Manufacturing method of annealing-free cold-drawing medium-carbon high-sulfur steel

Abstract

The invention relates to a manufacturing method of annealing-free cold-drawing medium-carbon high-sulfur steel which is round steel, takes Fe as a basic element and comprises the following chemical components: 0.40 to 0.50 percent of C, 0.15 to 0.35 percent of Si, 1.35 to 1.65 percent of Mn, less than or equal to 0.040 percent of P, 0.24 to 0.33 percent of S, less than or equal to 0.30 percent of Cr, less than or equal to 0.30 percent of Ni, less than or equal to 0.30 percent of Cu, less than or equal to 0.20 percent of Mo, less than or equal to 0.010 percent of Al and impurity elements. The production method comprises the steps of converter smelting, LF refining, continuous casting of billets, heating and cogging of the continuous casting billets, finishing of intermediate billets, high-speed wire rod rolling, acid pickling and film coating and combined drawing. Ferroaluminum and ferrous sulfide are added during converter tapping, the stability of [S] is improved, fine and dispersed aluminum oxide provides nucleation conditions for sulfide inclusions, aluminum deoxidation is not used in the refining process, the oxygen content in steel is increased, the oxygen content in molten steel is controlled to be 0.0020-0.0040%, the form of the sulfide inclusions in a casting blank is improved, and through bloom cogging and intermediate blank rolling, the content of the sulfide inclusions in the casting blank is increased. Sulfide inclusions which are small in size, uniform in distribution and low in length-width ratio are obtained through two times of high-temperature rolling, the appropriate drawing area reduction rate and the appropriate drawing speed are controlled, and the medium carbon steel cold drawing cracking phenomenon is restrained.

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 steel is widely used due to its good machinability, economy and environmental protection. Medium-carbon high-sulfur free-cutting steel is a hypoeutectoid structural steel with both mechanical properties and cutting properties. Because its surface finish is significantly better than other medium-carbon steels, it is mostly used to manufacture automobile engine sealing valves, hydraulic and automation components and systems. This type of steel not only has high requirements for cleanliness, but also has strict requirements for the sulfide morphology. Due to the high sulfur content in steel, sulfide inclusions act as stress concentration sources, cut off the continuity of the matrix, promote crack propagation, reduce tool friction, wrap hard points, etc., thereby improving the cutting performance of steel. However, there are still many problems in the control of sulfide morphology, especially the uniformity control of sulfides is the bottleneck restricting the improvement of product quality. Large-sized sulfide inclusions in continuous casting billets are easy to become long strips after high-speed wire rolling. The gaps generated by them in the steel matrix during deformation are connected to each other, which is easy to become a crack propagation source, destroying the transverse mechanical properties of the material, aggravating the anisotropy of the product, and easy to crack during the cold drawing process of wire rods. Summary of the invention

[0004] The purpose of the present invention is to provide a method for manufacturing medium carbon high sulfur steel, solve the technical problem of cold drawing cracking of medium carbon high sulfur steel, ensure the steel performance of medium carbon ultra-high sulfur steel, control the shape and distribution of sulfide inclusions in steel, obtain small and uniformly distributed short rod-shaped sulfide inclusions, and control the drawing reduction rate and drawing speed at the same time to meet the requirements of cold drawing of wire rod without annealing.

[0005] The technical solution adopted by the present invention is: a method for manufacturing annealing-free cold-drawn medium-carbon high-sulfur steel, obtaining medium-carbon high-sulfur cold-drawn round steel, wherein the round steel takes Fe as the basic element 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 inevitable impurity elements.

[0006] The manufacturing method comprises: converter smelting, LF refining, large square billet arc continuous casting, large square billet opening, intermediate billet surface finishing, high-speed wire rolling, and wire rod joint 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 final C content is controlled to be 0.10-0.25% when 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 control the stability of the S content in the molten steel. Finally, lime is added to refine the synthetic slag, and the converter slag is used to prevent oxidation slag from falling during tapping.

[0008] (2) LF refining: Fluorite is added to quickly deslag during refining to reduce the slag basicity and improve the slag fluidity. 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 molten steel are controlled during refining 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 above 390mm*510mm. During continuous casting, the liquid level of the crystallizer 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. The casting strand is subjected to light pressure reduction and electromagnetic stirring to improve the segregation of the bloom. The casting is protected throughout the continuous casting process to prevent secondary oxidation and ensure the stability of the bloom quality. The MnS inclusions in the bloom are mainly point-shaped and spherical.

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

[0011] (5) The surface of the intermediate billet is peeled, and the removal amount is more than 1.5 mm on the surface and more than 1.7 mm 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, the temperature of the first heating section is 900-980°C, the temperature of the second heating section is 1130-1250°C, the temperature of the soaking section is 1150-1240°C, the soaking section insulation time is not less than 40 minutes, the residual oxygen content in the furnace is ≤2.0%, after the insulation 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, rough rolling stage: the starting rolling temperature is 1050-1200°C; intermediate finishing rolling stage: the finishing rolling temperature is 930-1030°C; the sizing temperature is 950-1020°C; the wire laying 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 720Mpa. 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 surface 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 30-50m / min and the surface 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 process of blooming and rolling the intermediate billet is adopted. The blooming is carried out at high temperature. The MnS is deformed into long strips of MnS inclusions by 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 rollings 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 the production cost. During drawing, the surface reduction rate is controlled at 10-20% and the drawing speed is controlled at 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 in conjunction with examples. The examples are exemplary and intended to be used to explain the present invention, but should not 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, 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 refining is adopted, SiC diffusion deoxidation is used during refining, and the molten steel is cast into continuous casting billets under argon protection at a low superheat of 15 to 25°C.

[0028] Furthermore, the specific process of the above continuous casting billet heating, intermediate billet heating, rolling and cooling stages is as follows: the produced continuous casting billet is heated to 1250°C, kept warm for 6 hours, and opened into 200mm 2Intermediate billet: The intermediate billet is heated to 1220℃ and kept warm for 45min. 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-spinning 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. It is air-cooled to room temperature after leaving the cover.

[0029] Furthermore, the wire rod is pickled with lime and then transferred to the material rack. The wire rod head is sharpened and polished by a sharpening machine, and then inserted into a mold for shot blasting-combined drawing-cutting-straightening. The wire rod is drawn from Φ9mm to Φ8.5mm and 3m long round steel. 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 remainder 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 in the converter, the heating and opening of the continuous casting billet, the heating and cooling of the intermediate billet, and the subsequent drawing process, as follows:

[0033] Furthermore, the main raw materials are smelted in a 100t converter in turn, and 100kg of aluminum iron, 800kg of ferrosulfur, 400kg of lime and 500kg of synthetic slag are added in turn when the converter is tapped. LF refining is adopted, SiC diffusion deoxidation is used during refining, and 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 produced continuous casting billet is heated to 1240°C, 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. It is air-cooled to room temperature after leaving the cover.

[0035] Furthermore, the wire rod is pickled with lime and then transferred to the material rack. The wire rod head is sharpened and polished by a sharpening machine, and then inserted 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 modes. Any technical solutions formed by equivalent transformation or equivalent replacement should fall within the protection scope of the claims of the present invention.

Claims

1. A method for manufacturing cold-drawn medium-carbon high-sulfur steel without annealing, Features: include, Step 1, converter smelting: oxygen blowing and lime slag making in the converter, decarbonization and dephosphorization, the end point C content is controlled to be 0.10-0.25% when tapping, aluminum iron is first added along the steel flow for pre-deoxidation during tapping, and finely dispersed alumina provides nuclei for sulfide, and then sulfide iron is added to adjust the sulfur content, and finally lime is added to refine the synthetic slag, and the converter is slag blocked for tapping; Step 2, LF furnace refining: add fluorite to quickly slag, 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 above 390mm*510mm, and the MnS inclusions in the bloom are mainly in the form of dots and spheres; Step 4: Blooming: Bloom is reheated at 1200-1300℃ and shaped into 200mm square. 2 The intermediate billet is used to deform the MnS inclusion form from spherical to long strips by billet rolling. Step 5: Peeling the surface of the intermediate billet: removing surface defects to obtain a billet with a smooth surface; Step 6, high-speed wire rolling: the intermediate billet is heated and kept warm in the heating furnace, and is descaled by high-pressure water after being taken out of the furnace, and then rolled, the rough rolling stage: the rolling temperature is 1050-1200°C; the intermediate finishing rolling stage: the finishing temperature is 930-1030°C; the sizing temperature is 950-1020°C; the wire laying temperature is 860-950°C, 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 and then shot blasted and drawn. The drawing head is first pointed with a pointing machine and polished smooth. The drawing speed is controlled at 30-50m / min, and the surface reduction rate is controlled at 10-20% to ensure that no cracking occurs during drawing. After drawing, medium-carbon and high-sulfur cold-drawn round steel is obtained.

2. The method according to claim 1, Features: The round steel is based on Fe and also contains the following chemical components by weight: 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 inevitable impurity elements.

3. The method according to claim 1, Features: Step three: The continuous casting adopts induction heating in the tundish, and the superheat is controlled at 15-25°C. The casting strand is subjected to light pressure reduction and electromagnetic stirring to improve the segregation of the ingot. The entire continuous casting process is protected from secondary oxidation.

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

5. The method according to claim 1, Features: Step six, the intermediate billet is heated in three temperature sections in the heating furnace, the temperature of the first heating section is: 900-980°C, the temperature of the second heating section is: 1130-1250°C, the temperature of the soaking section is: 1150°C-1240°C, the holding time of the soaking section is not less than 40 minutes, and the residual oxygen content in the furnace is ≤2.0%.

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