High-precision medium-carbon steel hot-rolled wire rod for cold-rolled lead screws, preparation method and application

Abstract

The present application relates to a kind of high-precision cold-rolled screw with medium carbon steel hot rolled wire rod in metallurgical technology field, preparation method and application.The present application is aimed at solving the technical problems of low first processing qualified rate of cold-rolled screw caused by poor material organization uniformity and low purity.The chemical composition of the wire rod according to the present application is as follows: C 0.52-0.58%, Si 0.15-0.35%, Mn 0.70-0.90%, Cr 0.10-0.35%, Al 0.015-0.045%, Ti≤0.0025%, [O]≤0.0015%, [N]≤0.0060% and the like, with the balance being Fe.The preparation method of the present application obtains uniform organization with cross-section hardness range ≤20HV through the combined action of narrow component design, continuous casting end electromagnetic stirring, high temperature diffusion annealing and "fast first and slow later" controlled cooling process.The present application is mainly used for manufacturing high-precision ball screw, and the finished product precision reaches more than 5 levels, with first cold-rolled forming qualified rate ≥85%.

Description

Technical Field

[0001] This invention relates to the field of metallurgical technology, and in particular to a high-precision cold-rolled screw made of medium carbon steel hot-rolled wire rod, its preparation method and application. Background Technology

[0002] Lead screws are indispensable core transmission components in high-end equipment such as CNC machine tools, industrial robots, and precision measuring instruments. Their precision, wear resistance, and fatigue life directly determine the overall performance of the equipment. As the manufacturing industry develops towards high precision and sophistication, the requirements for the precision (such as grade 5 and above) and consistency of lead screws are becoming increasingly stringent.

[0003] Currently, the materials used domestically to manufacture high-precision cold-rolled lead screws, such as medium carbon steel equivalent to S55C in the Japanese standard JIS G4051, lag significantly behind imported materials in terms of microstructure uniformity, purity, and cold working performance. This gap leads to uneven deformation of the thread raceway and dimensional deviations during the cold rolling process due to the uneven microstructure of the material (such as banding and center segregation). As a result, the first-pass yield is often less than 60%, heavily relying on rework or scrap, significantly increasing manufacturing costs and severely hindering the localization process of high-end equipment in China.

[0004] To address the aforementioned issues, existing technologies have been researched. For example, Chinese patent application CN202511110302A discloses a method for increasing the sorbite content through rapid cooling after rolling. Its core principle is to provide a more ideal initial microstructure for subsequent spheroidizing annealing, thereby improving the quality of spheroidizing annealing and indirectly optimizing costs. However, this method has not effectively solved the problem of uneven microstructure and hardness in the wire rod cross-section caused by dendrite segregation in continuously cast billets, which is a key factor affecting the precision forming of cold-rolled screws.

[0005] Therefore, developing a high-precision cold-rolled wire rod for lead screws that has excellent uniformity of structure, high purity, excellent cold working performance, and eliminates the need for complex heat treatment processes has significant industrial value and strategic importance. Summary of the Invention

[0006] The technical problem this invention aims to solve is to overcome the shortcomings of existing technologies by providing a high-precision cold-rolled ball screw made of medium carbon steel hot-rolled wire rod with good microstructure uniformity, high purity, and excellent cold working performance, along with its preparation method and application. Screws manufactured using this wire rod exhibit high precision, with a first-pass yield of ≥85%, and finished product precision reaching grade 5 or higher. The key lies in the combined effect of narrow composition design, low superheat during continuous casting, end-stage electromagnetic stirring, and high-temperature diffusion annealing, which fundamentally reduces dendritic segregation in the cast billet. Combined with a subsequent controlled cooling process of "fast first, slow later," the banded structure in the hot-rolled wire rod is effectively suppressed, resulting in a uniform microstructure. The wire rod produced by this invention exhibits a cross-sectional hardness difference ≤20HV, high purity, and uniform microstructure. After spheroidizing annealing, it can be directly cold-rolled with large deformation for manufacturing high-precision ball screws, achieving a substitution for imported materials and demonstrating significant economic benefits.

[0007] The technical solution adopted by this invention to solve its technical problem is: a medium carbon steel hot-rolled wire rod for high-precision cold-rolled lead screws, wherein the chemical composition of the hot-rolled wire rod, by mass percentage, is: C: 0.52-0.58%, Si: 0.15-0.35%, Mn: 0.70-0.90%, P≤0.020%, S≤0.008%, Cr: 0.10-0.35%, Ni≤0.15%, Cu≤0.15%, Mo≤0.05%, Al: 0.015-0.045%, Ti≤0.0025%, [O]≤0.0015% (i.e., 15ppm), [N]≤0.0060%, with the balance being Fe and unavoidable impurity elements. Based on the chemical composition design, the present invention has the following key technical features: (1) High purity: By strictly controlling the content of P, S, O, N and harmful residual elements, non-metallic inclusions, especially brittle inclusions, are reduced, laying the foundation for improving fatigue life. (2) Uniformity of microstructure: Through the reasonable ratio of C, Mn and Cr, hardenability and strength are guaranteed; Al and N form AlN, refining the grains; by strictly controlling Ti, large-particle brittle TiN inclusions are avoided.

[0008] Further specified, the low-magnification structure and inclusions of the hot-rolled wire rod meet the following requirements (i.e., its microstructure meets the following requirements): Low-magnification structure rating: central porosity ≤ 1.0 grade, ingot segregation ≤ 1.0 grade, central segregation ≤ 1.0 grade; Non-metallic inclusions are tested according to GB / T10561 standard, non-metallic inclusion A (sulfides) fine series ≤ 2.0 grade, non-metallic inclusion A coarse series ≤ 1.5 grade; non-metallic inclusion B (alumina) fine series ≤ 1.5 grade, non-metallic inclusion B coarse series ≤ 1.0 grade; non-metallic inclusion C (silicates) fine series ≤ 0.5 grade, non-metallic inclusion C coarse series ≤ 0.5 grade; non-metallic inclusion D (spherical oxides) fine series ≤ 1.5 grade, non-metallic inclusion D coarse series ≤ 1.0 grade; non-metallic inclusion Ds (single spherical oxides) ≤ 1.0 grade. The mechanical properties of the hot-rolled wire rod meet the following requirements: hot-rolled tensile strength ≥700MPa, reduction of area ≥35%, hardness ≤250HBW, and the microhardness range of the wire rod cross section (diameter direction) ≤20HV.

[0009] This invention also provides a method for preparing medium carbon steel hot-rolled wire rod for high-precision cold-rolled screws, comprising the following process flow: molten iron pretreatment → converter or electric furnace smelting → LF ladle refining → RH or VD vacuum degassing treatment → continuous casting → continuous casting billet heating → billet rolling → intermediate billet surface treatment (peeling, grinding) → high-speed wire rod rolling → controlled cooling (Stelmore line) → (optional) spheroidizing annealing → finished wire rod.

[0010] The key process controls are as follows:

[0011] (1) In the LF ladle refining process, ensure precise control of composition and deep desulfurization and deoxidation;

[0012] (2) In the RH or VD vacuum degassing process, the RH or VD treatment time is ≥20 minutes, ensuring that [H]≤2.0ppm and [O]≤15ppm;

[0013] (3) In the continuous casting process, protective casting is adopted throughout the process, the superheat of the tundish is controlled at 10-30℃, and electromagnetic stirring in the crystallizer (M-EMS) and electromagnetic stirring at the solidification end (F-EMS) are adopted to optimize the internal quality of the continuous casting billet and reduce center segregation.

[0014] (4) In the continuous casting billet heating process, the specifications of the continuous casting billet are 220mm×260mm or 300mm×325mm large square billets. The heating temperature of the continuous casting billet is controlled at 1150~1250℃, and the soaking time is ≥150 minutes to ensure that the alloying elements diffuse fully and reduce the original dendrite segregation. The billet is rolled into an intermediate billet (e.g., 130~165mm square billet).

[0015] (5) Surface treatment of intermediate billets includes 100% peeling and / or grinding of intermediate billets to completely remove the surface defect layer and ensure the surface quality of the final wire rod.

[0016] (6) In the high-speed wire rod rolling process, the intermediate billet is reheated at a temperature of 1000-1150℃. The rolling process parameters are strictly controlled: the initial rolling temperature is 950-1050℃, the inlet temperature of the finishing mill is 850-920℃, the outlet (sizing) temperature of the finishing mill is controlled at ≤920℃, and the wire drawing temperature is 800-860℃;

[0017] (7) The cooling process is controlled by using the Stellmore cooling line and adopting a fast-then-slow cooling process: after the wire rod is spun out, the heat preservation cover is immediately closed and the fan is turned on at ≥80% for rapid cooling, so that the wire rod is rapidly cooled to the phase transformation temperature range before the phase transformation, which inhibits the excessive growth of proeutectoid ferrite and the formation of banded structure; then the fan is turned off and the heat preservation cover is turned on for slow cooling; the roller speed is controlled at 20-30 m / min to ensure that the phase transformation process is carried out slowly under heat preservation conditions to obtain a uniform pearlite + ferrite structure.

[0018] Further specifying that if the high-speed wire rod is directly used for cold-rolled lead screws after rolling, it requires subsequent spheroidizing annealing treatment. The process involves heating to 720–760°C, holding at that temperature for 8–15 hours, and then slowly cooling to 650°C at a rate of ≤30°C / h before air cooling. This process fully spheroidizes the lamellar pearlite, reduces hardness, and improves plasticity, meeting the requirements for large deformation cold rolling.

[0019] The present invention also provides an application in which the hot-rolled wire rod is used to manufacture high-precision ball screws, and the finished ball screws produced achieve a precision grade of 5 or above, with a single cold rolling forming pass rate of ≥85%.

[0020] The beneficial effects of this invention are:

[0021] 1. Significantly high uniformity of composition and microstructure: By combining narrow composition design, electromagnetic stirring at the end of continuous casting, and high-temperature diffusion annealing, dendritic segregation in the cast billet is fundamentally reduced. Combined with the subsequent controlled cooling process of "fast first, slow later," banded microstructure in the hot-rolled wire rod is effectively suppressed, ultimately resulting in a microhardness difference of ≤20HV in the wire rod cross-section. This extremely high microstructure uniformity is the key foundation for ensuring precise and synchronous forming of the thread raceway during the cold-rolled screw process.

[0022] 2. Excellent cold working performance, with no need for tempering: Through precise control of rolling and cooling processes, combined with subsequent spheroidizing annealing, the wire rod obtains an ideal spheroidized pearlite structure, with hot-rolled hardness controllable to ≤250HBW and significantly improved elongation. This allows the wire rod to be directly cold-rolled with large deformation, completely eliminating the energy-intensive and time-consuming tempering (quenching + high-temperature tempering) pretreatment process in traditional processes, greatly shortening the production cycle and reducing energy consumption and production costs.

[0023] 3. Significantly Improved Finished Product Precision and Yield: Benefiting from the high purity and uniformity of the materials, cold-rolled ball screws manufactured using the wire rods of this invention consistently achieve a precision grade of 5, with some reaching 3. The first-pass cold-rolling yield is over 85%, far exceeding the approximately 60% level of existing technologies. This not only effectively replaces imported high-end materials (such as S55C) but also significantly reduces scrap rates and rework costs, bringing significant economic benefits to enterprises.

[0024] 4. Excellent comprehensive mechanical properties: While ensuring excellent cold working performance, the material still has sufficient strength and hardness. After the final surface quenching treatment, it can meet the stringent requirements of wear resistance and fatigue resistance of the lead screw under high load. Attached Figure Description

[0025] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments recorded in this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0026] Figure 1 The image shows the microstructure (100×) of the hot-rolled wire rod prepared in Example 1 of this invention, which is a uniform ferrite + pearlite structure without obvious banding.

[0027] Figure 2 The image shows the microstructure (100×) of the hot-rolled wire rod prepared in Example 2 of this invention after spheroidizing annealing, indicating that the pearlite has been fully spheroidized.

[0028] Figure 3 The image (50×) shows the microstructure of the hot-rolled wire rod prepared in Comparative Example 2 after spheroidizing annealing, revealing a distinct banded structure.

[0029] Figure 4 This is a process flow diagram of the method for preparing medium carbon steel hot-rolled wire rod for high-precision cold-rolled screws according to the present invention. Detailed Implementation

[0030] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0031] Example 1: This example aims to produce a Φ21.0mm hot-rolled wire rod that can be used to manufacture high-precision ball screws.

[0032] 1. Chemical Composition Design: By mass percentage, the final product's chemical composition is precisely controlled as follows: C: 0.55%, Si: 0.22%, Mn: 0.80%, P: 0.012%, S: 0.004%, Cr: 0.18%, Ni: 0.03%, Cu: 0.05%, Mo: 0.01%, Al: 0.025%, Ti: 0.0015%, [O]: 0.0012%, [N]: 0.0045%, with the balance being Fe. This chemical composition design fully embodies the technical concepts of narrow composition control and high purity.

[0033] 2. Smelting and Continuous Casting: The process involves hot metal pretreatment → smelting in a 120t top and bottom blowing converter → LF refining (white slag holding time ≥30min to ensure deep desulfurization and deoxidation) → RH vacuum degassing (treatment time 25min, vacuum degree ≤0.5mbar, ensuring [H] ≤2.0ppm and [O] ≤15ppm) → slab continuous casting (section 220mm×260mm, tundish superheat precisely controlled at 25±3℃, using a combination of crystallizer electromagnetic stirring and solidification end electromagnetic stirring) to obtain continuously cast slabs with excellent internal quality.

[0034] 3. Heating and Billet Preparation: The continuously cast billet is heated in a walking beam furnace, with the soaking zone temperature precisely controlled at 1220℃ and held for up to 180 minutes to ensure full diffusion of alloying elements and fundamentally reduce the segregation of original dendrites. After exiting the furnace, it undergoes high-pressure water descaling to thoroughly remove the surface iron oxide scale, and is then sent to a billet mill to be rolled into 160mm×160mm intermediate billets.

[0035] 4. Surface treatment: After the intermediate billet cools, it is shot blasted and inspected for defects. The defective parts are then finely ground to ensure that the surface quality of the intermediate billet reaches the best state, laying the foundation for the high surface quality of the final wire rod.

[0036] 5. Wire Rod Rolling: The properly ground intermediate billet is fed into the high-speed wire rod mill production line. The temperature of the soaking zone in the heating furnace is controlled at 1120℃ and held for 60 minutes. The rolling process parameters are strictly controlled: the initial rolling temperature is 1020℃, the finishing rolling inlet temperature is 880℃, and the wire exit temperature is 840℃ to ensure the stability of the rolling process and the controllability of the microstructure evolution.

[0037] 6. Controlled Cooling: After the wire rod is spun, it enters the Stellmore cooling line, where a precise "fast first, slow later" control strategy is adopted: the insulation covers of sections 1-4 are fully closed (i.e., forced cooling), the fan is open at 100%, and the roller speed is 25m / min, so that the wire rod is rapidly cooled to about 650°C, effectively suppressing the excessive growth of proeutectoid ferrite and the formation of banded structures; then the insulation covers of sections 5-8 are fully opened and the fan is turned off, so that the wire rod is slowly cooled to room temperature under insulation conditions, ensuring that the phase transformation process is uniform and sufficient.

[0038] 7. Performance Testing: The resulting wire rod has a uniform and fine ferrite + pearlite microstructure, without banded structure (see...). Figure 1 The cross-sectional microhardness difference is only 15HV, the tensile strength reaches 760MPa, the reduction of area is 38%, and the hardness is 235HBW. All performance indicators are better than expected.

[0039] Example 2 is basically the same as Example 1, except that a spheroidizing annealing process is added after the wire is rolled in order to further improve the cold working performance.

[0040] 1. Chemical composition: Same as in Example 1 to ensure the accuracy of the comparison.

[0041] 2. Preparation process: The smelting, continuous casting, billet preparation, rolling, and cooling processes are exactly the same as in Example 1. After cooling to room temperature, the wire rod undergoes spheroidizing annealing treatment: the temperature is slowly increased to 740°C at a rate of 40°C / h and held for 12 hours to fully spheroidize the lamellar pearlite; then it is furnace cooled to 650°C at a rate of 20°C / h and then air-cooled to obtain the ideal spheroidized structure.

[0042] 3. Performance Testing: The resulting wire rod microstructure is a uniform mixture of spheroidized pearlite and ferrite (see...). Figure 2 The hardness is further reduced, and the plasticity is significantly improved. This wire rod is more suitable for subsequent cold rolling processes with large deformation, providing the optimal material state for the forming of high-precision lead screws.

[0043] Comparative Example 1: This comparative example uses the same chemical composition and smelting and continuous casting process as Example 1, but conventional processes are used in the wire rolling and cooling stages to illustrate the superiority of the process of the present invention.

[0044] 1. Preparation process:

[0045] Wire rod rolling: initial rolling temperature 1050℃, finishing rolling inlet temperature 900℃, and wire drawing temperature 860℃.

[0046] Controlled cooling: After the wire rod is spun out, all the insulation covers on the Stellmore line are fully opened, and the roller speed is 15m / min, so that the wire rod is slowly cooled at a higher temperature.

[0047] 2. Performance Testing: The obtained wire rod has a coarse ferrite + pearlite microstructure with obvious banded structure. The cross-sectional microhardness range is as high as 45HV, the tensile strength is only 680MPa, the reduction of area is 28%, and the hardness is 220HBW. The overall performance is significantly worse than that of Example 1.

[0048] Comparative Example 2: This comparative example corresponds to Example 2, and uses the same wire rod as Comparative Example 1 for the same spheroidizing annealing treatment to illustrate that annealing alone cannot eliminate the original structural defects.

[0049] 1. Preparation process: The wire rod obtained in Comparative Example 1 was subjected to the same spheroidizing annealing treatment as in Example 2 (see...). Figure 3 ).

[0050] 2. Performance Testing: After annealing, the original banded structure was severe and could not be eliminated by spheroidizing annealing. The microstructure was uneven, with lamellar pearlite still present in some areas. In subsequent cold rolling simulation tests, uneven deformation led to dimensional deviations, failing to meet the forming requirements of high-precision lead screws.

[0051] Table 1 details the performance comparison data between the embodiments and the comparative examples, fully demonstrating the significant effect of the technical solution of the present invention.

[0052] category Band-like tissue (rating) Cross-sectional hardness range (HV) Tensile strength in hot-rolled / annealed state (MPa) Reduction of area (%) Cold rolling forming pass rate (%) Example 1 Level 1.0 15 760 38 70 Example 2 Level 1.0 10 (560 after annealing) 58 92 Comparative Example 1 Level 3.5 45 680 28 30 Comparative Example 2 Level 3.5 25 (610 after annealing) 45 75

[0053] Conclusion: As shown in the table and figures above, this invention effectively eliminates banded structures and achieves excellent microstructure uniformity by optimizing chemical composition and key process parameters, particularly by employing a precise cooling strategy of "fast followed by slow (strong cooling + gentle cooling)". This results in the material of this invention (especially Example 2 after spheroidizing annealing) exhibiting excellent cold deformation properties, meeting the forming requirements of high-precision cold-rolled screws, significantly improving the first-pass yield, and demonstrating significantly better overall performance than the comparative example, thus achieving an effective substitution for imported high-end materials.

[0054] The above are merely preferred embodiments of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A medium carbon steel hot-rolled wire rod for high-precision cold-rolled lead screws, characterized in that, The chemical composition of the hot-rolled wire rod, by mass percentage, is as follows: C: 0.52-0.58%, Si: 0.15-0.35%, Mn: 0.70-0.90%, P≤0.020%, S≤0.008%, Cr: 0.10-0.35%, Ni≤0.15%, Cu≤0.15%, Mo≤0.05%, Al: 0.015-0.045%, Ti≤0.0025%, [O]≤0.0015%, [N]≤0.0060%, with the balance being Fe and unavoidable impurity elements.

2. The high-precision cold-rolled lead screw using medium carbon steel hot-rolled wire rod according to claim 1, characterized in that, The low-magnification microstructure and inclusions of the hot-rolled wire rod satisfy the following: Low-magnification tissue grading: Central looseness ≤ 1.0 grade, spindle segregation ≤ 1.0 grade, central segregation ≤ 1.0 grade; Non-metallic inclusions of Class A fine series ≤ 2.0 grade, non-metallic inclusions of Class A coarse series ≤ 1.5 grade; Non-metallic inclusions of Class B fine series ≤ 1.5 grade, non-metallic inclusions of Class B coarse series ≤ 1.0 grade; Non-metallic inclusions of Class C fine series ≤ 0.5 grade; non-metallic inclusions of Class C coarse series ≤ 0.5 grade; Non-metallic inclusions of Class D fine series ≤ 1.5 grade, non-metallic inclusions of Class D coarse series ≤ 1.0 grade; Non-metallic inclusions, Ds class ≤ 1.

0.

3. The high-precision cold-rolled lead screw using medium carbon steel hot-rolled wire rod according to claim 1 or 2, characterized in that, The mechanical properties of the hot-rolled wire rod meet the following requirements: hot-rolled tensile strength ≥700MPa, reduction of area ≥35%, hardness ≤250HBW, and microhardness range of the wire rod cross section ≤20HV.

4. A method for preparing high-precision cold-rolled lead screws using medium carbon steel hot-rolled wire rods as described in any one of claims 1 to 3, characterized in that, The process includes the following steps: hot metal pretreatment → converter or electric furnace smelting → LF refining → RH or VD vacuum degassing → continuous casting → continuous casting billet heating → billet rolling → intermediate billet surface treatment → high-speed wire rod rolling → controlled cooling. in: (1) In the continuous casting process, the superheat of the tundish is controlled to be 10-30℃, and electromagnetic stirring of the crystallizer and electromagnetic stirring at the end of solidification are adopted. (2) In the continuous casting billet heating process, the heating temperature of the continuous casting billet is controlled at 1150~1250℃, and the soaking time is ≥150 minutes; (3) In the high-speed wire rod rolling process, the initial rolling temperature is 950-1050℃, the inlet temperature of the finishing mill is 850-920℃, and the wire drawing temperature is 800-860℃; (4) The cooling process adopts the Stellmore cooling line and adopts a fast-then-slow cooling process: after the wire rod is spun, the insulation cover is closed first and the fan at ≥80% is turned on for rapid cooling. Then the fan is turned off and the insulation cover is turned on for slow cooling. The speed of the roller conveyor is controlled to be 20-30m / min.

5. The preparation method according to claim 4, characterized in that, In the RH or VD vacuum degassing process, the RH or VD treatment time is ≥20 minutes, ensuring that [H] ≤2.0ppm and [O] ≤15ppm.

6. The preparation method according to claim 4, characterized in that, The intermediate billet surface treatment includes 100% peeling and / or grinding of the intermediate billet.

7. The preparation method according to claim 4, characterized in that, In the high-speed wire rod rolling process, the exit temperature of the finishing mill is controlled at ≤920℃.

8. The preparation method according to claim 4, characterized in that, In the high-speed wire rod rolling process, the intermediate billet is reheated at a temperature of 1000–1150°C.

9. The preparation method according to claim 4, characterized in that, After the high-speed wire is rolled, it also includes a spheroidizing annealing process: heating to 720-760℃, holding for 8-15 hours, and then slowly cooling to 650℃ at a rate of ≤30℃ / h before being taken out of the furnace and air-cooled.

10. The high-precision cold-rolled lead screw made directly from medium carbon steel hot-rolled wire rod according to any one of claims 4 to 9, characterized in that, The hot-rolled wire rod is used to manufacture high-precision ball screws, and the finished ball screws produced have a precision grade of 5 or above, with a one-time cold rolling forming qualification rate of ≥85%.

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