Low-carbon cold forging steel for low-strength and high-plasticity flat wires and production method of low-carbon cold forging steel

Abstract

The invention belongs to the technical field of iron and steel smelting, and relates to low-carbon cold forging steel for low-strength high-plasticity flat wires and a production method of the low-carbon cold forging steel. The low-carbon cold forging steel comprises the following chemical components in percentage by mass: less than or equal to 0.05% of C, less than or equal to 0.05% of Si, 0.10-0.30% of Mn, less than or equal to 0.010% of P, less than or equal to 0.010% of S, more than or equal to 0.04% of Al, less than or equal to 15 ppm of total oxygen, less than or equal to 35 ppm of nitrogen, less than or equal to 0.05% of Cu, less than or equal to 0.03% of Ni and the balance of Fe and impurities. The production method comprises the following steps: S1, smelting and continuous casting; s2, rolling process; through the synergistic effect of component optimization and controlled rolling and controlled cooling processes, the comprehensive performance that the tensile strength is smaller than or equal to 340 MPa, the ductility is larger than or equal to 45%, the percentage reduction of area is larger than or equal to 78%, and the grain size is larger than or equal to 9 levels is achieved, and the problems of bending cracking and surface roughness of traditional low-carbon cold heading steel for cold-drawn flat wires are solved.

Description

Technical Field

[0001] The present invention belongs to the technical field of steel smelting, and relates to a low-carbon cold-heading steel for low-strength and high-plasticity flat wires and a production method thereof. Background Art

[0002] Although the existing low-carbon cold-heading steels have good plasticity, they have problems such as coarse grains and high total oxygen content resulting in excessive inclusions, and it is difficult to meet the requirements of high-end cold-drawn flat wire materials for low strength and ultra-high plasticity. Most of the traditional steels for flat wires use medium-carbon steel or low-carbon steel, but there are the following problems:

[0003] 1. A high carbon content (C≥0.06%) is likely to cause the risk of cracking after cold drawing and bending, and high alloying significantly increases the cost.

[0004] 2. The tensile strength is too high (350 MPa), resulting in serious work hardening during subsequent cold drawing of flat wires and easy cracking during bending and forming after drawing;

[0005] 3. Coarse grains (grain size ≤ 8 grades) affect the material uniformity and surface quality;

[0006] 4. The ordinary rolling process has inaccurate temperature control, resulting in poor tissue uniformity.

[0007] Patent CN115161545A provides a high-plasticity and low-strength medium-carbon cold-heading steel fine wire and a production method thereof. The components and weight percentages contained in the medium-carbon cold-heading steel fine wire are as follows: C: 0.30% - 0.40%, Si: 0.15% - 0.35%, Mn: 0.60% - 1.20%, P ≤ 0.015%, S ≤ 0.010%, Ni ≤ 0.20%, Cu ≤ 0.20%, Mo ≤ 0.10%, N ≤ 0.0060%, O ≤ 0.0015%, H ≤ 0.0002%, and also contains one or several of Cr: 0.25% - 0.45%, Al: 0.015% - 0.050%, Ti: 0.02% - 0.05%, B: 0.0008 - 0.0035%, V: 0.02% - 0.05%, Nb: 0.01% - 0.04%, and the rest is Fe and impurities; The production method includes the following steps: S1, hot-rolling wire rods; S2, pickling and phosphating; S3, rough drawing. During the rough drawing process, the rough drawing area reduction rate is controlled at 30% - 40%; S3, spheroidizing annealing. The spheroidizing annealing process adopts isothermal spheroidizing annealing: first heat up to 600 - 650°C and hold for 1 - 1.5 h, then heat up to 740 - 750°C at a rate not higher than 100°C / h and hold for 5 - 7 h, then cool down to 690 - 710°C at a rate not higher than 25°C / h and hold for 4 - 6 h, and finally cool down to below 550°C at a rate not higher than 25°C and air-cool; S4, pickling and phosphating; S5, finish drawing. During the finish drawing process, the finish drawing area reduction rate is controlled at 3% - 6%. Although low strength and high plasticity can be achieved, it is suitable for medium-carbon cold-heading steel and must go through the spheroidizing annealing step. Summary of the Invention

[0008] To solve the above technical problems, the present invention provides a cold-heading steel with low strength and high plasticity and a preparation method thereof by optimizing the composition design and rolling process, significantly improving the processing performance. The tensile strength ≤ 340 MPa, elongation ≥ 45%, reduction of area ≥ 78%, grain size ≥ grade 9, and no cracks in 180° bending, which is suitable for flat wire products with high requirements for grain refinement and forming performance.

[0009] The technical solution adopted by the present invention is as follows:

[0010] In the first aspect, the present invention provides a production method of low-strength and high-plasticity flat wire made of low-carbon cold-heading steel. The chemical composition and mass percentage of the low-carbon cold-heading steel are as follows: C ≤ 0.05%, Si ≤ 0.05%, Mn: 0.10% - 0.30%, P ≤ 0.010%, S ≤ 0.010%, Al ≥ 0.04%, total oxygen ≤ 15 ppm, nitrogen ≤ 35 ppm, Cu ≤ 0.05%, Ni ≤ 0.03%, and the balance is Fe and impurities;

[0011] The production method includes the following steps:

[0012] S1: Smelting and continuous casting;

[0013] S2: Rolling process;

[0014] In the rolling process, the rough rolling starting temperature is 900 - 1000 °C, and the cumulative reduction ratio is ≥ 65%;

[0015] The finish rolling temperature of finish rolling is 800 - 850 °C, and the wire laying temperature is 750 - 850 °C;

[0016] Cooling is carried out by first quickly cooling to 690 °C and then slowly cooling;

[0017] The Stelmor air cooling line: 3 - 8 fans are turned on, the air volume is 30% - 80%, and the coiling temperature is ≤ 600 °C.

[0018] Furthermore, the described S1: Smelting and continuous casting is specifically as follows: Converter smelting is adopted, and the end point C ≤ 0.04%, P ≤ 0.015% are controlled; The total time of molten steel in LF is ≥ 55 min; The tundish with an integral nozzle is used for continuous casting, and a low-carbon steel protective slag is adopted, and the cooling rate of the casting blank is ≤ 10 °C / s.

[0019] Preferably, the cooling rate of quick cooling is 7 - 10 °C / s, and the cooling rate of slow cooling is ≤ 3 °C / s.

[0020] Furthermore, the heating temperature of the continuous casting billet is 1100 - 1200 °C, and the heating time is ≥ 2 hours.

[0021] In the second aspect, the present invention also provides a low-strength and high-plasticity flat wire low-carbon cold heading steel, and the chemical composition and mass percentage of the low-carbon cold heading steel are: C ≤ 0.05%, Si ≤ 0.05%, Mn: 0.10% - 0.30%, P ≤ 0.010%, S ≤ 0.010%, Al ≥ 0.04%, total oxygen ≤ 15 ppm, nitrogen ≤ 35 ppm, Cu ≤ 0.05%, Ni ≤ 0.03%, and the balance is Fe and impurities.

[0022] Furthermore, the tensile strength of the low-carbon cold heading steel is ≤ 340 MPa, the elongation is ≥ 45%, the reduction of area is ≥ 70%, the grain size is ≥ 9 grades, and there are no cracks in the 180° bending.

[0023] In the third aspect, the present invention provides an application of the low-strength and high-plasticity flat wire low-carbon cold heading steel in automotive seat belt buckles, precision gaskets, and steel for kitchenware pull-out baskets.

[0024] The beneficial effects of the present invention are:

[0025] (1) Through the synergistic control of ultra-low carbon (C ≤ 0.05%) and high-aluminum (Al ≥ 0.04%) killed steel design and deformation-induced phase transformation, the traditional process limits are broken through in terms of both composition design and controlled rolling and controlled cooling: micro-alloying with manganese (0.10 - 0.30%) and hot rolling start at high temperature (900 - 1000°C) promote dynamic recrystallization and avoid mixed grain structure.

[0026] (2) Low-temperature wire laying (750 - 850°C) combined with controlled cooling inhibits grain growth and promotes the formation of uniform ferrite; fine grain strengthening and high cleanliness (total oxygen ≤ 15 ppm, nitrogen ≤ 35 ppm) are used to reduce cold drawing cracks.

[0027] (3) The present invention realizes the synergy of "contradictory properties" of fine grains (above grade 9) and low strength (≤ 340 MPa), providing an ideal base material for high-precision cold-drawn flat wire. Description of the Drawings

[0028] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for use in the description of the embodiments or the prior art. Obviously, the drawings in the following description are only some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can be obtained based on these drawings.

[0029] Figure 1 It is the metallographic structure diagram of Example 1;

[0030] Figure 2 It is the diagram of no cracking in flat bending of Example 1;

[0031] Figure 3 It is the diagram of no cracking in side bending of Example 1;

[0032] Figure 4 It is the metallographic structure diagram of Comparative Example 1;

[0033] Figure 5 It is the diagram of cracking in flat bending of Comparative Example 1;

[0034] Figure 6 It is the diagram of cracking in side bending of Comparative Example 1. Detailed Embodiments

[0035] The following will clearly and completely describe the technical solutions in the embodiments of the present invention with reference to the drawings in the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, rather than all embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts belong to the scope of protection of the present invention.

[0036] The present invention provides a production method for low-carbon cold-heading steel for low-strength and high-plasticity flat wires. The chemical composition and mass percentage of the low-carbon cold-heading steel are as follows: C ≤ 0.05%, Si ≤ 0.05%, Mn: 0.10% - 0.30%, P ≤ 0.010%, S ≤ 0.010%, Al ≥ 0.04%, total oxygen ≤ 15 ppm, nitrogen ≤ 35 ppm, Cu ≤ 0.05%, Ni ≤ 0.03%, and the balance is Fe and impurities;

[0037] The specific functions of the above elements in the present invention are as follows:

[0038] Carbon is the main alloying element of cold-heading steel and has a significant impact on the strength and hardness of steel. As the carbon content increases, the hardness and strength of the steel increase, but the plasticity and toughness decrease. In the present invention, C ≤ 0.05% is selected to control the carbon content to reduce strength and improve plasticity.

[0039] Silicon is a common alloying element in cold-heading steel, which can improve the strength and hardness of steel, improve the heat treatment effect, and enhance the wear resistance and corrosion resistance of steel. In the present invention, Si ≤ 0.05% is selected to reduce oxide inclusions and improve surface quality.

[0040] Manganese can improve the strength and hardness of steel and improve cold working and heat treatment properties. In the present invention, Mn: 0.10 - 0.30% is selected to balance solid solution strengthening and toughness.

[0041] Phosphorus is an impurity element in cold-heading steel, which increases the cold brittleness of steel. In the present invention, P ≤ 0.010% is selected to reduce cold brittleness.

[0042] Sulfur is a harmful impurity element, which can cause hot brittleness of steel, reduce fatigue strength and dynamic load strength. In the present invention, S ≤ 0.010% is selected to reduce the influence of sulfides on plasticity.

[0043] The main function of aluminum in cold-heading steel is to refine grains and improve plasticity. However, too high aluminum content may generate alumina inclusions, which will instead have a negative impact on performance. In the present invention, Al ≥ 0.04% is selected to refine grains and promote deoxidation.

[0044] The production method includes the following steps:

[0045] S1: Smelting and continuous casting

[0046] Converter smelting is adopted, and the end-point C ≤ 0.04% and P ≤ 0.015% are controlled at the end-point.

[0047] The total time of the molten steel in LF is ≥ 55 min.

[0048] Continuous casting adopts an integral nozzle tundish, uses a low-carbon steel protective slag, and the cooling rate of the casting billet is ≤ 10 °C / s to avoid composition segregation.

[0049] S2: Rolling Process

[0050] Heating Stage: The continuous casting billet is heated to 1100 - 1200 °C, and the total heating time is ≥ 2 hours;

[0051] Rough Rolling: The starting rolling temperature is 900 - 1000 °C, and the cumulative reduction ratio is ≥ 65%;

[0052] Finish Rolling: The final rolling temperature is 800 - 850 °C, and the laying temperature is 750 - 850 °C (key control point);

[0053] Cooling: First, quickly cool to 690 °C and then slowly cool. The rapid cooling rate is 7 - 10 °C / s, and the slow cooling rate is ≤ 3 °C / s;

[0054] Stelmor Air Cooling Line: Turn on 3 - 8 fans, the air volume is 30% - 80%, and the coiling temperature is ≤ 600 °C.

[0055] The technical solution of the invention will be further described below in conjunction with embodiments.

[0056] Example 1: Producing 6.5 mm Specification

[0057] The chemical composition and mass percentage of the steel are: C: 0.03%, Si: 0.02%, Mn: 0.13%, P: 0.006%, S: 0.004%, Al: 0.049%, total oxygen: 12 ppm, nitrogen: 31 ppm, Cu: 0.02%, Ni: 0.01%, and the rest is Fe and inevitable impurity elements.

[0058] Key Process Steps:

[0059] (1) Converter end point C: 0.04%, P: 0.01%

[0060] (2) The total time of molten steel in LF is 62 min.

[0061] (3) The tundish of the continuous casting uses an integral nozzle, and low-carbon steel protective slag is used. The cooling rate of the continuous casting billet is 5 °C / s,

[0062] (4) Rolling: Heating temperature 1180 °C, heating time 125 min, starting rolling temperature 980 °C, rough rolling reduction ratio 89%; final rolling temperature 830 °C, laying temperature 790 °C, cooling rate 7 °C / s, turn on 3 fans, air volume 30%, coiling temperature 580 °C.

[0063] The measured performance and grain size are shown in Table 1, and the metallographic structure is as Figure 1 shown, and the presence or absence of cracking in flat bending is as Figure 2 shown, and the presence or absence of cracking in side bending is as Figure 3 shown.

[0064] Example 2: Producing 22 mm Specification

[0065] The chemical composition and mass percentage of the steel are as follows: C: 0.04%, Si: 0.02%, Mn: 0.12%, P: 0.009%, S: 0.005%, Al: 0.052%, total oxygen: 13 ppm, nitrogen: 32 ppm, Cu: 0.02%, Ni: 0.01%, and the rest is Fe and inevitable impurity elements.

[0066] Key process steps:

[0067] (1) End point of converter: C: 0.05%, P: 0.011%

[0068] (2) Total time of molten steel in LF is 60 min.

[0069] (3) For continuous casting, an integral nozzle tundish is used, a low-carbon steel protective slag is adopted, and the cooling rate of the billet is 7 °C / s.

[0070] (4) Rolling: Heating temperature is 1175 °C, heating time is 122 min, rolling start temperature is 970 °C, reduction ratio of rough rolling is 85%; finishing rolling temperature is 820 °C, spinning temperature is 800 °C, cooling rate is 9 °C / s, 4 fans are turned on, air volume is 60%, and coiling temperature is 560 °C.

[0071] The measured properties and grain size are shown in Table 1.

[0072] Comparative example 1: Producing a 6.5 mm specification

[0073] The mass percentage composition of the steel is: C: 0.072%, Si: 0.039%, Mn: 0.28%, P: 0.013%, S: 0.008%, Al: 0.028%, there are no requirements for total oxygen and nitrogen, and the rest is Fe and inevitable impurity elements.

[0074] Key process steps: Rolling: Heating temperature is 1220 °C, heating time is 95 min, rolling start temperature is 1120 °C, spinning temperature is 950 °C, and coiling temperature is 720 °C. Other steps are the same as those in Example 1.

[0075] The measured properties and grain size are shown in Table 1, the metallographic structure is as Figure 4 shown, whether there is cracking in flat bending is as Figure 5 shown, and whether there is cracking in side bending is as Figure 6 shown.

[0076] Comparative example 2: Producing a 22 mm specification

[0077] The mass percentage composition of the steel is as follows: C: 0.074%, Si: 0.037%, Mn: 0.29%, P: 0.010%, S: 0.009%, Al: 0.029%. There are no requirements for total oxygen and nitrogen, and the rest are Fe and inevitable impurity elements.

[0078] Key process steps: Rolling: Heating temperature 1215°C, heating time 92 min, rolling start temperature 1125°C, wire laying temperature 930°C, coiling temperature 710°C. Other steps are the same as those in Example 2.

[0079] The measured properties and grain size are shown in Table 1.

[0080] Table 1: Measured properties and grain size of examples and comparative examples

[0081]

[0082] In summary, it is difficult for the existing process to balance low tensile strength and high plasticity without cracking. Through the synergistic effect of composition optimization (C ≤ 0.05%, Al ≥ 0.04%) and controlled rolling and cooling process (rolling start temperature 900 - 1000°C, wire laying temperature 750 - 850°C), the present invention achieves comprehensive properties with tensile strength ≤ 340 MPa, elongation ≥ 45%, reduction of area ≥ 78%, and grain size ≥ 9 grades, solving the problems of bending cracking and surface roughness of low-carbon cold heading steel for traditional cold-drawn flat wire, and being applicable to the processing of precision cold-drawn flat wire with high requirements for grain refinement and forming performance, such as automotive seat belt buckles, precision gaskets, and kitchenware wire baskets, etc., featuring low cost and high-efficiency forming.

[0083] The above are only the preferred embodiments of the present invention and are not intended to limit the protection scope of the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention are included in the protection scope of the present invention.

Claims

1. A method for producing low-carbon cold heading steel for low-strength and high-plasticity flat wire, characterized in that: The chemical composition and mass percentage of the low carbon cold heading steel are: C≤0.05%, Si≤0.05%, Mn: 0.10%-0.30%, P≤0.010%, S≤0.010%, Al≥0.04%, total oxygen≤15ppm, nitrogen≤35ppm, Cu≤0.05%, Ni≤0.03%, and the balance is Fe and impurities; The production method comprises the following steps: S1: smelting and continuous casting; S2: rolling process; The rough rolling process has a starting temperature of 900-1000°C and a cumulative reduction rate of ≥65%; Finishing rolling temperature is 800-850℃, spinning temperature is 750-850℃; Cooling is done by first cooling quickly to 690℃ and then cooling slowly; Stelmore air cooling line: turn on 3 to 8 fans, air volume 30% to 80%, coiling temperature ≤ 600℃.

2. The method for producing low-carbon cold heading steel for low-strength and high-plasticity flat wire according to claim 1, characterized in that: The S1: smelting and continuous casting are specifically as follows: using converter smelting, controlling the end point C≤0.04%, P≤0.015%; the total time of molten steel in LF≥55min; using an integral nozzle tundish for continuous casting, using low-carbon steel protective slag, and the cooling rate of the ingot is ≤10℃ / s.

3. The method for producing low-carbon cold heading steel for low-strength and high-plasticity flat wire according to claim 1, characterized in that: The fast cooling rate is 7-10℃ / s, and the slow cooling rate is ≤3℃ / s.

4. The method for producing low-carbon cold heading steel for low-strength and high-plasticity flat wire according to claim 2, characterized in that: The continuous casting slab heating temperature is 1100-1200℃, and the heating time is ≥2 hours.

5. A low-carbon cold heading steel for low-strength and high-plasticity flat wire produced by the production method according to any one of claims 1 to 4, characterized in that: The chemical composition and mass percentage of the low carbon cold heading steel are: C≤0.05%, Si≤0.05%, Mn: 0.10%-0.30%, P≤0.010%, S≤0.010%, Al≥0.04%, total oxygen≤15ppm, nitrogen≤35ppm, Cu≤0.05%, Ni≤0.03%, and the balance is Fe and impurities.

6. The low-carbon cold heading steel for low-strength and high-plasticity flat wire according to claim 5, characterized in that: The low-carbon cold heading steel has a tensile strength of ≤340MPa, an elongation of ≥45%, a cross-sectional shrinkage of ≥70%, a grain size of ≥9, and no cracks when bent at 180°.

7. Use of the low-carbon cold heading steel for low-strength and high-plasticity flat wire as claimed in claim 6 in automobile safety belt buckles, precision gaskets, and steel for kitchen pull-out baskets.

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