A method for manufacturing cold-drawn medium-carbon high-sulfur steel without annealing

Abstract

The present invention relates to a method for manufacturing non-annealed cold-drawn medium-carbon, high-sulfur steel. The steel is round steel, based on Fe, and contains the following chemical components: C: 0.40-0.50%, Si: 0.15-0.35%, Mn: 1.35-1.65%, P: ≤0.040%, S: 0.24-0.33%, Cr: ≤0.30%, Ni: ≤0.30%, Cu: ≤0.30%, Mo: ≤0.20%, Al: ≤0.010%, and impurity elements. The production method comprises converter smelting, LF refining, continuous casting, continuous casting heating and blanking, intermediate billet finishing, high-speed wire rolling, pickling and coating, and combined drawing. The addition of ferroaluminum and ferrosulfur during converter steelmaking improves the stability of [S]. Finely dispersed alumina provides nucleation conditions for sulfide inclusions. Aluminum deoxidation is not used during the refining process, which increases the oxygen content in the steel and controls the oxygen content in the molten steel to 0.0020-0.0040%. This improves the morphology of sulfide inclusions in the ingot. Through blooming and intermediate rolling, two high-temperature rolling steps are performed to obtain small, evenly distributed sulfide inclusions with a low aspect ratio. Appropriate drawing reduction ratios and drawing speeds are controlled to suppress cold-drawing cracking in medium-carbon steel.

Description

Technical Field

[0001] The invention belongs to the technical field of metallurgy, and in particular relates to a method for manufacturing medium-carbon high-sulfur steel. Background Art

[0002] With the vigorous development of machinery manufacturing and new energy vehicles and the increasingly high-speed and automated production, especially the popularization and application of CNC machine tools, free-cutting steel with the advantages of easy chip breaking, high surface finish, low tool wear, high production efficiency and low energy consumption has achieved rapid development.

[0003] Sulfur-based free-cutting steels are widely used for their excellent machinability, cost-effectiveness, and environmental friendliness. Medium-carbon, high-sulfur free-cutting steels are hypoeutectoid steels that combine excellent mechanical and machinability. Their surface finish significantly surpasses that of other medium-carbon steels, making them popular in the manufacture of automotive engine sealing valves, hydraulic, and automation components and systems. This steel not only requires high cleanliness but also stringent requirements for sulfide morphology. Due to the high sulfur content in the steel, sulfide inclusions act as stress concentrators, disrupting the continuity of the matrix, promoting crack propagation, reducing tool friction, and encapsulating hard particles, thereby improving the steel's machinability. However, controlling sulfide morphology remains a major challenge, particularly in ensuring uniformity, a bottleneck hindering product quality. Large sulfide inclusions in continuous castings tend to become elongated after high-speed wire rolling. During deformation, these inclusions connect with gaps in the steel matrix, becoming a source of crack propagation, degrading the material's transverse mechanical properties, exacerbating product anisotropy, and potentially causing cracking during cold wire drawing. Summary of the Invention

[0004] The present invention aims to provide a method for manufacturing medium-carbon, high-sulfur steel to address the technical challenge of cold-drawing cracking in medium-carbon, high-sulfur steel. This method ensures the performance of medium-carbon, ultra-high-sulfur steel by controlling the morphology and distribution of sulfide inclusions in the steel to produce small, evenly distributed, short-rod-shaped sulfide inclusions. Furthermore, the method controls the drawing area reduction and drawing speed to meet the requirements for cold drawing of wire rod without annealing.

[0005] The technical solution adopted by the present invention is: a method for manufacturing medium-carbon high-sulfur steel by cold-drawing without annealing, obtaining medium-carbon high-sulfur cold-drawn round steel, wherein the round steel is based on Fe and also contains the following chemical components (wt%): C: 0.40-0.50%, Si: 0.15-0.35%, Mn: 1.35-1.65%, P: ≤0.040%, S: 0.24-0.33%, Cr: ≤0.30%, Ni: ≤0.30%, Cu: ≤0.30%, Mo: ≤0.20%, Al: ≤0.010%, and unavoidable impurity elements.

[0006] The manufacturing method includes: converter smelting, LF refining, large square billet arc continuous casting, large square billet blooming, intermediate billet surface finishing, high-speed wire rolling, and wire rod combined drawing. The specific process steps are as follows:

[0007] (1) Converter smelting: oxygen is blown into the converter and lime is added to make slag, decarbonize and dephosphorize. The end point C content is controlled at 0.10-0.25% during tapping. During the tapping process, aluminum iron is first added along the steel flow for pre-deoxidation. In the subsequent smelting process, aluminum deoxidation is no longer used. Sulfur iron is then added for alloying to regulate the stability of the S content in the molten steel. Finally, lime is added and synthetic slag is refined. The converter slag is blocked during tapping to prevent oxidation slag.

[0008] (2) LF refining: Fluorite is added to the refining process to quickly disintegrate the slag, reduce the basicity of the slag and improve the fluidity of the slag. Carbon powder and SiC powder are used for deoxidation (aluminum deoxidation is not applicable) to control the oxygen content in the molten steel to 0.0020-0.0040%. The argon flow rate and voltage level of the stirring molten steel during refining are controlled to make the composition and temperature of the molten steel uniform.

[0009] (3) Bloom arc continuous casting: The cross-sectional size of the bloom is 390mm*510mm or more. During continuous casting, the mold liquid level is kept stable and the superheat is kept low. To stabilize the superheat, the tundish induction heating technology is used. The superheat is controlled at 15-25°C. Soft pressure reduction and electromagnetic stirring are applied to the casting strand to improve the segregation of the cast strand. The casting is protected throughout the continuous casting process to prevent secondary oxidation and ensure stable cast strand quality. The MnS inclusions in the bloom are mainly point-shaped and spherical.

[0010] (4) Blooming: Blooms are reheated at 1200-1300°C and rolled into 200mm thick pieces. 2 In the billet, the MnS inclusions in the billet are deformed from spherical to long strips through high temperature rolling;

[0011] (5) The surface of the intermediate billet is peeled, and the amount of removal is: more than 1.5mm on the surface and more than 1.7mm on the corners to ensure that the surface of the billet is smooth and avoid severe decarburization during the heating process;

[0012] (6) High-speed wire rolling: The intermediate billet is heated in three temperature sections in the heating furnace, with the temperature of the first heating section at 900-980°C, the temperature of the second heating section at 1130-1250°C, and the temperature of the soaking section at 1150-1240°C. The soaking section holding time is not less than 40 minutes, and the residual oxygen content in the furnace is ≤2.0%. After the holding is completed, it is subjected to high-pressure water descaling to remove the oxide scale formed on the surface due to heating, and then rolled. In the rough rolling stage, the starting rolling temperature is 1050-1200°C; in the intermediate finishing rolling stage, the finishing rolling temperature is 930-1030°C; the sizing temperature is 950-1020°C; the wire drawing temperature is 860-950°C, the coil is air-cooled on the roller, the roller speed is 0.20m / s-0.30m / s, and ≥4 insulation covers are opened;

[0013] (7) Wire rod combined drawing: Since this type of steel has a high C content, which is at the level of medium carbon steel, the strength of the hot-rolled wire rod is about 720 MPa. The plasticity of the MnS inclusions in the wire rod is worse than that of the matrix during cold drawing. Therefore, it is necessary to control the drawing speed and area reduction rate. The wire rod is first pickled and coated and then shot blasted and drawn. The drawing head is first pointed with a pointing machine and then ground smooth. The drawing speed is 30-50 m / min and the area reduction rate is controlled at 10-20% to ensure that no cracking occurs during drawing.

[0014] Compared with the prior art, the advantages of the present invention are:

[0015] (1) The smelting method is to add aluminum iron and ferrosulfur in advance before the steel is tapped from the converter to improve the stability of [S] control in the steel. The finely dispersed Al2O3 provides nuclei for MnS inclusions, allowing the easily deformed MnS to adhere to the hard Al2O3 surface as a shell, forming a composite inclusion that is soft on the outside and hard on the inside, reducing its plasticity, thereby achieving the effect of controlling sulfide inclusions.

[0016] (2) The refining process only diffuses deoxidation, which increases the dissolved oxygen content in the molten steel, creates conditions for the formation of point-shaped and spherical MnS inclusions, and improves the morphology of sulfide inclusions in the ingot;

[0017] (3) The blooming + intermediate billet rolling process is adopted. Through the high-temperature blooming of the bloom, the MnS is deformed into long strips of MnS inclusions in the first rolling. The intermediate billet is then subjected to a second high-temperature rolling. The long strips of MnS are split into several short MnS inclusions. The two high-temperature rolling processes obtain sulfide inclusions with small size, uniform distribution, and low aspect ratio.

[0018] (4) The wire rod can be directly cold drawn without annealing, which reduces production costs. During drawing, the area reduction rate is controlled at 10-20% and the drawing speed is 30-50 m / min, which effectively improves the cold drawing cracking of medium carbon high sulfur steel. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1The morphology of the bloom sulfide of the embodiment of the present invention (×100);

[0020] Figure 2 The morphology of the intermediate billet sulfide of the embodiment of the present invention (×100);

[0021] Figure 3 This is the sulfide morphology of the hot-rolled wire rod according to an embodiment of the present invention (×500). DETAILED DESCRIPTION

[0022] The present invention is further described in detail below with reference to the examples. The examples are illustrative and intended to explain the present invention, but are not to be construed as limiting the present invention.

[0023] Example 1

[0024] The medium-carbon, high-sulfur cold-drawn round steel involved in this embodiment has a specification of Φ9mm, and the components and their mass percentages included are: C: 0.41%, Si: 0.18%, Mn: 1.51%, P: 0.011%, S: 0.28%, Cr: 0.10%, Al: 0.002%, Ni: 0.03%, Cu: 0.02%, Mo: 0.01%, O: 0.0032%, and the remainder is iron and unavoidable impurity elements.

[0025] The production process of the medium carbon high sulfur cold drawn round steel is as follows:

[0026] The smelting raw materials are configured according to the chemical composition of the cold-drawn round steel and are sequentially subjected to converter smelting, LF refining, continuous casting, warm or hot delivery of continuous casting billets, continuous casting billet heating and billet opening, continuous casting billet surface cleaning, heating, high-pressure water descaling, controlled rolling, controlled cooling, wire rod making, pickling and coating, and combined drawing.

[0027] Furthermore, the main raw materials are smelted in a 100t converter in turn, and 100kg of aluminum iron, 750kg of ferrosulfur, 400kg of lime and 500kg of synthetic slag are added in turn when the converter is tapped. LF off-furnace refining is adopted, and SiC diffusion deoxidation is used during refining. The molten steel is cast into continuous casting billets under argon protection at a low superheat of 15-25°C throughout the process.

[0028] Furthermore, the specific process of the above continuous casting billet heating, intermediate billet heating, rolling and cooling stages is as follows: the continuous casting billet produced is heated to 1250℃, kept warm for 6 hours, and opened into 200mm 2Intermediate billet: The intermediate billet is heated to 1220℃ and kept warm for 45 minutes. The residual oxygen content in the furnace is 1.0%. After being taken out of the furnace, it is descaled with high-pressure water and then rolled. The starting rolling temperature is 1100℃. The water tank pressure and water valve are adjusted. After cooling, it passes through several rolling mills to make the surface and core temperature of the rolled piece uniform. The finishing rolling temperature is 980℃, the sizing temperature is 950℃, the wire drawing temperature is 910℃, the roller speed is increased by 0.20m / s and increased by 0.02m / s. 6 insulation covers are opened and the rest are closed. The piece is air-cooled to room temperature after being taken out of the cover.

[0029] Furthermore, the wire rod is pickled with lime and then transferred to a material rack. The wire rod head is pointed and polished smooth with a pointing machine, and then passed into a mold for shot blasting-combined drawing-cutting-straightening. The wire rod is drawn from Φ9mm wire rod to Φ8.5mm round steel with a length of 3m. The drawing reduction rate is 10.8% and the drawing speed is 45m / min.

[0030] Example 2

[0031] The specifications of the medium carbon high sulfur cold drawn round steel involved in this embodiment are The components contained and their mass percentages are: C: 0.43%, Si: 0.19%, Mn: 1.53%, P: 0.020, S: 0.27%, Cr: 0.12%, Al: 0.002%, Ni: 0.02%, Cu: 0.01%, Mo: 0.01%, O: 0.0036%, and the balance is iron and unavoidable impurity elements.

[0032] The production process of cold-drawn round steel is basically the same as that of Example 1, but there are differences in the amount of alloy added during converter tapping, continuous casting billet heating and opening, intermediate billet heating, cooling, and subsequent drawing process, as follows:

[0033] Furthermore, the main raw materials are smelted in a 100t converter in sequence, and 100kg of aluminum iron, 800kg of ferrosulfur, 400kg of lime and 500kg of synthetic slag are added in sequence when the converter is tapped. LF refining is adopted, and SiC diffusion deoxidation is used during refining. The molten steel is cast into continuous casting billets under argon protection at a low superheat of 15-25°C throughout the process.

[0034] Furthermore, the specific process of the above continuous casting billet heating, intermediate billet heating, rolling and cooling stages is as follows: the continuous casting billet produced is heated to 1240℃, kept warm for 6 hours, and opened into 200mm 2Intermediate billet: The intermediate billet is heated to 1230°C and kept warm for 42 minutes. The residual oxygen content in the furnace is 1.0%. After being taken out of the furnace, it is descaled with high-pressure water and then rolled. The starting rolling temperature is 1120°C. The water tank pressure and water valve are adjusted. After cooling, it passes through several rolling mills to make the surface and core temperature of the rolled piece uniform. The finishing rolling temperature is 1000°C, the sizing temperature is 960°C, the wire drawing temperature is 915°C, the roller speed is increased by 0.20m / s, and increased by 0.02m / s. 6 insulation covers are opened and the rest are closed. The piece is air-cooled to room temperature after being taken out of the cover.

[0035] Furthermore, the wire rod is pickled with lime and then transferred to a material rack. The wire rod head is pointed and polished smooth with a pointing machine, and then passed into a mold for shot blasting, combined drawing, cutting, and straightening. The wire rod is drawn from Φ16.5mm wire rod to Φ14.8mm round steel with a length of 3m. The drawing reduction rate is 19.5% and the drawing speed is 40m / min.

[0036] The finished cold-drawn round steel produced by the above process has excellent surface quality, no cracking during drawing, uniform sulfide distribution, good strength and surface shrinkage. Its mechanical properties and cracking ratio are shown in Table 1.

[0037] Table 1

[0038]

[0039] Note: Room temperature tensile testing is carried out in accordance with standard GB / T228.

[0040] In addition to the above embodiments, the present invention also includes other implementation methods. Any technical solutions formed by equivalent transformation or equivalent replacement should fall within the scope of protection of the claims of the present invention.

Claims

1. A method for producing cold-drawn medium-carbon high-sulfur steel without annealing, characterized by: include, Step 1: Converter smelting: oxygen is blown into the converter and lime is added to form slag for decarburization and dephosphorization. The final carbon content is controlled at 0.10-0.25% during tapping. During the tapping process, aluminum iron is first added along the steel flow for pre-deoxidation. At the same time, finely dispersed alumina provides nuclei for sulfide formation. Then, ferrosulfur is added to control the sulfur content. Finally, lime is added and the synthetic slag is refined. The converter is used to block the slag for tapping. Step 2, LF furnace refining: add fluorite to quickly deslag, reduce the slag basicity and improve the slag fluidity, use carbon powder and SiC powder for deoxidation, and control the oxygen content in the molten steel to 0.0020-0.0040%. Step 3: Bloom arc continuous casting: The cross-sectional size of the bloom is 390mm*510mm or larger. The MnS inclusions in the bloom are mainly point-shaped and spherical. The continuous casting adopts induction heating in the tundish, and the superheat is controlled at 15-25℃. The casting strand is subjected to soft pressure reduction and electromagnetic stirring to improve the segregation of the cast strand. The casting is protected throughout the continuous casting process to prevent secondary oxidation. Step 4: Blooming: Bloom is reheated at 1200-1300℃ and shaped into 200mm square pieces. 2 The intermediate billet is rolled to deform the MnS inclusion shape from spherical to long strips; Step 5: Peeling the surface of the intermediate billet: removing surface defects to obtain a smooth surface billet; Step 6, high-speed wire rolling: the intermediate billet is heated and kept warm in a heating furnace, and the intermediate billet is heated in three temperature sections in the heating furnace, the temperature of the first heating section: 900~980℃, the temperature of the second heating section: 1130~1250℃, the temperature of the soaking section: 1150℃~1240℃, the soaking section holding time is not less than 40min, the residual oxygen content in the furnace is ≤2.0%, and it is descaled with high-pressure water after being taken out of the furnace, and then rolled, rough rolling stage: the starting rolling temperature is 1050~1200℃; medium and finishing rolling stage: the finishing rolling temperature is 930~1030℃; the sizing temperature is 950~1020℃; the wire drawing temperature is 860℃~950℃, the coil is cooled on the roller, the roller speed is 0.20m / s~0.30m / s, and the number of insulation covers on the roller is opened ≥4; Step 7: Wire rod combined drawing: The wire rod is pickled and coated, then shot blasted and drawn. The drawing head is first pointed with a pointing machine and then ground smooth. The drawing speed is controlled at 30-50m / min, and the area reduction rate is controlled at 10-20% to ensure that no cracking occurs during drawing. After drawing, medium-carbon, high-sulfur cold-drawn round steel is obtained; The round steel is based on Fe, and the other chemical components wt% are: C: 0.40~0.50%, Si: 0.15~0.35%, Mn: 1.35~1.65%, P: ≤0.040%, S: 0.24~0.33%, Cr: 0.1~0.30%, Ni: 0.02~0.30%, Cu: 0.01~0.30%, Mo: 0.01~0.20%, Al: ≤0.010%, and inevitable impurity elements.

2. The method according to claim 1, wherein: Step 5: Peeling amount of the intermediate blank surface: more than 1.5mm on the surface and more than 1.7mm on the corners.

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