Medium carbon steel hot-rolled wire rod and manufacturing method thereof

Abstract

The invention discloses a medium carbon steel hot-rolled wire rod, which comprises the following chemical elements in percentage by mass: 0.25 to 0.60 percent of C, 0.1 to 0.5 percent of Si, 0.5 to 0.9 percent of Mn, 0.02 to 0.06 percent of Al, 0.015 to 0.05 percent of Ti and the balance of Fe. And the balance of Fe and other inevitable impurities. Correspondingly, the invention also provides a manufacturing method of the medium carbon steel hot-rolled wire rod, which comprises the following steps: (1) smelting and pouring; (2) rolling a high-temperature wire rod to obtain a wire rod; (3) controlled cooling: firstly, rapidly cooling to 700-750 DEG C, and controlling the rapid cooling speed to be 5-10 DEG C / s; and then continuously and slowly cooling to 500-550 DEG C, and controlling the slow cooling speed to be less than 1.0 DEG C / s. By the adoption of the method, a uniform metallographic structure can be obtained, and mixed crystals are reduced.

Description

Technical Field

[0001] The invention relates to a hot-rolled wire rod and a manufacturing method thereof, in particular to a medium carbon steel wire rod and a manufacturing method thereof. Background Art

[0002] Medium carbon steel hot rolled wire rod is often used to make structural parts, fasteners, and special-shaped parts with certain strength requirements. With the development of the market, downstream enterprises of steel products often directly cold-deform the hot rolled wire rod to reduce production costs when using medium carbon steel wire rod to produce parts. However, due to the limitations of the organization and mechanical properties of medium carbon steel wire rod, the direct cold deformation process of hot rolled wire rod is prone to cracking due to poor plasticity, which has adverse effects on both processing enterprises and steel mills.

[0003] Research shows that the poor direct cold working performance of medium carbon steel is not only related to the mechanical properties of hot rolled wire rods, but also the abnormal pearlite agglomerates that often exist in medium carbon steel hot rolled wire rods are also important factors causing plastic cracking or fracture. This type of abnormal pearlite agglomerate structure is manifested in the microstructure as increased pearlite content, disordered lamellae, and coarse grains. In severe cases, Widmanstätten structures can be found in the pearlite agglomerates. Based on this, it is expected to reduce or even eliminate this type of abnormal pearlite structure in hot rolled wire rods.

[0004] For example, the Chinese patent document with publication number CN102784799A, publication date November 21, 2012, and titled "A hot rolling method for reducing the coarse grain structure of low carbon steel wire rod", believes that the structure of low carbon steel square billet has a certain heredity, and believes that the different recrystallization mechanisms of the surface rigid-plastic interface and rigid deformation zone during the billet rolling process cause the coarse grain of the wire rod. However, the patent document involves hot-rolled wire rods of low carbon steel, not medium carbon steel, and the structure is controlled by adjusting the hole type and roll gap.

[0005] For another example, the Chinese patent document with publication number CN114622071A and publication date June 14, 2022, entitled "A medium carbon steel with uniform grain structure and its production process" involves a medium carbon steel hot-rolled wire rod, which adopts a two-stage heating method with a heating time controlled at 150-200 minutes. Its improvement mechanism is high-temperature heating of the steel billet to homogenize the structure, large rolling reduction, and post-rolling temperature control. Summary of the invention

[0006] One of the purposes of the present invention is to provide a medium carbon steel hot rolled wire rod, which aims to solve the problem of abnormal growth of pearlite structure of medium carbon steel, obtain uniform metallographic structure and reduce the generation of mixed crystals through specific component design and manufacturing process.

[0007] In order to achieve the above object, the present invention provides a medium carbon steel hot-rolled wire rod, the mass percentage of each chemical element of which is:

[0008] C: 0.25-0.60%, Si: 0.1-0.5%, Mn: 0.5-0.9%, Al: 0.02-0.06%, Ti: 0.015-0.05%; the balance is Fe and other inevitable impurities.

[0009] Furthermore, in the medium carbon steel hot-rolled wire rod of the present invention, the mass percentage of each chemical element further satisfies at least one of the following items:

[0010] C: 0.3~0.5%,

[0011] Mn: 0.6~0.8%,

[0012] Al: 0.025~0.05%,

[0013] Ti: 0.03~0.05%.

[0014] In the medium carbon steel hot-rolled wire rod of the present invention, the design principles of each chemical element are specifically described as follows:

[0015] Carbon: The C element is an essential component of medium carbon steel. As the carbon content increases, the yield strength and tensile strength of the steel increase, but the plasticity decreases. In order to ensure the toughness of the material of the present invention and meet the cold deformation requirements, the carbon content of the present invention is controlled to be 0.25-0.60%, and can be further controlled to be 0.30-0.50%.

[0016] Silicon: Si is the residue of deoxidizer during steel smelting, which can ensure a lower oxygen content in the steel. However, too high Si content will increase the brittleness of the steel and is detrimental to the plasticity of the steel, especially the cold plastic deformation of the steel. Therefore, the present invention controls the silicon content to 0.1-0.5%.

[0017] Manganese: Mn can improve the hardenability of steel, so a certain Mn content can make the steel have a better match between strength and toughness, but too high a manganese content will lead to poor thermoplasticity of the steel, so the present invention controls the manganese content to 0.50-0.90%, and can further be controlled to 0.60-0.80%.

[0018] Aluminum: Al is mainly used for deoxidation and grain refinement. Al oxide formed at high temperature can refine the solidified structure of molten steel, and the formed oxide particles can prevent the growth of austenite during high-temperature heating. However, if the Al content is too high, large harmful inclusions are easily formed, and smelting and casting are also difficult. Therefore, the Al content of the present invention is controlled at 0.02-0.06%, and can be further controlled to 0.025-0.050%.

[0019] Titanium: Ti oxides, nitrides and carbides form particles at relatively high temperatures, which are beneficial to the equiaxed crystallization of the solidified structure of molten steel. The particles can also inhibit the growth of austenite grains in the subsequent hot working process. However, too high a Ti content will affect the morphology of inclusions in the steel and further affect the impact plasticity and fatigue resistance of the steel, and also affect the pouring of molten steel. Therefore, the present invention controls the Ti content to be 0.015-0.05%, and can further be controlled to a Ti content of 0.03-0.05%.

[0020] Furthermore, among the inevitable impurities of the medium carbon steel hot-rolled wire rod described in the present invention, P≤0.02%, S≤0.015%, N≤0.006%, and O≤0.002%.

[0021] The inevitable impurities in the present invention mainly include P, S, N and O. Generally, it is desired that the content of these elements in steel is as low as possible.

[0022] Specifically, phosphorus has a great influence on the cold deformation performance of steel and is likely to cause crystal structure segregation during the casting process. Therefore, phosphorus is controlled to be ≤ 0.02% in the steel of the present invention.

[0023] Sulfur will deteriorate the hot working properties of steel and also have a negative effect on the corrosion resistance of the material, so in the steel of the present invention, S is controlled to be ≤ 0.015%.

[0024] The atomic radius of nitrogen is much smaller than that of metal elements, and even smaller than that of carbon. The N element is dissolved in steel as interstitial atoms, which is not conducive to cold deformation and easily causes aging brittleness of the material. Therefore, the present invention controls N≤0.006%. The present invention adds a certain amount of Ti to form TiO and TiN particles at high temperature, which can refine the solidification structure of molten steel and improve the uniformity of material structure on the one hand, and reduce the harm of N on the other hand.

[0025] Oxygen will deteriorate the desulfurization effect of molten steel, increase the hot brittleness tendency of steel, and will also combine with easily oxidizable alloy elements to form inclusions. Therefore, the present invention controls O≤0.0020%.

[0026] Furthermore, in the medium carbon steel hot-rolled wire rod described in the present invention, the average grain size is above grade 9.

[0027] Furthermore, in the medium carbon steel hot-rolled wire rod described in the present invention, the grain size difference is below 2 levels.

[0028] Furthermore, the medium carbon steel hot-rolled wire rod described in the present invention has a tensile strength of ≥540 MPa, an elongation of ≥25%, and a cross-sectional shrinkage of ≥45%.

[0029] Another object of the present invention is to provide a method for manufacturing medium carbon steel hot-rolled wire rod, which addresses the problem of abnormal growth of pearlite structure of medium carbon steel. Based on the component system of the present invention, by controlling the manufacturing process parameters, especially the cooling process, a uniform metallographic structure can be obtained and the generation of mixed crystals can be reduced.

[0030] Based on the above invention object, the present invention provides a method for manufacturing a medium carbon steel hot-rolled wire rod, which comprises the steps of:

[0031] (1) Smelting and casting;

[0032] (2) High temperature wire rolling to obtain wire rod;

[0033] (3) Controlled cooling: First, rapidly cool to 700-750°C, and control the rapid cooling rate to 5-10°C / s; then continue to slowly cool to 500-550°C, and control the slow cooling rate to less than 1.0°C / s.

[0034] In the manufacturing method described in the present invention, in the cooling step, the hot-rolled wire rod is first rapidly cooled to 700-750°C at a cooling rate of 5-10°C / s, and then a slow cooling method is performed, so that the wire rod temperature can quickly pass through the austenite recovery growth temperature range to avoid abnormal growth of pearlite in large-grained austenite. After rapid cooling, slow cooling is performed at a specific temperature to obtain a hot-rolled wire rod with good plasticity. This cooling process can make the temperature of the medium carbon steel after rolling quickly pass through the austenite recovery growth area, avoid uneven growth of grains during the austenite recovery process, and reduce uneven precipitation of carbides.

[0035] Furthermore, in step (2) of the manufacturing method described in the present invention, the heating temperature is controlled to be 1000-1050° C., and the heating time is 90-120 min.

[0036] In this embodiment, the above-mentioned heating temperature and heating time can further reduce the local abnormal growth of austenite that may occur in the medium carbon steel ingot or billet during the heating process.

[0037] Furthermore, in step (2) of the manufacturing method of the present invention, the finishing temperature is controlled to be 860-910°C, the sizing temperature is controlled to be 830-880°C, and the spinning temperature is controlled to be 830-860°C.

[0038] In this embodiment, the above temperature control parameters are used to further reduce the progress and speed of austenite recovery recrystallization.

[0039] Furthermore, the sizing temperature is controlled to be 830-850°C.

[0040] Furthermore, in step (2) of the manufacturing method of the present invention, the total surface reduction rate of the sizing mill is controlled to be 10-25%.

[0041] Furthermore, the total surface reduction rate of the sizing mill is controlled to be 15-25%.

[0042] In the manufacturing method of the present invention, the size of the steel billet or steel ingot obtained in step (1) after blanking can be 120×120 to 150×150 mm.

[0043] The medium carbon steel hot-rolled wire rod and the manufacturing method thereof of the present invention have the following beneficial effects:

[0044] The medium carbon steel hot-rolled wire rod of the present invention has uniform metallographic structure and reduces the generation of mixed crystals.

[0045] The average grain size of the medium carbon steel hot-rolled wire rod described in the present invention is above level 9, and the grain size difference is below level 2.

[0046] The present invention adopts reasonable chemical composition design and reduces the damage of interstitial atomic element N to the plasticity of steel materials in a Ti alloying manner. TiO and TiN formed in the smelting and pouring process refine and uniform the crystal structure of the ingot / bill, and TiO and TiN particles hinder the growth of grains in the subsequent ingot / bill heating process and austenite recovery and recrystallization process.

[0047] By adopting the controlled cooling process of the present invention, i.e., first performing rapid cooling and then performing slow cooling at a specific temperature, the wire rod temperature can quickly pass through the austenite recovery growth temperature range, avoiding abnormal growth of pearlite in large-grained austenite, so as to obtain hot-rolled wire rod with good plasticity. DETAILED DESCRIPTION

[0048] The medium carbon steel hot-rolled wire rod and the method for manufacturing the same described in the present invention will be further explained and illustrated below in conjunction with specific embodiments. However, such explanation and illustration do not constitute an improper limitation on the technical solution of the present invention.

[0049] Examples 1-6 of the present invention and comparative examples 1-3 are all prepared by the following steps:

[0050] (1) Molten steel is smelted, alloyed, and poured to obtain steel ingots or steel billets. The chemical composition contents of the steel ingots or steel billets of various embodiments and comparative examples are shown in Table 1.

[0051] Specifically, the smelting process first adopts an electric arc furnace or a converter for rough refining, and the rough molten steel is then refined in a ladle refining furnace. The basicity of the refining furnace slag is controlled at 5-9. After Si-Fe and Al are fully deoxidized, titanium iron is added to the molten steel for Ti alloying. The yellow-white slag is kept for more than 20 minutes. Thereafter, the molten steel that has been fully calmed, for example, for 20 minutes, is poured into an ingot or a billet in an oxygen-free protective atmosphere.

[0052] In some more preferred embodiments, VD or RH vacuum refining and degassing treatment is performed after refining, which effectively promotes the deoxidation of molten steel and can also effectively reduce the H and N contents in the molten steel, further reduce harmful impurities in the molten steel, and improve the purity of the molten steel.

[0053] (2) High-temperature wire rolling to obtain wire rod: The 140×140mm continuous casting billet or the billet after the ingot is opened is rolled into a wire rod on the high-speed wire. The heating temperature is controlled to be 1000-1050℃, the heating time is 90-120min, the finishing temperature is 860-910℃, the sizing temperature is 830-880℃, the total reduction rate of the sizing unit is 10-25%, and the high-speed wire drawing temperature is 830-860℃.

[0054] (3) Controlled cooling: First, rapidly cool to 700-750°C, and control the rapid cooling rate to 5-10°C / s; then continue to slowly cool to 500-550°C, and control the slow cooling rate to less than 1.0°C / s.

[0055] In some embodiments, rapid cooling may be performed by air cooling.

[0056] It should be noted that the chemical element composition and related process design of the medium carbon steel hot-rolled wire rods of Examples 1-6 all meet the design specification requirements of the present invention, while the chemical element composition and related process design of the hot-rolled wire rods of Comparative Examples 1-3 do not meet the design specification requirements of the present invention.

[0057] Table 1 lists the mass percentages of various chemical elements in the medium carbon steel hot rolled wire rods of Examples 1-6 and the hot rolled wire rods of Comparative Examples 1-3.

[0058] Table 1. (wt%, the balance is Fe and other inevitable impurities except P, S, N, O)

[0059]

[0060]

[0061] Table 2 lists the specific process parameters of the medium carbon steel hot rolled wire rods of Examples 1-6 and the hot rolled wire rods of Comparative Examples 1-3 during high temperature wire rolling and controlled cooling.

[0062] Table 2.

[0063]

[0064] Note: The rapid cooling speed and the temperature after rapid cooling of Comparative Examples 1-3 in Table 2 are all marked with “—”, indicating that the comparative examples do not have a rapid cooling step.

[0065] In this field, if the relative grain size difference in the metallographic detection field is greater than or equal to 3 levels, it is considered that the material has mixed crystal structure. Samples were made for each embodiment and comparative example, and the cross section of the sample was taken for metallographic detection. The samples with abnormal structure were evaluated according to the method of GB / T 24177 "Dual Grain Size Characterization and Determination Method" and GB / T 6394 "Method for Determination of Average Grain Size of Metals". The samples without significant abnormal structure were evaluated according to the method of GB / T 6394 "Method for Determination of Average Grain Size of Metals". The number of samples in each group was 5, and the test results were the average of 5 samples, which are listed in Table 3.

[0066] Table 3.

[0067] serial number result Example 1 Horizontal, average 10 levels Example 2 Horizontal, average 10 levels Example 3 Horizontal, average level 9 Example 4 Horizontal, average 10 levels Example 5 Horizontal, average level 11 Example 6 Horizontal, average level 11 Comparative Example 1 Horizontal, double, ALA, average 9 levels, maximum 3 levels Comparative Example 2 Horizontal, double, ALA, average 9 levels, maximum 4 levels Comparative Example 3 Horizontal, average level 9, maximum level 7

[0068] Note: Example 1 of the result description "transverse, double, ALA, average 9 levels, maximum 3 levels" means "the cross section of the test specimen, the ALA state of double grain size, the average grain size is 9 levels, and the grain size of abnormally large grains is 3 levels".

[0069] From the test results of the embodiments and comparative examples in Table 3, it can be seen that in the embodiments of the present invention, since Ti is added in the composition design, TiN and TiO formed in the molten steel and the solidification process prevent the growth of casting grains, and it is easy to obtain a uniform equiaxed crystal solidification structure. At the same time, the billet is heated for a short time and at a relatively low temperature, the rolling process temperature is relatively low, the final rolling deformation is relatively large (i.e., the area reduction rate), and the hot-rolled wire rod is immediately cooled by air after spinning, etc., to prevent the abnormal growth of austenite. No abnormal pearlite agglomerates are found in the hot-rolled wire rod, the metallographic structure is relatively uniform, and the grain size is basically above level 9.

[0070] The comparative samples have poor uniformity, especially comparative sample 1, which has the most uneven structure.

[0071] Accordingly, after completing the above observations and analyses, the obtained medium carbon steel hot-rolled wire rods of Examples 1-6 and Comparative Examples 1-3 can be further sampled again, and the wire rod samples of each embodiment and comparative example can be subjected to relevant mechanical property tests. The obtained mechanical property test results are listed in Table 4.

[0072] The relevant mechanical properties test methods are as follows:

[0073] Tensile test: at room temperature, according to GBT 228.1-2010 "Tensile test of metallic materials Part 1: Room temperature test method" standard, the yield strength, tensile strength and area reduction ratio of the medium carbon steel hot-rolled wire rod of each embodiment and comparative example are detected.

[0074] Table 4 lists the mechanical property test results of the hot-rolled wire rods of various embodiments and comparative examples.

[0075] Table 4.

[0076]

[0077] It can be seen from Table 4 that the tensile strength of each embodiment of the present invention is ≥540MPa, the elongation is ≥25%, and the cross-sectional shrinkage is ≥45%. In addition, the cross-sectional shrinkage of the plasticity index is higher than that of the comparative example with similar carbon content by more than 3%.

[0078] It should be noted that the combination of the various technical features in this case is not limited to the combination described in the claims of this case or the combination described in the specific embodiments. All technical features recorded in this case can be freely combined or combined in any way unless there is a contradiction between them.

[0079] It should also be noted that the above-listed embodiments are only specific embodiments of the present invention. Obviously, the present invention is not limited to the above embodiments, and similar changes or modifications made therewith can be directly derived or easily associated with the contents disclosed by those skilled in the art from the present invention, and all should belong to the protection scope of the present invention.

Claims

1. A medium carbon steel hot-rolled wire rod, characterized in that: The mass percentage of each chemical element is: C: 0.25~0.60%, Si: 0.1~0.5%, Mn: 0.5~0.9%, Al: 0.02~0.06%, Ti: 0.015-0.05%; the balance is Fe and other inevitable impurities.

2. The medium carbon steel hot rolled wire rod according to claim 1, characterized in that: The mass percentage of each chemical element further satisfies at least one of the following items: C：0.3～0.5％， Mn: 0.6~0.8%, Al：0.025～0.05％， Ti: 0.03~0.05%.

3. The medium carbon steel hot rolled wire rod according to claim 1, characterized in that: Among the inevitable impurities, P ≤ 0.02%, S ≤ 0.015%, N ≤ 0.006%, and O ≤ 0.002%.

4. The medium carbon steel hot rolled wire rod according to claim 1, characterized in that: Its average grain size is above grade 6.

5. The medium carbon steel hot rolled wire rod according to claim 1, characterized in that: The grain size difference is below level 2.

6. The medium carbon steel hot rolled wire rod according to claim 1, characterized in that: Its tensile strength is ≥540MPa, elongation is ≥25%, and section shrinkage is ≥45%.

7. The method for manufacturing medium carbon steel hot rolled wire rod according to any one of claims 1 to 6, characterized in that: It includes the steps of: (1) Smelting and casting; (2) High temperature wire rolling to obtain wire rod; (3) Controlled cooling: First, rapidly cool to 700-750°C, and control the rapid cooling rate to 5-10°C / s; then continue to slowly cool to 500-550°C, and control the slow cooling rate to less than 1.0°C / s.

8. The manufacturing method according to claim 7, characterized in that: In step (2), the heating temperature is controlled to be 1000-1050° C. and the heating time is controlled to be 90-120 min.

9. The manufacturing method according to claim 7, characterized in that: In step (2), the finishing temperature is controlled to be 860-910°C, the sizing temperature is controlled to be 830-880°C, and the spinning temperature is controlled to be 830-860°C.

10. The manufacturing method according to claim 7, characterized in that: In step (2), the total surface reduction rate of the sizing mill is controlled to be 10-25%.

11. The manufacturing method according to claim 9, characterized in that: In step (2), the sizing temperature is controlled to be 830-850°C.

12. The manufacturing method according to claim 10, characterized in that: In step (2), the total surface reduction rate of the sizing mill is controlled to be 15-25%.

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