Non-quenched and tempered steel wire rod and its production method and use

Abstract

The application belongs to the technical field of steel metallurgy, and particularly relates to a non-quenched and tempered steel wire rod and a preparation method and application thereof. The preparation method of the non-quenched and tempered steel wire rod provided by the application improves the strength of the steel by designing reasonable contents of carbon, silicon, manganese and chromium; meanwhile, nickel and vanadium are added for micro-alloying, so as to further improve the strength and plasticity of the steel through solid solution strengthening and fine-grain strengthening. Single LF refining means + a reasonable cooling system after rolling is adopted, so that the wire rod has excellent mechanical properties; through accurate control of the binary basicity of the refining slag and the content of aluminum oxide in the refining slag, the RH refining step is omitted, and the process is simplified.

Description

Technical Field

[0001] This invention belongs to the field of iron and steel metallurgy technology, specifically relating to a non-quenched and tempered steel wire rod, its preparation method, and its application. Background Technology

[0002] Steering racks are common components in the automotive industry, widely used in passenger cars and mini and light trucks. Currently, the commonly used material for steering racks is 45# steel. The manufacturing process involves heat treatment of 45# steel followed by drawing, heat treatment, machining, induction hardening of the rack portion, and low-temperature tempering. This process is time-consuming and costly. Because 45# steel has poor hardenability, its strength after heat treatment is low, with a tensile strength of 750-780 MPa. In actual use, the low strength and poor wear resistance of the rack result in a short service life. Therefore, steering racks made from 45# steel cannot meet the requirements of high-end vehicle models.

[0003] With the increasing demands for lightweighting and stringent environmental protection requirements in the automotive industry, automotive steering racks are gradually evolving towards high-strength, non-quenched and tempered steel. This has led to a growing need for high-strength non-quenched and tempered steel wire rods for automotive steering racks. Non-quenched and tempered steel, developed in the 1970s and 80s, is an energy-saving and environmentally friendly steel. Compared to traditional quenched and tempered steel, it eliminates the quenching and tempering process, reducing environmental pollution from heat treatment. Furthermore, the addition of sulfur improves machinability and reduces machining costs. Consequently, non-quenched and tempered steel has gained widespread application in industries such as automotive, construction machinery, and fasteners.

[0004] For example, existing technology discloses a non-quenched and tempered steel for automotive steering racks and its manufacturing method. The composition design concept involves vanadium microalloying, along with the addition of reinforcing elements such as carbon, silicon, manganese, and chromium to improve the steel's strength and plasticity. The steel is produced using an LF+RH refining process. By setting reasonable initial rolling temperature, final rolling temperature, and controlling the cooling rate of the cooling bed, the resulting non-quenched and tempered steel bar product achieves a yield strength ≥580MPa, tensile strength ≥820MPa, elongation ≥16%, reduction of area ≥40%, room temperature U-shaped impact energy ≥45J, and Brinell hardness 240-280HBW. The mechanical properties of the non-quenched and tempered steel bar are superior to or comparable to those of quenched and tempered steel No. 45. It can replace quenched and tempered steel No. 45 in the steering rack production process, eliminating the need for the quenching and tempering process. However, its production process using LF+RH refining methods results in high production costs, hindering product promotion. Furthermore, it discloses a non-quenched and tempered steel bar for automotive steering racks, not wire rod. Bar products require straightening during rack production, involve larger machining depths, or result in significant waste at the beginning and end of the drawing process, leading to lower yield and production efficiency compared to wire rod products. Additionally, the disclosed bar does not require drawing and can be used directly, resulting in high alloy content and high alloy costs. Moreover, the disclosed non-quenched and tempered steel for automotive steering racks contains sulfur as an unavoidable impurity element, with a sulfur content ≤0.02%. There is no intention to add sulfur to improve machinability, thus failing to improve the steel's machinability and hindering cost reduction during rack machining. Summary of the Invention

[0005] Therefore, the technical problem to be solved by the present invention is to overcome the above-mentioned defects of non-quenched and tempered steel in the prior art, such as low yield, low production efficiency and poor machinability, so as to provide a non-quenched and tempered steel wire rod, its preparation method and application.

[0006] Therefore, the present invention provides the following technical solution:

[0007] This invention provides a method for preparing non-quenched and tempered steel wire rod, comprising the following steps: converter steelmaking, LF refining, large billet continuous casting, billet opening, large coil rolling, and cooling process, wherein the cooling process adopts a stack cooling method; the binary basicity of the refining slag is controlled to be 2.8-3.2, and the Al2O3 content in the refining slag is 20-25%;

[0008] The non-quenched and tempered steel wire rod comprises, by mass percentage: C 0.43–0.46%, Si 0.10–0.30%, Mn 1.10–1.50%, P ≤0.020%, S 0.030–0.060%, Cr 0.1–0.25%, Ni 0.05–0.20%, V 0.04–0.07%, Al 0.015–0.045%, N 0.0030–0.0070%, with the remainder being Fe and unavoidable impurities.

[0009] The design incorporates a CaO-SiO2-Al2O3 ternary refining slag system with a binary basicity (CaO / SiO2 weight ratio) of 2.8–3.2 and an Al2O3 content of 20–25%. This results in a low-basicity, low-melting-point refining slag, which facilitates rapid slag formation during the refining process, maintains good fluidity of the refining slag, enhances its deoxidation and inclusion adsorption capabilities, and reduces sulfur content loss during refining.

[0010] Optionally, in the converter steelmaking process, when the weight of the steel tapped from the converter is 30-35%, silicon-manganese alloy, aluminum blocks, lime, and low-basicity pre-melted slag with a binary basicity of 0.8-1.1 are added in sequence. The amount of aluminum blocks added is 1.0-2.0 kg / t, the amount of lime added is 3-3.5 kg / t, and the amount of low-basicity pre-melted slag with a binary basicity of 0.8-1.1 added is 2.5-3.0 kg / t. After the steel is tapped from the converter, it is allowed to stand for 5-8 minutes, and the bottom blowing argon pressure is controlled at 6-8 MPa.

[0011] And / or, the sulfur content of the molten iron fed into the converter is 0.020-0.060%, and the temperature of the molten iron is ≥1300℃.

[0012] Optionally, in the LF refining process, the amount of ferrous sulfate added to the molten steel upon arrival at the station is 2.2-2.8 kg / t; before argon gas soft stirring, a silicon-calcium wire is fed in, with the amount of silicon-calcium wire added being 0.55-0.75 m / t; and the argon gas soft stirring time is 15-25 min.

[0013] Optionally, in the large billet continuous casting process, the superheat of molten steel is 35-45℃, the casting speed is 0.60-0.65m / min, the secondary cooling water ratio is 0.18-0.20L per kilogram of molten steel, the continuous casting billet size is 300mm×390mm, and the total reduction under light pressure is 14-16mm.

[0014] Optionally, in the billet-opening process, the billet-opening heating temperature is 1220-1250℃, the heating time is 260-300min, and the final rolling speed is 0.75-0.80m / s.

[0015] Optionally, in the large coil rolling process, the heating temperature is 1080-1120℃, the initial rolling temperature is 970-1000℃, the finishing rolling inlet temperature is 870-900℃, and the wire drawing temperature is 870-900℃.

[0016] Optionally, the cooling process involves using a wire rod stacking cooling method, turning on fans 1-6 at 95-100% capacity, and allowing the wire rod temperature to drop below 600°C before it enters the insulation hood.

[0017] Optionally, the wire rod temperature drops to 550-600℃ before entering the insulation cover.

[0018] The present invention also provides a non-quenched and tempered steel wire rod prepared by the above-described preparation method.

[0019] Optionally, the non-quenched and tempered steel wire rod has a tensile strength of 750-850 MPa and a reduction of area of ​​30-45%.

[0020] The present invention also provides an application of the above-mentioned non-quenched and tempered steel wire rod in the fields of automobiles, construction machinery or fasteners;

[0021] Optionally, in the application of steering gear racks, the non-quenched and tempered steel wire rods obtained by the present invention have a wire rod yield of ≥92% during the processing of steering gear racks.

[0022] The following explains the role and rationale for limiting each element in non-quenched and tempered steel wire rod:

[0023] C is the most basic and cheapest strengthening element in steel, but as the carbon content increases, the plasticity of steel deteriorates. Therefore, the carbon content in this invention is limited to 0.43 to 0.46%.

[0024] Silicon (Si) is a strengthening and deoxidizing element in steel, but excessive silicon can reduce the plasticity and drawing properties of steel. In this invention, the silicon content ranges from 0.10% to 0.30%.

[0025] Mn is a strengthening element in steel, but it reacts with sulfur to form manganese sulfide, which weakens its strengthening effect. To ensure the strength of the steel, the manganese content in this invention is controlled at 1.10–1.50%.

[0026] Phosphorus (P) is an impurity element in steel. Its segregation at grain boundaries causes grain boundary embrittlement, thereby reducing the strength and plasticity of the steel. This invention controls the P content to below 0.020%.

[0027] Sulfur (S) can improve the machinability of steel, but its segregation at grain boundaries can cause grain boundary embrittlement, thereby reducing the strength and plasticity of the steel, ultimately leading to poorer plasticity. This invention controls the S content to be between 0.030% and 0.060%.

[0028] Cr is a strengthening element in steel, which can improve the hardenability of steel. In this invention, the chromium content is controlled at 0.1% to 0.25%.

[0029] Ni is a non-carbide-forming element that can exist in ferrite or austenite in a mutually soluble form with Fe, thereby strengthening the steel through solid solution and improving its plasticity by refining the ferrite grains. However, since nickel is an expensive alloying element, resulting in high alloy costs, the nickel content in this invention is controlled at 0.05–0.20%.

[0030] Vanadium forms carbides or carbonitrides in steel, which can precipitate on ferrite and play a role in precipitation strengthening. In addition, carbides or carbonitrides pin the grains and refine the grain size. Therefore, adding a small amount of vanadium can improve the strength and plasticity of steel. However, too much vanadium will form coarse precipitates, which will reduce the plasticity and strength of steel. In this invention, the vanadium content is controlled at 0.04 to 0.07%.

[0031] Al is a deoxidizing element in steel, effectively removing oxygen and improving its cleanliness. Al combines with nitrogen to form fine AlN particles, refining the grain size. In this invention, the aluminum content is controlled between 0.015% and 0.045%.

[0032] Nitrogen (N) combines with elements such as vanadium and aluminum in steel to form nitrides, refining the grain size and increasing the steel's strength. However, excessive nitrogen can lead to decreased plasticity in the steel. In this invention, the nitrogen content is controlled between 0.0030% and 0.0070%.

[0033] The technical solution of this invention has the following advantages:

[0034] The method for preparing non-quenched and tempered steel wire rod provided by this invention improves the strength of the steel by designing reasonable carbon, silicon, manganese, and chromium contents; simultaneously, the addition of nickel and vanadium for microalloying further enhances the strength and plasticity of the steel through solid solution strengthening and grain refinement strengthening; and the intentional addition of sulfur improves the machinability of the wire rod. The use of a single LF refining method combined with a reasonable post-rolling cooling regime results in excellent mechanical properties of the wire rod. Precise control of the binary basicity of the LF refining slag, the alumina content in the refining slag, and a reasonable calcium treatment process achieves precise control of sulfur content, preserving sulfur and aluminum, adsorbing inclusions, and improving the castability of molten steel. This eliminates the need for the RH refining step, simplifying the process. Furthermore, since bar products require straightening and have a large machining depth during rack production, or due to the large amount of scrap at the beginning and end of the bar drawing process, wire rod improves the yield and production efficiency compared to existing bar technologies.

[0035] The non-quenched and tempered steel wire rod provided by this invention has better mechanical properties than quenched and tempered steel No. 45 after drawing. In practical applications, especially in the production of steering gear racks, this wire rod can omit the quenching and tempering process, reduce the environmental pollution caused by heat treatment, and at the same time, the material can obtain higher strength. It can replace the commonly used quenched and tempered steel No. 45 wire rod and meet increasingly stringent environmental protection requirements. Attached Figure Description

[0036] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0037] Figure 1 This is a metallographic diagram of the non-quenched and tempered steel wire rod provided in Embodiment 1 of the present invention.

[0038] Figure 2 This is a diagram of machining chips from steel after drawing non-adjustable wire rod, provided in Embodiment 3 of the present invention.

[0039] Figure 3 This is a diagram of machining chips from the steel after wire rod drawing, provided in Comparative Example 3 of this invention. Detailed Implementation

[0040] The following embodiments are provided to better understand the present invention and are not limited to the preferred embodiments described. They do not constitute a limitation on the content and scope of protection of the present invention. Any product that is the same as or similar to the present invention, derived by any person under the guidance of the present invention or by combining the features of the present invention with other prior art, falls within the protection scope of the present invention.

[0041] Where specific experimental steps or conditions are not specified in the examples, they can be performed according to the conventional experimental steps or conditions described in the literature in this field. Reagents or instruments whose manufacturers are not specified are all commercially available conventional reagent products.

[0042] Example 1

[0043] This embodiment provides a method for preparing non-quenched and tempered steel wire rod, employing a converter + LF + large billet continuous casting + billet preparation + large coil rolling + cooling production process. The composition control of the wire rod is shown in Table 1, and the specific process parameters for each step are controlled as follows:

[0044] The sulfur content of the molten iron fed into the converter is 0.030%, and the temperature of the molten iron is 1320℃.

[0045] When the converter reaches 33% tapping, silicon-manganese alloy, aluminum blocks, lime, and low-basicity pre-melted slag (binary basicity of 1.0, CaO and SiO2 content of 96%, with unavoidable impurities of Al2O3 and MgO) are added in sequence. The amount of aluminum blocks added is 1.5 kg / t, the amount of lime added is 3.4 kg / t, and the amount of low-basicity pre-melted slag added is 2.7 kg / t. After tapping, the mixture is allowed to stand for 5 minutes, and the bottom-blown argon pressure is controlled at 6 MPa.

[0046] LF refining: 2.5 kg / t of ferrous sulfate is added to the molten steel upon arrival at the refining station. The binary basicity of the refining slag is controlled at 3.1, and the Al2O3 content in the refining slag is 23%. No ferrous sulfate or aluminum ingots are added during the later stages of refining. Before argon gas soft stirring, a silicon-calcium wire is fed in at a rate of 0.65 m / t. The argon gas soft stirring time is 18 min.

[0047] In the large billet continuous casting process, the molten steel superheat is 44℃, the casting speed is 0.62m / min, the secondary cooling water ratio is 0.19L per kilogram of molten steel, the continuous casting billet size is 300×390mm, and the total reduction under light pressure is 15mm.

[0048] The billet heating temperature is 1240℃, the heating time is 295min, and the final rolling speed is 0.80m / s.

[0049] The heating temperature of the large coil heating furnace is 1100℃, the initial rolling temperature is 980℃, the finishing rolling inlet temperature is 880℃, the wire drawing temperature is 870℃, and the rolling specification is 30mm.

[0050] Post-rolling cooling adopts the stack cooling method. Fans 1-6 are turned on at 100% air volume, and the wire rod temperature drops to 595℃ before entering the insulation cover.

[0051] Figure 1 The image shows the metallographic structure of the obtained steel wire rod. As can be seen from the image, the wire rod structure is ferrite + pearlite, without hard phase structures such as bainite and martensite.

[0052] Example 2

[0053] This embodiment provides a method for preparing non-quenched and tempered steel wire rod, employing a converter + LF + large billet continuous casting + billet preparation + large coil rolling + cooling production process. The composition control of the wire rod is shown in Table 1, and the specific process parameters for each step are controlled as follows:

[0054] The sulfur content of the molten iron fed into the converter is 0.035%, and the temperature of the molten iron is 1330℃.

[0055] When the converter reaches 35% of its steel output, silicon-manganese alloy, aluminum blocks, lime, and low-basicity pre-melted slag (binary basicity of 0.9, CaO and SiO2 content of 95%, with unavoidable impurities of Al2O3 and MgO) are added in sequence. The amount of aluminum blocks added is 1.65 kg / t, the amount of lime added is 3.1 kg / t, and the amount of low-basicity pre-melted slag added is 2.9 kg / t. After tapping the steel from the converter, it is allowed to stand for 6 minutes, and the bottom-blown argon pressure is controlled at 7 MPa.

[0056] LF refining: 2.4 kg / t of ferrous sulfate is added to the molten steel upon arrival at the refining station. The binary basicity of the refining slag is controlled at 2.95, and the Al2O3 content in the refining slag is 21%. No additional ferrous sulfate or aluminum ingots are added during the later stages of refining. Before argon gas soft stirring, a silicon-calcium wire is fed in at a rate of 0.60 m / t. The argon gas soft stirring time is 17 min.

[0057] Large billet continuous casting: the superheat of molten steel is 40℃, the casting speed is 0.63m / min, the secondary cooling water ratio is 0.20L per kilogram of molten steel, the continuous casting billet size is 300×390mm, and the total reduction under light pressure is 15mm.

[0058] The billet heating temperature is 1235℃, the heating time is 290min, and the final rolling speed is 0.80m / s.

[0059] The heating temperature of the large coil heating furnace is 1110℃, the initial rolling temperature is 990℃, the finishing rolling inlet temperature is 870℃, the wire drawing temperature is 870℃, and the rolling specification is 28mm.

[0060] Post-rolling cooling adopts the stack cooling method. Fans 1-6 are turned on at 100% air volume, and the wire rod temperature drops to 590℃ before entering the insulation cover.

[0061] Example 3

[0062] This embodiment provides a method for preparing non-quenched and tempered steel wire rod, employing a converter + LF + large billet continuous casting + billet preparation + large coil rolling + cooling production process. The composition control of the wire rod is shown in Table 1, and the specific process parameters for each step are controlled as follows:

[0063] The sulfur content of the molten iron fed into the converter was 0.032%, and the temperature of the molten iron was 1310℃.

[0064] When the converter reaches 31% of its steel output, silicon-manganese alloy, aluminum blocks, lime, and low-basicity pre-melted slag (binary basicity of 0.8, CaO and SiO2 content of 95%, with unavoidable impurities such as Al2O3 and MgO) are added in sequence. The amount of aluminum blocks added is 1.95 kg / t, the amount of lime added is 3.0 kg / t, and the amount of low-basicity pre-melted slag added is 2.6 kg / t. After tapping the steel from the converter, it is allowed to stand for 6 minutes, and the bottom-blown argon pressure is controlled at 7 MPa.

[0065] LF refining: 2.6 kg / t of ferrous sulfate is added to the molten steel upon arrival at the refining station. The binary basicity of the refining slag is controlled at 3.05, and the Al2O3 content of the refining slag is 25%. No additional ferrous sulfate or aluminum ingots are added during the later stages of refining. Before argon gas soft stirring, a silicon-calcium wire is fed in at a rate of 0.72 m / t. The argon gas soft stirring time is 20 min.

[0066] Large billet continuous casting: the superheat of molten steel is 42℃, the casting speed is 0.65m / min, the secondary cooling water ratio is 0.18L per kilogram of molten steel, the continuous casting billet size is 300×390mm, and the total reduction under light pressure is 15mm.

[0067] The billet heating temperature is 1240℃, the heating time is 295min, and the final rolling speed is 0.80m / s.

[0068] The heating temperature of the large coil heating furnace is 1090℃, the initial rolling temperature is 970℃, the finishing rolling inlet temperature is 890℃, the wire drawing temperature is 870℃, and the rolling specification is 24mm.

[0069] Post-rolling cooling adopts the stack cooling method. Fans 1-6 are turned on at 100% air volume, and the wire rod temperature drops to 585℃ before entering the insulation cover.

[0070] Comparative Example 1

[0071] This comparative example provides a method for preparing steel wire rod, employing a converter + LF + large billet continuous casting + billet preparation + large coil rolling + cooling production process. The composition control of the wire rod is shown in Table 1, and the specific process parameters for each step are controlled as follows:

[0072] The sulfur content of the molten iron entering the converter is 0.015%, and the temperature of the molten iron is 1280℃. When the steel reaches 33% of its output from the converter, silicon-manganese alloy, aluminum blocks, and lime are added in sequence, with the amount of aluminum blocks added being 2.5 kg / t and the amount of lime added being 4.7 kg / t.

[0073] LF refining: Since sulfur in molten steel is an impurity element, there is no need to intentionally add sulfur or ensure a certain content of sulfur. The refining slag can control a high binary basicity to achieve deoxidation and desulfurization. The binary basicity is 4.3. Before argon soft stirring, pure calcium wire is fed in at a rate of 1.3 m / t. The argon soft stirring time is 18 min.

[0074] Large billet continuous casting: the superheat of molten steel is 30℃, the casting speed is 0.68m / min, the secondary cooling water ratio is 0.20L per kilogram of molten steel, the continuous casting billet size is 300×390mm, and the total reduction under light pressure is 13mm.

[0075] The parameters for billet preparation, rolling, and cooling are controlled in the same way as in Example 1.

[0076] Comparative Example 2

[0077] This comparative example provides a method for preparing steel wire rod, employing a converter + LF + large billet continuous casting + billet preparation + large coil rolling + cooling production process. The composition control of the wire rod is shown in Table 1, and the specific process parameters for each step are controlled as follows:

[0078] The sulfur content of the molten iron fed into the converter is 0.018%, and the temperature of the molten iron is 1280℃. When the converter has 35% of its steel output, silicon-manganese alloy, aluminum blocks, and lime are added in sequence. The amount of aluminum blocks added is 2.6 kg / t, the amount of lime added is 4.8 kg / t, and the amount of fluorite added is 0.3 kg / t.

[0079] LF refining: Since sulfur in molten steel is an impurity element, there is no need to intentionally add sulfur or ensure a certain content of sulfur. The refining slag can control a higher binary basicity to achieve deoxidation and desulfurization. The binary basicity of the refining slag is 4.7. Before argon soft stirring, pure calcium wire is fed in at a rate of 1.5 m / t. The argon soft stirring time is 15 min.

[0080] Large billet continuous casting: the superheat of molten steel is 30℃, the casting speed is 0.65m / min, the secondary cooling water ratio is 0.19L per kilogram of molten steel, the continuous casting billet size is 300×390mm, and the total reduction under light pressure is 14mm.

[0081] The control of billet preparation, rolling, and cooling parameters is the same as in Example 2.

[0082] Comparative Example 3

[0083] This comparative example provides a method for preparing steel wire rod, employing a converter + LF + large billet continuous casting + billet preparation + large coil rolling + cooling production process. The composition control of the wire rod is shown in Table 1, and the specific process parameters for each step are controlled as follows:

[0084] The sulfur content of the molten iron fed into the converter is 0.017%, and the temperature of the molten iron is 1290℃. When the weight of the steel tapped from the converter is 31%, silicon manganese alloy, aluminum blocks, and lime are added in sequence, with the amount of aluminum blocks added being 2.8 kg / t and the amount of lime added being 4.3 kg / t.

[0085] LF refining: Since sulfur in molten steel is an impurity element, there is no need to intentionally add sulfur or ensure a certain content of sulfur. The refining slag can control a higher binary basicity to achieve deoxidation and desulfurization. The binary basicity of the refining slag is 4.3. Before argon soft stirring, pure calcium wire is fed in at a rate of 1.6 m / t. The argon soft stirring time is 17 min.

[0086] Large billet continuous casting: the superheat of molten steel is 30℃, the casting speed is 0.66m / min, the secondary cooling water ratio is 0.21L per kilogram of molten steel, the continuous casting billet size is 300×390mm, and the total reduction under light pressure is 15mm.

[0087] The control of billet preparation, rolling, and cooling parameters is the same as in Example 3.

[0088] Comparative Example 4

[0089] This comparative example provides a method for preparing steel wire rod, employing a converter + LF + large billet continuous casting + billet preparation + large coil rolling + cooling production process. The composition of the wire rod is controlled as follows: C 0.38%, Si 0.42%, Mn 1.30%, P 0.012%, S 0.008%, Cr 0.23%, Ni 0.04%, Mo 0.01%, Cu 0.04%, V 0.07%, Al 0.008%, B 0.0002%, N 0.0095%, with the remainder being Fe and unavoidable impurities. The specific process parameters for each step are controlled as in Comparative Example 1.

[0090] Comparative Example 5

[0091] This comparative example provides a method for preparing steel bars. Using the billet obtained in Example 1 after billet preparation, the steel bars are produced on a bar rolling line. The furnace heating temperature is 1100℃, the initial rolling temperature is 980℃, the finishing mill inlet temperature is 880℃, the upper cooling bed temperature is 870℃, and the rolling diameter is 30mm. The cooling bed insulation cover is lowered, and the cooling rate is controlled at 1.6℃ / s. After cooling to 400℃, the steel bars are removed from the line for further cooling.

[0092] Table 1

[0093]

[0094]

[0095] The mechanical properties of the wire rods obtained in Comparative Examples 1-5 and Examples 1-3 are shown in Table 2. The tensile strength of the wire rods obtained in Examples 1-2 is 795-805 MPa, and the reduction of area is 35-38%. The tensile strength of the 45 steel wire rods obtained in Comparative Examples 1-2 is 635-650 MPa, and the reduction of area is 32-38%. The mechanical properties of the non-quenched and tempered steel wire rods obtained in Examples 1-2 are significantly better than those of the 45 steel wire rods. Compared with Example 3, Comparative Example 3 did not intentionally add sulfur content, and the mechanical properties of the hot-rolled wire rods obtained were basically the same. The wire rods obtained in Comparative Example 4 have a high strength of 920 MPa, but poor plasticity and a reduction of area of ​​28%. Due to the high alloy content in Comparative Example 4 and the use of post-rolling air cooling process, the cooling intensity is higher than that of the bar cooling process in Comparative Example 5, resulting in high wire rod strength and poor plasticity. Obviously, this does not meet the requirements of the present invention for a wire rod tensile strength of 750-850 MPa and a reduction of area of ​​30-45%. The high strength of the wire rod obtained in Comparative Example 4 increases the difficulty of drawing it, and the drawn rod has even higher strength but lower plasticity, resulting in it not meeting the mechanical performance requirements of the gear rack. The bar obtained in Comparative Example 5 has lower strength (742 MPa), better plasticity, and a high reduction of area (41%), but its strength is lower than that of the quenched and tempered 45# steel, failing to meet the strength requirements of the 45# steel gear rack. The bar obtained in Comparative Example 5 requires quenching and tempering or drawing to improve its strength. Using quenching and tempering is not feasible as it cannot eliminate the quenching and tempering process, making it undesirable. Using drawing to improve strength will further reduce the utilization rate of steel and increase production costs.

[0096] Table 2

[0097] Wire rod specifications / mm Tensile strength / MPa Reduction of area / % Example 1 30 795 35 Comparative Example 1 30 635 38 Example 2 28 805 38 Comparative Example 2 28 650 32 Example 3 24 820 39 Comparative Example 3 24 825 37 Comparative Example 4 30 920 28 Comparative Example 5 30 742 41

[0098] Examples 1-3 were drawn to 26mm, 24mm, and 20mm respectively, and the mechanical properties of the drawn wires were measured. Comparative Examples 1-3 were drawn to 26mm, 24mm, and 20mm respectively, and the drawn wires obtained from Comparative Examples 1-2 were then subjected to quenching and tempering treatment. The mechanical properties of the 45 steel obtained from Comparative Examples 1-2 after quenching and tempering treatment were then tested. Comparative Example 4 was drawn to 26mm, and the mechanical properties of the drawn wire were measured. The bar obtained from Comparative Example 5 was subjected to quenching and tempering treatment and then drawn to 26mm, and the mechanical properties were measured. The results are shown in Table 3. The quenching and tempering parameters for the comparative examples were: quenching temperature 880±30℃, holding time 60±5min; tempering temperature 500±30℃, tempering time 90±5min.

[0099] After drawing, the wire rods obtained in Examples 1-2 have a tensile strength of 880-900 MPa, a reduction of area of ​​30-33%, and a yield of 95-97%. After quenching and tempering, the wire rods obtained in Comparative Examples 1-2 have a tensile strength of 765-770 MPa, a reduction of area of ​​32-34%, and a yield of 95-98%. After drawing, the wire rods obtained in Example 3 have a tensile strength of 935 MPa, a reduction of area of ​​35%, and a yield of 98%. During machining, the iron filings that fall off are mainly short iron filings (such as...). Figure 2 As shown in the figure, this indicates good machinability; after drawing the wire rod obtained in Comparative Example 3, the tensile strength of the drawn steel wire is 938 MPa, the reduction of area is 33%, the yield is 98%, and the steel wire is machined. The iron chips that fall off during machining are mainly long iron chips (such as...). Figure 3 As shown in the figure, this indicates poor cutting performance; in the machining of steel wire in Examples 1-2, the iron chips that fall off during machining are mainly short iron chips, which are similar to... Figure 2 Similar to the machining of steel wires in proportions 1-2 and 4, the iron filings detached during machining were mainly long iron filings, and... Figure 3 The details are similar and will not be shown individually. After drawing, the wire rod obtained in Comparative Example 4 has a tensile strength of 1042 MPa and a reduction of area of ​​18%, exhibiting poor plasticity and failing to meet the mechanical requirements of a gear rack (gear rack mechanical requirements: tensile strength ≥ 750 MPa, high-end vehicle gear rack tensile strength ≥ 830 MPa; reduction of area ≥ 25%). The wire rod obtained in Comparative Example 4 can be used directly for gear rack production without drawing, but its yield is basically the same as that of the bar stock, significantly lower than that of this invention. The bar stock obtained in Comparative Example 5, after heat treatment, has a tensile strength of 890 MPa and a reduction of area of ​​30%, meeting the mechanical performance requirements of a gear rack, but its yield is only 86%. Without heat treatment, the bar stock obtained in Comparative Example 5, through drawing to improve strength, has a tensile strength of 850 MPa and a reduction of area of ​​31%, meeting the mechanical requirements of a gear rack, but its yield further decreases to 83%.

[0100] Therefore, the non-quenched and tempered steel wire rod disclosed in this invention exhibits excellent mechanical properties. The strength and machinability of the drawn wire are better than those of quenched and tempered 45# steel or previously disclosed technologies. The reduction of area is comparable to that of quenched and tempered 45# steel, and it meets the mechanical performance requirements of gear racks. Compared to quenched and tempered 45# steel, the wire rod obtained by this invention, while maintaining comparable plasticity (reduction of area), significantly improves the tensile strength of the gear rack, further enhancing its wear resistance and service life, meeting the requirements of high-end vehicle models. Compared to bar products, the yield is significantly improved. Using the wire rod obtained by this invention to produce steering gear racks eliminates the need for a quenching and tempering process, while achieving high strength, high plasticity, and excellent mechanical properties, along with low production costs.

[0101] Table 3

[0102]

[0103]

[0104] Obviously, the above embodiments are merely illustrative examples for clear explanation and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations here. However, obvious variations or modifications derived therefrom are still within the scope of protection of this invention.

Claims

1. A method for preparing non-quenched and tempered steel wire rod, characterized in that, In order, they include: The steelmaking process includes converter steelmaking, LF refining, continuous casting of large billets, billet preparation, large coil rolling, and cooling. The cooling process employs a stack cooling method. The binary basicity of the refining slag is controlled at 2.8–3.2, and the Al₂O₃ content in the refining slag is 20–25%. The non-quenched and tempered steel wire rod comprises, by mass percentage: C 0.43~0.46%, Si 0.10~0.30%, Mn 1.10~1.50%, P≤0.020%, S 0.030~0.060%, Cr 0.1~0.25%, Ni 0.05~0.20%, V 0.04~0.07%, Al 0.015~0.045%, N 0.0030~0.0070%, with the remainder being Fe and unavoidable impurities.

2. The method for preparing non-quenched and tempered steel wire rod according to claim 1, characterized in that, In the converter steelmaking process, when the weight of the steel tapped from the converter is 30-35%, silicon-manganese alloy, aluminum blocks, lime, and low-basicity pre-melted slag with a binary basicity of 0.8-1.1 are added in sequence. The amount of aluminum blocks added is 1.0-2.0 kg / t, the amount of lime added is 3-3.5 kg / t, and the amount of low-basicity pre-melted slag with a binary basicity of 0.8-1.1 added is 2.5-3.0 kg / t. After the steel is tapped from the converter, it is allowed to stand for 5-8 minutes, and the bottom blowing argon pressure is controlled at 6-8 MPa. And / or, the sulfur content of the molten iron fed into the converter is 0.020~0.060%, and the temperature of the molten iron is ≥1300℃.

3. The method for preparing non-quenched and tempered steel wire rod according to claim 1, characterized in that, In the LF refining process, the amount of ferrous sulfate added to the molten steel upon arrival at the station is 2.2-2.8 kg / t; before argon gas soft stirring, a silicon-calcium wire is fed in, with an addition amount of 0.55-0.75 m / t; the argon gas soft stirring time is 15-25 min.

4. The method for preparing non-quenched and tempered steel wire rod according to claim 1, characterized in that, In the large billet continuous casting process, the superheat of molten steel is 35~45℃, the casting speed is 0.60~0.65m / min, the secondary cooling water ratio is 0.18~0.20L per kilogram of molten steel, the continuous casting billet size is 300mm×390mm, and the total reduction under light pressure is 14-16mm.

5. The method for preparing non-quenched and tempered steel wire rod according to claim 1, characterized in that, In the billet opening process, the billet heating temperature is 1220~1250℃, the heating time is 260~300min, and the final rolling speed is 0.75-0.80m / s.

6. The method for preparing non-quenched and tempered steel wire rod according to claim 1, characterized in that, In the large coil rolling process, the heating temperature is 1080-1120℃, the initial rolling temperature is 970-1000℃, the finishing rolling inlet temperature is 870-900℃, and the wire drawing temperature is 870-900℃.

7. The method for preparing non-quenched and tempered steel wire rod according to any one of claims 1-6, characterized in that, The cooling process involves using a wire rod stacking cooling method, turning on fans 1-6 at 95-100% capacity, and allowing the wire rod temperature to drop below 600℃ before it enters the insulation hood.

8. The method for preparing non-quenched and tempered steel wire rod according to claim 7, characterized in that, The coil cooling process involves reducing the coil temperature to 550-600℃ before introducing it into an insulation hood.

9. A non-quenched and tempered steel wire rod prepared by the preparation method according to any one of claims 1-8.

10. The non-quenched and tempered steel wire rod according to claim 9, characterized in that, The non-quenched and tempered steel wire rod has a tensile strength of 750~850MPa and a reduction of area of ​​30~45%.

11. The application of the non-quenched and tempered steel wire rod as described in claim 9 or 10 in the fields of automobiles, construction machinery or fasteners.

12. The application of the non-quenched and tempered steel wire rod according to claim 11 in the automotive, construction machinery, or fastener fields, characterized in that, In the steering gear rack processing, the yield rate of the non-quenched and tempered steel wire rod is ≥92%.

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