A medium carbon steel hot-rolled wire rod and its manufacturing method

Abstract

This invention discloses a medium-carbon steel hot-rolled wire rod, the mass percentage of each chemical element being: 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 being Fe and other unavoidable impurities. Accordingly, this invention also provides a method for manufacturing this medium-carbon steel hot-rolled wire rod, comprising the steps of: (1) smelting and casting; (2) high-temperature wire rod rolling to obtain the wire rod; (3) controlled cooling: first, rapid cooling to 700-750℃, controlling the rapid cooling rate at 5-10℃ / s; then continued slow cooling to 500-550℃, controlling the slow cooling rate at less than 1.0℃ / s. Using this invention, a uniform metallographic structure can be obtained, and the generation of mixed crystals can be reduced.

Description

Technical Field

[0001] This invention relates to a hot-rolled wire rod and its manufacturing method, and more particularly to a medium carbon steel wire rod and its manufacturing method. Background Technology

[0002] Medium carbon steel hot-rolled wire rod is commonly used to manufacture structural components, fasteners, and irregularly shaped parts requiring a certain strength. With market development, downstream steel product manufacturers often directly cold-deform the hot-rolled wire rod to reduce production costs when producing parts. However, due to the limitations of the microstructure and mechanical properties of medium carbon steel wire rod, direct cold deformation processing of hot-rolled wire rod is prone to causing cracking due to poor plasticity, negatively impacting both processing companies and steel mills.

[0003] Studies show that the poor direct cold working performance of medium carbon steel is not only related to the mechanical properties of hot-rolled wire rod, but also significantly influenced by abnormal pearlite agglomerates frequently present in hot-rolled medium carbon steel wire rod, which contribute to ductile cracking or fracture. These abnormal pearlite agglomerates are characterized by increased pearlite content, disordered lamellar structure, and coarse grains in the microstructure; in severe cases, Widmanstätten structure can be observed within the pearlite agglomerates. Therefore, the aim is to mitigate or even eliminate these abnormal pearlite structures in hot-rolled wire rod.

[0004] For example, Chinese patent document CN102784799A, published on November 21, 2012, entitled "A Hot Rolling Method for Reducing Coarse-Grained Structure of Low-Carbon Steel Wire Rods," suggests that the microstructure of low-carbon steel billets has a certain degree of heritability, and that the different recrystallization mechanisms at the surface rigid-plastic interface and in the rigid deformation zone during the billet rolling process cause the coarse grains in the wire rods. However, this patent document deals with hot-rolled low-carbon steel wire rods, not medium-carbon steel, and it controls the microstructure by adjusting the roll pass and roll gap.

[0005] For example, Chinese patent document CN114622071A, published on June 14, 2022, entitled "A Medium Carbon Steel with Uniform Grain Structure and Its Production Process," relates to a hot-rolled medium carbon steel wire rod that employs a two-stage heating method with a heating time controlled at 150-200 minutes. Its improvement mechanism involves high-temperature heating of the billet to homogenize the microstructure, large reduction during rolling, and controlled post-rolling temperature. Summary of the Invention

[0006] One of the objectives of this invention is to provide a medium carbon steel hot-rolled wire rod that addresses the problem of abnormal growth of pearlite in medium carbon steel by using specific composition design and manufacturing process to obtain a uniform metallographic structure and reduce the generation of mixed crystals.

[0007] To achieve the above objectives, the present invention provides a medium carbon steel hot-rolled wire rod, wherein the mass percentage content of each chemical element is as follows:

[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%; balance is Fe and other unavoidable impurities.

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

[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] The design principles of each chemical element in the medium carbon steel hot-rolled wire rod of the present invention are as follows:

[0015] Carbon: C 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 strength and toughness of the material of this invention and meet the requirements of cold deformation, the carbon content of this invention is controlled at 0.25-0.60%, and can be further controlled at 0.30-0.50%.

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

[0017] Manganese: Mn can improve the hardenability of steel. Therefore, a certain Mn content can make the steel have a better strength and toughness balance. However, too high a manganese content will lead to poor hot plasticity of the steel. Therefore, the present invention controls the manganese content at 0.50-0.90%, and can further control it at 0.60-0.80%.

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

[0019] Titanium: The oxides, nitrides, and carbides of Ti form particles at higher temperatures, which is beneficial to the equiaxed crystallization of the solidification structure of molten steel. These particles can also inhibit the growth of austenite grains during subsequent hot working. However, excessive Ti content will affect the morphology of inclusions in steel, thereby affecting the impact plasticity and fatigue resistance of steel, and also affecting the casting of molten steel. Therefore, this invention controls the Ti content to be 0.015-0.05%, and can be further controlled to be 0.03-0.05%.

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

[0021] The unavoidable impurities in this invention mainly include P, S, N, and O. Generally, it is desirable for these elements to have a lower content in the steel.

[0022] Specifically, phosphorus has a significant impact on the cold deformation properties of steel, and it can also easily cause segregation of crystalline structures during the casting process. Therefore, the phosphorus content in the steel of this invention is controlled to be ≤0.02%.

[0023] Sulfur can deteriorate the hot working properties of steel and also has a negative effect on the corrosion resistance of materials. Therefore, the sulfur content in the steel of this invention is controlled to be ≤0.015%.

[0024] Nitrogen atoms have a radius much smaller than those of metallic elements, and even smaller than that of carbon. Nitrogen atoms are dissolved in steel as interstitial atoms, which is not conducive to cold deformation and easily causes aging brittleness of the material. Therefore, this invention controls N to be ≤0.006%. This invention adds a certain amount of Ti to form TiO and TiN particles at high temperature. On the one hand, this refines the solidification structure of the molten steel and improves the uniformity of the material structure. On the other hand, it can also reduce the harm of N.

[0025] Oxygen can worsen the desulfurization effect of molten steel, exacerbate the hot brittleness tendency of steel, and combine with easily oxidized alloying elements to form inclusions. Therefore, this invention controls O ≤ 0.0020%.

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

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

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

[0029] Another objective of this 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 in medium carbon steel. Based on the composition system of this 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-mentioned objectives, this invention provides a method for manufacturing medium carbon steel hot-rolled wire rod, comprising the following steps:

[0031] (1) Smelting and casting;

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

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

[0034] In the manufacturing method described in this invention, during 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, followed by slow cooling. This allows the wire rod temperature to quickly pass through the austenite recovery and growth temperature range, preventing abnormal pearlite growth within large-grained austenite. The rapid cooling followed by slow cooling at a specific temperature yields a hot-rolled wire rod with good plasticity. This cooling process allows the temperature of the rolled medium-carbon steel to quickly pass through the austenite recovery and growth region, avoiding uneven grain growth during austenite recovery and reducing uneven carbide precipitation.

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

[0036] In this embodiment, by using the above-mentioned heating temperature and heating time, the local abnormal growth of austenite that may occur in medium carbon steel ingots and billets during the heating process can be further reduced.

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

[0038] In this embodiment, by using the above-mentioned temperature control parameters, the process and speed of austenite recovery recrystallization can be further reduced.

[0039] Furthermore, the sizing temperature is controlled at 830–850℃.

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

[0041] Furthermore, the total reduction rate of the sizing unit is controlled at 15-25%.

[0042] In the manufacturing method described in this invention, the size of the billet or ingot obtained in step (1) after billeting can be 120×120~150×150mm.

[0043] The medium carbon steel hot-rolled wire rod and its manufacturing method described in this invention have the following beneficial effects:

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

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

[0046] This invention reduces the damage to the plasticity of steel materials caused by interstitial element N through Ti alloying by adopting a reasonable chemical composition design. The TiO and TiN formed during the smelting and casting process refine and homogenize the crystal structure of the ingot / billet. Furthermore, the TiO and TiN particles hinder grain growth during the subsequent heating process of the steel ingot / billet and the austenite recovery recrystallization process.

[0047] The controlled cooling process of this invention, which involves rapid cooling followed by slow cooling at a specific temperature, allows the wire rod temperature to quickly pass through the austenite recovery and growth temperature range, preventing abnormal growth of pearlite within large-grained austenite and thus obtaining hot-rolled wire rods with good plasticity. Detailed Implementation

[0048] The following will provide further explanation and description of the medium carbon steel hot-rolled wire rod and its manufacturing method according to the present invention with reference to specific embodiments. However, such explanation and description do not constitute an improper limitation on the technical solution of the present invention.

[0049] Examples 1-6 and Comparative Examples 1-3 of the present invention were all prepared using the following steps:

[0050] (1) The molten steel is smelted, alloyed and cast to obtain steel ingots or billets. The chemical composition of the steel ingots or billets in each embodiment and comparative example is shown in Table 1.

[0051] Specifically, the smelting process first uses an electric arc furnace or converter for roughing, and then the roughed steel is refined in a ladle refining furnace. The basicity of the slag in the refining furnace is controlled at 5-9. After Si-Fe and Al are fully deoxidized, ferrotitanium is added to the molten steel for Ti alloying. The yellow-white slag is kept for more than 20 minutes. Then, under an oxygen-free protective atmosphere, the molten steel that has been fully killed, for example for 20 minutes, is poured into steel ingots or billets.

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

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

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

[0055] In some implementations, rapid cooling can be achieved using 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 in Examples 1-6 all meet the requirements of the design specifications of this invention, while the chemical element composition and related process design of the hot-rolled wire rods in Comparative Examples 1-3 do not meet the requirements of the design specifications of this invention.

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

[0058] Table 1. (wt%, balance Fe and other unavoidable impurities other than P, S, N, and O)

[0059]

[0060]

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

[0062] Table 2.

[0063]

[0064] Note: In Table 2, the rapid cooling rate and the temperature after rapid cooling for Comparative Examples 1-3 are indicated by "—" to show that the comparative examples did not have a rapid cooling step.

[0065] In this field, a relative grain size difference of 3 or more within the field of view in metallographic inspection is considered to indicate the presence of a mixed-grain structure in the material. Samples were prepared for each embodiment and comparative example, and cross-sections of the samples were subjected to metallographic inspection. Samples exhibiting abnormal structures were evaluated according to GB / T 24177 "Dual Grain Size Characterization and Determination Method" and GB / T 6394 "Method for Determination of Average Grain Size of Metals". Samples without significant abnormal structures were evaluated according to GB / T 6394 "Method for Determination of Average Grain Size of Metals". Five samples were tested in each group, and the results were the average of the five samples, 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 11 levels Example 6 Horizontal, average 11 levels 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 Horizontally, average level 9, maximum level 7

[0068] Note: The example description of the results in Comparative Example 1, "Transverse, Dual, ALA, Average 9, Maximum 3", means "The cross-section of the test sample is examined for dual grain size ALA state, with an average grain size of 9 and an abnormally large grain size of 3".

[0069] As can be seen from the test results of the embodiments and comparative examples in Table 3, the embodiments of the present invention, due to the addition of Ti in the composition design, the TiN and TiO formed during the molten steel and solidification process prevent the growth of casting grains, making it easy to obtain a uniform equiaxed solidification structure. At the same time, the billet heating adopts short time and relatively low temperature heating, relatively low rolling process temperature, relatively large final rolling deformation (i.e., reduction in surface area), and immediate air cooling after hot-rolled wire rod is used to hinder the abnormal growth of austenite. No abnormal pearlite agglomerates were found in the hot-rolled wire rod, the metallographic structure is relatively uniform, and the grain size is basically above grade 9.

[0070] The comparative samples showed poor homogeneity, especially Comparative Example 1, which had the least uniform microstructure.

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

[0072] The relevant mechanical property testing methods are as follows:

[0073] Tensile test: Under room temperature conditions, the yield strength, tensile strength and reduction of area of ​​the medium carbon steel hot-rolled wire rods of each embodiment and comparative example were tested according to GBT 228.1-2010 "Metallic materials - Tensile testing - Part 1: Room temperature test method".

[0074] Table 4 lists the mechanical property test results of the hot-rolled wire rods of each embodiment and comparative example.

[0075] Table 4.

[0076]

[0077] As can be seen from Table 4, the tensile strength of each embodiment of the present invention is ≥540MPa, the elongation is ≥25%, and the reduction of area is ≥45%. Moreover, compared with the comparative examples with similar carbon content, the reduction of area in terms of plasticity is more than 3% higher.

[0078] It should be noted that the combination of the technical features in this case is not limited to the combination methods described in the claims of this case or the combination methods described in the specific embodiments. All technical features described in this case can be freely combined or combined in any way, unless they contradict each other.

[0079] It should also be noted that the embodiments listed above are merely specific embodiments of the present invention. Obviously, the present invention is not limited to the above embodiments, and similar changes or modifications made thereto are those that can be directly derived or easily conceived by those skilled in the art from the content disclosed in the present invention, and should all fall within the protection scope of the present invention.

Claims

1. A medium carbon steel hot-rolled wire rod, characterized in that, Its mass percentage content of each chemical element is as follows: 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%; balance Fe and other unavoidable impurities; The medium carbon steel hot-rolled wire rod is produced by the following steps: (1) Smelting and casting; (2) High-temperature wire rod rolling to obtain wire rod; (3) Controlled cooling: First, rapidly cool to 700-750℃, controlling the rapid cooling rate at 5-10℃ / s; then continue to slowly cool to 500-550℃, controlling the slow cooling rate at less than 1.0℃ / s.

2. The medium carbon steel hot-rolled wire rod as described in claim 1, characterized in that, Its mass percentage content of each chemical element further satisfies at least one of the following conditions: 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 as described in claim 1, characterized in that, In unavoidable impurities, P ≤ 0.02%, S ≤ 0.015%, N ≤ 0.006%, and O ≤ 0.002%.

4. The medium carbon steel hot-rolled wire rod as described in claim 1, characterized in that, Its average grain size is above level 6.

5. The medium carbon steel hot-rolled wire rod as described in claim 1, characterized in that, Its grain size difference is below level 2.

6. The medium carbon steel hot-rolled wire rod as described in claim 1, characterized in that, Its tensile strength is ≥540MPa, elongation is ≥25%, and reduction of area is ≥45%.

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

8. The manufacturing method as described in claim 7, characterized in that, In step (2), the heating temperature is controlled at 1000-1050℃ and the heating time is 90-120min.

9. The manufacturing method as described in claim 7, characterized in that, In step (2), the finishing rolling temperature is controlled at 860-910℃, the sizing temperature is controlled at 830-880℃, and the wire drawing temperature is controlled at 830-860℃.

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

11. The manufacturing method as described in claim 9, characterized in that, In step (2), the sizing temperature is controlled to be 830-850℃.

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

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