

[0003] The key to improving free-cutting performance is to control the distribution of sulfide forms. Existing research has found that the following measures are usually adopted:

[0004] Chinese patent publication number CN107287504A discloses "medium-carbon free-cutting non-quenched and tempered steel containing sulfur and tellurium and its production process method," which improves the steel's free-cutting performance to a certain extent. However, the high price of the tellurium element is not conducive to cost reduction. In addition, tellurium has a certain degree of toxicity, and its compounds are all toxic. Tellurium vapor seriously damages human health during smelting.

[0005] Chinese patent document with publication number CN104404386A discloses “a method for preparing an iron alloy”, wherein the iron alloy is high in carbon and has limited improvement on the material's free-cutting performance.

[0006] Chinese patent publication number CN101597725A discloses "free-cutting chromium stainless steel for ballpoint pen tips," which contains the alloying element Pb. Although it can increase the cutting performance of steel, it is toxic and pollutes the environment.

[0007] Chinese patent publication number CN202311581444A discloses a "method for producing ultra-high sulfur free-cutting steel bars." The steel's chemical composition by weight is: C ≤ 0.08%, Si ≤ 0.05%, Mn 2.00% to 2.80%, P 0.04% to 0.09%, S 0.50% to 0.70%, with the remainder being Fe and unavoidable impurities. The steel is suitable for production of gauges from 20 to 65 mm, exhibiting a tensile strength (Rm) of 400 to 540 MPa, a reduction of area ≥ 45%, and an elongation ≥ 22%. The steel can be used as a replacement for lead-based free-cutting steel. However, its high manganese content can easily lead to severe positive segregation, impairing performance. Summary of the Invention

[0008] In order to overcome the shortcomings of the existing technology, the present invention provides a medium-carbon free-cutting steel bar and a manufacturing method thereof, which belongs to the field of lead-free and environmentally friendly free-cutting steel. In order to obtain excellent free-cutting performance, without changing the comprehensive mechanical properties of the steel, the distribution and morphology of sulfides in the steel are controlled by adding free-cutting elements, thereby significantly improving the cutting performance of the medium-carbon steel and meeting the cutting performance requirements of the medium-carbon free-cutting structural steel.

[0009] A medium-carbon free-cutting steel bar comprises the following chemical compositions by weight: C: 0.25%-0.34%, Si: 0.05%-0.28%, Mn: 1.65%-2.80%, P: 0.05-0.08%, S: 0.15%-0.24%, TO: 0.0065%-0.012%; the bar further comprises at least two of: Ti: 0.01%-0.030%, V: 0.01%-0.019%, and Nb: 0.01%-0.019%; Se: 0.05%-0.25%, and the balance being Fe and unavoidable impurities.

[0010] The selection of the addition amount of each element of the bar and its function are explained as follows:

[0011] C: Carbon is the most fundamental matrix-strengthening element in steel and directly affects its toughness and weldability. Increasing carbon content significantly improves steel's strength and hardness, but excessive carbon content significantly reduces toughness and ductility, leading to poor machinability. Carbon is added to the steel to ensure the strength of the free-cutting steel. A carbon content below 0.25% fails to guarantee the strength of the bar, while a carbon content above 0.34% significantly impairs free-cutting performance. Therefore, the carbon content in the present invention is controlled within the range of 0.25% to 0.34%.

[0012] Si: Silicon is commonly used to improve the hardenability of the carburized layer, which contributes to strength. However, the silicon added in the present invention acts as a deoxidizer, working together with Mn to deoxidize the steel. When the silicon content is below 0.05%, the deoxidation effect is poor; when the silicon content exceeds 0.28%, the machinability is significantly reduced. Therefore, the silicon content in the present invention is controlled within the range of 0.05% to 0.28%.

[0013] Mn: Manganese increases the stability of supercooled austenite, inhibits pearlite transformation, and improves hardenability. It is a key component of Class A inclusions in free-cutting steel, contributing to improved free-cutting performance. In addition to increasing the strength of the free-cutting steel and acting as a deoxidizer, manganese is added to the steel to produce free-cutting inclusions called manganese sulfide and manganese selenide, enhancing the free-cutting performance of the bar. A manganese content below 1.65% fails to provide additional strength, while a manganese content above 2.50% results in excessive hardness, poor plasticity, and a significant reduction in free-cutting performance. Therefore, the manganese content in the present invention is controlled within the range of 1.65% to 2.50%.

[0014] P: For structural steel, the addition of phosphorus will cause cold brittleness and destroy the toughness and plasticity of the material. However, for free-cutting structural steel, appropriate phosphorus can improve the free-cutting performance. Therefore, the phosphorus content of the present invention is controlled to be less than 0.03%.

[0015] S: Sulfur is a major element that causes hot brittleness in steel. However, for the free-cutting structural steel of the present invention, sulfur is a beneficial element. Adding an appropriate amount of sulfur to form compounds with manganese and selenium can significantly improve free-cutting performance. When the sulfur content is less than 0.15%, it fails to form compounds with manganese and selenium that contribute to cutting performance. When the sulfur content exceeds 0.24%, the hot workability of the steel is reduced. Therefore, the sulfur content of the present invention is controlled within the range of 0.15% to 0.24%.

[0016] Oxygen is typically added in appropriate amounts to free-cutting steel as a nucleation site. When the oxygen content is below 0.0065%, it is difficult to form oxidized species for nucleation of MnS, MnSe, etc., which is detrimental to improving free-cutting performance. When the oxygen content is above 0.010%, it tends to produce excessive amounts of secondary deoxidation products during cooling, degrading machinability. Therefore, the oxygen content in the present invention is controlled within the range of 0.0065% to 0.010%.

[0017] Nb: Trace amounts of niobium can enhance steel strength through precipitation strengthening without compromising ductility. At least one of niobium oxides, carbides, nitrides, and carbonitrides will moderately precipitate in the steel, serving as nucleation sites for MnS and MnSe inclusions, thereby improving machinability. However, this effect is not evident when the niobium content is less than 0.01%. However, when the niobium content exceeds 0.019%, machinability is significantly reduced. Therefore, the niobium content in the present invention is controlled within the range of 0.01% to 0.019%.

[0018] Vanadium (V) is a key functional element in non-quenched and tempered steel, significantly improving its toughness and fatigue resistance. However, a vanadium content below 0.01% fails to achieve these benefits. A vanadium content above 0.019% increases smelting costs and reduces machinability. Therefore, the vanadium content in this invention is controlled within the range of 0.01% to 0.019%.

[0019] Ti: Trace amounts of titanium can induce precipitation strengthening and provide nucleation for the formation of free-cutting compounds. This effect is not significant when the titanium content is below 0.01%, while a titanium content above 0.030% can reduce free-cutting performance. Therefore, the titanium content in the present invention is controlled within the range of 0.01% to 0.030%.

[0020] Se: Selenium is an environmentally friendly free-cutting element, belonging to the same main group as S. It is mainly used in lead-free free-cutting steel to form compounds that help increase free-cutting properties. The selenium added in the present invention is mainly used to improve the cutting performance of free-cutting structural steel and provide better plasticity. By forming selenides such as FeSe and MnSe present in the steel, it reduces cutting resistance and cutting heat, makes it easier to remove chips, significantly improves the machinability of the steel, and can also obtain good processing surface roughness. When the selenium content is less than 0.05%, the effect of improving free-cutting properties and plasticity cannot be achieved; when the selenium content exceeds 0.25%, the free-cutting properties can no longer be further improved and the smelting cost increases. Therefore, the selenium content of the present invention is controlled within the range of 0.05% to 0.25%.

[0021] When the selenium content in steel is low, it primarily dissolves in MnS inclusions. As the selenium content increases, the MnS inclusions gradually assume a spindle shape. When the selenium content in the MnS inclusions reaches saturation, MnSe forms, encapsulating the MnS inclusions. Simultaneously, FeSe forms, inhibiting MnS deformation during hot working, providing lubrication during cutting, and improving the material's toughness and ductility. The microstructure of the medium-carbon, sulfur-containing, selenium-rich, free-cutting steel bar produced using the process of the present invention primarily consists of MnS with Se dissolved in it and composite inclusions encapsulated by MnSe. Furthermore, it also contains a certain amount of FeSe and MnS compounds. The dissolved Se reduces the aspect ratio of the MnS inclusions, improving free-cutting performance. MnSe encapsulating MnS prevents deformation and elongation along the rolling direction during hot working, while also providing lubrication for the cutting tool during cutting, contributing to improved free-cutting performance.

[0022] The present invention forms dispersed, spindle-shaped selenides and tinides, so that chips are easily removed during cutting and the cutting performance is significantly improved. The cutting performance index is that under the cutting conditions of feed speed f=0.06-0.09 mm / r and rotation speed 600-800 r / min, the relative cutting coefficient Kr of steel is 2.62-2.95, and the cutting type is C-type chip.

[0023] The present invention also provides a method for manufacturing a medium-carbon free-cutting steel bar. The process flow of the manufacturing method includes: converter smelting → LF refining → VD vacuum degassing → continuous casting → heating in a heating furnace → rolling and cooling the continuously cast billet; wherein the element Se is added in the form of selenium powder cored wire, pure selenium particles or manganese selenide at the end of LF furnace refining, and specifically includes the following steps:

[0024] 1) Converter smelting: The converter molten iron is not desulfurized, only decarbonized and dephosphorized, and the amount of lime added is controlled at 2.0-2.4 tons per 100 tons of steel, and the phosphorus content of the tapped steel is controlled at 0.05-0.08%. Silicon manganese is used for weak deoxidation during the tapping process, and the total oxygen content is controlled at 80-120ppm;

[0025] 2) LF refining: The molten steel is deoxidized on the slag surface with ferrosilicon and silicon carbide, argon is blown, and after white slag treatment, silicon calcium barium iron, low-phosphorus ferromanganese, ferrosulfur and selenium powder cored wire, pure selenium particles or manganese selenide are added to the molten steel for alloying. The alloying refining time is 20-25 minutes, and the white slag refining time is maintained at 12-15 minutes. The process temperature is controlled at 1595-1600℃.

[0026] 3) VD vacuum degassing: vacuum treatment time is 30 to 40 minutes to ensure that hydrogen is below 2.0ppm and oxygen is below 30ppm, in order to reduce hydrogen cracks and Class B inclusions;

[0027] 4) Continuous casting: The continuous casting billets need to be stacked and slowly cooled to room temperature. Hot charging and hot delivery are strictly prohibited. The continuous casting billets are chamfered to reduce round bar defects caused by micro cracks at the corners of the billets. The casting speed is controlled at 0.40m / min to 0.80m / min to ensure the surface quality of the billets and prevent cracks. Only through the above treatment method can the beneficial effects of the present invention be achieved and the free-cutting steel bars of the present invention can be obtained in combination with subsequent processes.

[0028] 5) Continuous casting billet heating: The continuous casting billet is cold loaded and heated to above 1200℃ in the heating furnace. The heating and holding time of the continuous casting billet is 1.5-3.0h;

[0029] 6) Continuous Casting Slab Rolling and Cooling: The continuous casting slab undergoes 8-11 rough rolling passes, which are then rolled into rough slabs. Round bars are then formed through 5-7 finish rolling passes, with the final rolling pass temperature at 940-990°C. After finish rolling, the bars are air-cooled to room temperature. This rolling process introduces a large number of deformation dislocations, which then break up recrystallized grains. Recrystallization nucleation occurs again, followed by the introduction of a large number of dislocations, which then break up recrystallized grains again. This results in dislocation pinning of grain boundaries, hindering boundary expansion and refining austenite grains. This results in both grain refinement and dislocation strengthening, improving the strength and toughness of the round bars.

[0030] Furthermore, in step 4), the cross-sectional size of the continuous casting billet is (280-320) mm×(380-410) mm, and the large square billet is used to ensure the compression ratio.

[0031] Furthermore, in step 4), the continuous casting billet is chamfered on four sides.

[0032] Furthermore, in step 6), the continuous casting billet is descaled by high-pressure water and then subjected to rough rolling.

[0033] Furthermore, in step 6), the cross-sectional size of the rough rolled billet is (100-150) mm×(150-200 mm).

[0034] Further, in step 6), the finishing rolling is performed Round rod.

[0035] Compared with the prior art, the present invention has the following beneficial effects:

[0036] First, the present invention utilizes the alloying effect of manganese, sulfur, and selenium to weaken the precipitation of sulfides along grain boundaries, forming MnSe that encapsulates MnS inclusions and simultaneously forming FeSe. This can inhibit MnS deformation during hot working, effectively improving the morphology of inclusions in free-cutting steel bars that are beneficial to cutting performance, transforming them from long strips to spindles or spheres, reducing the size of built-up edge on the tool and making the edge uniform, thereby significantly improving the cutting performance of free-cutting steel bars.

[0037] Secondly, the present invention addresses the technical bottleneck of achieving both free-cutting performance and surface quality for high-grade free-cutting steel bars. This environmentally friendly free-cutting steel meets the current trend of lead-free, environmentally friendly free-cutting steels. Its market launch promises promising economic and environmental benefits for businesses. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] Figure 1 This is the microstructure diagram of the free-cutting steel bar prepared in Example 6. DETAILED DESCRIPTION

[0039] The following non-limiting examples may enable those skilled in the art to more fully understand the present invention, but are not intended to limit the present invention in any way.

[0040] Examples 1-13

[0041] A medium carbon free-cutting steel bar, the specific chemical composition of which is shown in Table 1, with the balance being Fe and inevitable impurities.

[0042] The manufacturing method of the above-mentioned medium carbon free-cutting steel bar comprises the following process flow: converter smelting → LF furnace refining → VD vacuum degassing → continuous casting → continuous casting billet heating → continuous casting billet rolling and cooling; wherein the element Se is added in the form of selenium powder cored wire, pure selenium granules or manganese selenide at the end of LF furnace refining, and specifically comprises the following steps:

[0043] 1) Converter smelting: The converter molten iron is not desulfurized, only decarbonized and dephosphorized. The amount of lime added is controlled at 2.0-2.4 tons per 100 tons of steel, and the phosphorus content of the tapped steel is controlled at 0.05-0.08%. During the tapping process, silicon-manganese alloy is used for weak deoxidation, and the total oxygen content is controlled at 80-120 ppm.

[0044] 2) LF furnace refining: The molten steel is deoxidized on the slag surface with ferrosilicon and silicon carbide, argon is blown, and after white slag treatment, silicon calcium barium iron, low phosphorus ferromanganese, ferrosulphur and selenium powder cored wire are added to the molten steel for alloying. The alloying refining time is 25 minutes, and the white slag refining time is maintained for 12 minutes. The process temperature is controlled at 1595-1600℃;

[0045] 3) VD vacuum degassing: vacuum treatment time is 30 to 40 minutes, ensuring that gases such as hydrogen and oxygen are below 2.0ppm and 30ppm respectively;

[0046] 4) Continuous casting: Continuous casting uses 280mm×380mm large-section billets; the billets need to be stacked and slowly cooled to room temperature, hot charging and hot delivery are strictly prohibited, and the billets must be chamfered on all four sides; the casting speed is controlled at 0.40m / min~0.80m / min.

[0047] 5) Continuous casting billet heating: The continuous casting billet is cold loaded and heated to above 1200℃ in the heating furnace. The heating and holding time of the continuous casting billet is 1.5-3.0h;

[0048] 6) Continuous casting billet rolling and cooling: The billet is descaled by high-pressure water, and then rough rolled by 8 passes to form a 150mm*150mm cross-section rough rolled billet, and then finished by 5 passes. For round bars, the final rolling temperature is 940-990℃; after finishing rolling, it is air-cooled to room temperature.

[0049] Among them, the key parameters for steelmaking, heating and rolling process control are shown in Table 2.

[0050] The cutting performance effects in the embodiments of the present invention are shown in Table 3.

[0051] Table 1 Specific chemical composition (weight percentage (wt%))

[0052]

[0053]

[0054] Table 2 Key control points for steelmaking, heating and rolling processes

[0055]

[0056] Table 3 Statistics of billet end cracking, surface scarring, bubble defects, relative cutting coefficient Kr and chip type

[0057]

[0058]

Claims

1. A medium carbon free-cutting steel bar, characterized in that: The chemical composition of the rod is as follows by weight: C: 0.25% to 0.34%, Si: 0.05% to 0.28%, Mn: 1.65% to 2.80%, P: 0.05-0.08%, S: 0.15% to 0.24%, TO: 0.0065% to 0.012%; it also contains at least two of Ti: 0.01% to 0.030%, V: 0.01% to 0.019%, and Nb: 0.01% to 0.019%; Se: 0.05% to 0.25%, and the balance is Fe and unavoidable impurities.

2. The medium carbon free-cutting steel bar according to claim 1, characterized in that: Under the cutting conditions of a feed rate of 0.06-0.09 mm / r and a rotation speed of 600-800 r / min, the relative cutting coefficient of the bar is 2.62-2.95, and the cutting type is C-type cutting.

3. The method for producing a medium carbon free-cutting steel bar according to claim 1 or 2, characterized in that: The manufacturing method comprises: converter smelting → LF refining → VD vacuum degassing → continuous casting → continuous casting billet heating → continuous casting billet rolling and cooling; the specific steps are as follows: 1) Converter smelting: The converter molten iron is not desulfurized, only decarbonized and dephosphorized, and the amount of lime added is controlled at 2.0-2.4 tons per 100 tons of steel, and the phosphorus content of the tapped steel is controlled at 0.05-0.08%. Silicon manganese is used for weak deoxidation during the tapping process, and the total oxygen content is controlled at 80-120ppm; 2) LF refining: The molten steel is deoxidized on the slag surface with ferrosilicon and silicon carbide, argon is blown, and after white slag treatment, silicon calcium barium iron, low phosphorus ferromanganese, ferrosulfur and selenium powder cored wire, pure selenium particles or manganese selenide are added to the molten steel for alloying. The alloying refining lasts for 20-25 minutes, and the white slag refining is maintained for 12-15 minutes. The process temperature is controlled at 1595-1600℃. 3) VD vacuum degassing: vacuum treatment for 30 to 40 minutes to ensure that hydrogen is less than 2.0ppm and oxygen is less than 30ppm; 4) Continuous casting: The continuous casting billet stack is slowly cooled to room temperature, and the continuous casting billet is chamfered; the casting speed is controlled at 0.40m / min to 0.80m / min; 5) Continuous casting billet heating: The continuous casting billet is cold loaded and heated to above 1200℃ in the heating furnace. The continuous casting billet is heated and kept warm for 1.5-3.0h; 6) Rolling and cooling of continuous casting slab: The continuous casting slab is subjected to 8 to 11 rough rolling passes, and then the continuous casting slab is rolled into a rough rolled slab, which is then subjected to 5 to 7 finish rolling passes to form a round bar, with the final rolling pass temperature being 940 to 990°C; after finish rolling, it is air-cooled to room temperature.

4. The manufacturing method according to claim 3, characterized in that In step 4), the cross-sectional size of the continuous casting billet is (280-320) mm×(380-410) mm.

5. The manufacturing method according to claim 3, characterized in that In step 4), the continuous casting billet is chamfered on four sides.

6. The manufacturing method according to claim 3, characterized in that In step 6), the continuous casting billet is descaled by high-pressure water and then subjected to rough rolling.

7. The manufacturing method according to claim 3, characterized in that In step 6), the cross-sectional size of the rough rolled billet is (100-150) mm×(150-200) mm.

8. The manufacturing method according to claim 3, characterized in that In step 6), the finishing rolling is performed Round rod.

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