Cold heading wire rod for 10.9-grade non-quenched and tempered fastener and production method of cold heading wire rod

Abstract

The cold heading wire rod comprises the following components in percentage by mass: 0.20%-0.26% of C, less than or equal to 0.35% of Si, 1.2%-1.6% of Mn, less than or equal to 0.02% of P, less than or equal to 0.005% of S, 0.04%-0.14% of V, 0.02%-0.10% of Nb, less than or equal to 0.2% of Cr, less than or equal to 0.1% of Ni, 0.02%-0.06% of Al and the balance of Fe and inevitable impurities, the V + Nb is greater than or equal to 0.09%, the C / 4 + Si + Mn / 2 + Cr / 2 + 2Ni is greater than or equal to 1.1% and less than or equal to 1.3%, the F + P is greater than or equal to 95%, the F grain size is greater than or equal to grade 12, the longitudinal pearlite strip The steel plate prepared by the method does not crack at 1 / 5 of a hot rolling state, can meet the requirements of a user on strength and plasticity of cold heading forming after 5-50% of cold drawing deformation, and can meet the requirements of a 10.9-grade high-strength fastener on fatigue and delayed fracture resistance at the same time.

Description

Technical Field

[0001] The present invention belongs to the technical field of steel for fasteners, and particularly relates to a cold-heading wire rod for 10.9-grade non-quenched and tempered fasteners and a production method thereof. Background Art

[0002] Traditional 10.9-grade high-strength fasteners are often produced using materials such as 10B35, SCM435, and SCM440. Their manufacturing process includes pickling and phosphating - drawing - spheroidizing annealing - drawing - cold heading - quenching and tempering, etc. The spheroidizing annealing and quenching and tempering heat treatment processes consume a large amount of energy, increase the production process and manufacturing cost, and there is also a risk of bending of parts such as long bolts and threaded rods after quenching and tempering treatment, which requires additional straightening treatment. As a green and energy-saving product, non-quenched and tempered cold-heading steel can be used to replace traditional quenched and tempered steel to prepare high-strength fasteners, omitting the spheroidizing annealing and quenching and tempering processes of raw materials, greatly omitting the manufacturing process, and having the advantages of reducing manufacturing cost and saving energy consumption. Therefore, it is necessary to conduct technical development and application verification for non-quenched and tempered steel for 10.9-grade fasteners.

[0003] At present, non-quenched and tempered steel for 8.8-grade fasteners has been promoted and applied in the market. Users have a further demand for non-quenched and tempered steel for 10.9-grade fasteners, but related products are still relatively lacking. Although some cold-heading wire rods for 10.9-grade non-quenched and tempered fasteners have been proposed in the prior art, there are still the following problems:

[0004] In order to balance material cost, strength, and drawing performance, in the existing production methods of 10.9-grade non-quenched and tempered steel, medium-high carbon alloy steel with C≥0.32% is used to make ferrite + sorbite structure. For example, a cold-drawn steel wire for 10.9-grade non-quenched and tempered U-bolts and its manufacturing method disclosed in Patent CN116904877A adopt a C-Si-Mn-Cr-Nb composition design, and combine EDC water bath cooling after wire laying and Stelmor cooling line cooling after coiling to manufacture wire rods. However, due to the relatively high C and Si contents in the components, the plasticity of the wire rods is insufficient, which easily leads to work hardening and a sharp increase in deformation resistance. There is a risk of large die wear and even cold-heading cracking during the cold-heading process. For example, it is easy to crack when producing large-deformation flange bolts. And reducing the C and Si contents will result in a large strength loss, affecting the final performance grade of the fasteners.

[0005] In order to balance material strength and cold drawing performance, in the existing production methods of 10.9 grade non-quenched and tempered steel, low-carbon alloy steel with C≤0.15% is used to make bainitic wire rods. For example, a 10.9 grade non-quenched and tempered cold heading steel wire rod for fasteners and its production method disclosed in Patent CN106480376A adopt the composition design of C-Si-Mn-Cr-B-Ti, and are made into granular bainite structure by first air cooling and then air cooling after low-temperature wire laying. However, on the one hand, there are problems of high alloy element content and high cost. On the other hand, since bainite structure forms continuously in a relatively wide temperature range and upper bainite and lower bainite structures coexist, the tissue uniformity is poor, the obtained bainite structure has a high distortion, the wire rod is prone to insufficient plasticity, increasing the risks of brittle fracture during manufacturing, die loss during cold heading and even cold heading cracking. Summary of the Invention

[0006] The present invention aims to solve at least one of the above technical problems to some extent. The present invention provides a cold heading wire rod for 10.9 grade non-quenched and tempered fasteners and its production method, which can balance material cost, meet the requirements of the user for the strength and plasticity of the wire rod for non-quenched and tempered cold heading forming, and at the same time meet the requirements of 10.9 grade high-strength fasteners for fatigue and resistance to delayed fracture performance.

[0007] The technical solution adopted by the present invention to solve its technical problems is as follows:

[0008] A cold heading wire rod for 10.9 grade non-quenched and tempered fasteners, the composition of the cold heading wire rod includes, by mass percentage: C: 0.20% - 0.26%, Si: ≤0.35%, Mn: 1.2% - 1.6%, P: ≤0.02%, S: ≤0.005%, V: 0.04% - 0.14%, Nb: 0.02% - 0.10%, Cr: ≤0.2%, Ni: ≤0.1%, Al: 0.02% - 0.06%, the balance being Fe and inevitable impurities, where V+Nb≥0.09%, 1.1%≤C / 4+Si+Mn / 2+Cr / 2+2Ni≤1.3%; the volume ratio of ferrite + pearlite in the microstructure of the cold heading wire rod is ≥95%.

[0009] The inventors have found through research that carbon, silicon, manganese, phosphorus, sulfur, vanadium, and niobium are the most critical chemical elements determining the plasticity of the cold heading wire rod. The above cold heading wire rod adopts the design concept of "low-medium carbon design + microalloying of Mn, V, Nb + coordinated control of phase transformation deformation" to solve the problems of insufficient plasticity of medium-high carbon alloy steel with C≥0.32% and easy cracking when producing large-deformation flange bolts, as well as the problems of high alloy element content, high cost, and poor tissue uniformity of low-carbon alloy steel with C≤0.15%. The composition and mass percentage design basis of the cold heading wire rod include:

[0010] (1) Carbon: C is the main strengthening element in non-quenched and tempered steel, existing in the form of interstitial solid solution or carbide. Increasing the C content helps to improve the matrix strength and hardenability; if the C content is too low, the strength will be insufficient. When it exceeds 0.26%, the plasticity and toughness and cold working performance will deteriorate, and the work hardening rate of the steel will increase. To balance the material cost, the tensile strength in the hot-rolled state, and the fatigue performance, and to facilitate the reasonable control of the microstructure, the carbon content is appropriately reduced. Therefore, the carbon content is controlled at 0.20% - 0.26% by mass percentage.

[0011] (2) Manganese: Mn mainly exists in the form of solid solution in steel, which can improve the strength and hardness of the steel. Manganese can refine the grains of the steel, increase the grain boundary area, thus increasing the resistance to dislocation movement and resulting in work hardening. At the same time, Mn can improve the stability of the austenite structure and delay the pearlite phase transformation and promote the bainite phase transformation. An appropriate amount of manganese can ensure the strength and work hardening rate of the steel. However, when the mass fraction of Mn in non-quenched and tempered steel exceeds 1.60%, it is very easy to produce bainite structure, resulting in poor tissue uniformity and increased control difficulty, seriously affecting the plasticity of the wire rod. Therefore, the mass percentage of Mn is controlled at 1.20% - 1.60%.

[0012] (3) Silicon: Si can strengthen ferrite, increase the resistance to dislocation movement, thus resulting in work hardening and then increasing the tensile and yield strengths of the steel. However, too high Si content will cause a sharp increase in the deformation resistance of the steel and significantly increase the consumption of the die during cold drawing and cold heading. Therefore, the mass percentage of Si is controlled at ≤0.35%.

[0013] (4) Phosphorus: P has a strong segregation tendency and segregates at the grain boundaries during high-temperature heating, increasing the cold brittleness of the steel and reducing the plasticity, which is harmful to the uniformity of the product structure and performance. However, phosphorus is an inevitable element. Through research by the inventor, it is found that when the content of P is controlled below 0.020%, it does not cause cold brittleness and damage to plasticity and toughness of the steel. Therefore, the mass percentage of P is limited to ≤0.020%.

[0014] (5) Sulfur: S is also an inevitable harmful element in steel. Sulfur easily forms MnS inclusions in the steel, causing the steel to have hot brittleness. At the same time, MnS inclusions have a certain damage to the toughness and processing performance of the steel. Therefore, the mass percentage of S is limited to ≤0.005%.

[0015] (6) Vanadium: V is a strengthening element in steel. V has a very strong affinity with both C and N and mainly exists in the form of carbides in steel. Due to the precipitation strengthening of VC and V(CN), the main role of V in steel is to refine the grain size of the microstructure. As the grain refinement of the material, the strength of the steel increases and the sensitivity to cracks is greatly reduced, which can improve the anti-delayed fracture performance of the material. However, when the V content is relatively high, the cost is high, and when the content is less than 0.04%, the effect is very small. Therefore, the mass percentage of V is controlled at 0.04% - 0.14%.

[0016] (7) Niobium: Nb is similar to V and has a very strong affinity with both C and N, forming VC and V(CN), which are dispersed during the rolling stage of the steel and the low-temperature aging stage after the bolt is upset and formed, improving the strength and anti-delayed fracture performance. When the content is less than 0.01%, it has no effect, and when it is higher than 0.1%, the effect reaches saturation. Therefore, the mass percentage of Nb is controlled at 0.02% - 0.10%. At the same time, further considering the roles of V and Nb and the material cost, it is controlled that V + Nb ≥ 0.09%.

[0017] (8) Aluminum: Al is an effective deoxidizer and can form AlN to refine the grains. An appropriate amount of aluminum can improve the cold heading performance of cold heading steel, making the deformation of the metal more uniform during the cold heading process of cold heading steel. When the Al content is less than 0.02%, the effect is not obvious. When it is higher than 0.060%, it is easy to form coarse inclusions, deteriorating the plastic toughness of the steel and increasing the cost. Therefore, the mass percentage of Al is controlled at 0.02% - 0.06%.

[0018] Based on the low and medium carbon, low silicon, Mn, V, Nb microalloying strengthening composition system, further considering the requirements of non-quenched and tempered cold heading forming for the strength and plasticity of the wire rod, and being able to meet the fatigue performance requirements of 10.9 grade high-strength bolts, C / 4 + Si + Mn / 2 + Cr / 2 + 2Ni is further controlled. Through the research of the inventor, it is found that when this value is relatively low, the work hardening rate of the cold heading wire rod is insufficient and it is difficult to reach the required strength grade of the finished fastener. When this value is relatively high, the cold heading performance of the cold heading wire rod will deteriorate and the cold heading is prone to cracking. Therefore, it is controlled that 1.1% ≤ C / 4 + Si + Mn / 2 + Cr / 2 + 2Ni ≤ 1.3%. At the same time, further coordinating the phase transformation deformation control, controlling the volume ratio of ferrite + pearlite in the microstructure ≥ 95%, which can improve the tissue uniformity, avoid the adverse effects of other abnormal tissues such as martensite and bainite on drawing and cold heading, and then through the reasonable regulation of chemical composition design and microstructure, a good strength and plasticity matching can be obtained, especially improving the plasticity of non-quenched and tempered cold heading steel. In the preferred embodiment, the diameter of the cold heading wire rod It is 5 to 28 mm, the tensile strength in the hot-rolled state is ≥680 MPa, and the reduction of area is ≥55%, so as to save the user's annealing and quenching and tempering processes, improve the mold wear in the cold heading process, reduce the manufacturing cost of 10.9-grade fasteners, the risk of cold heading cracking and save energy consumption.

[0019] The ferrite has good plasticity and toughness. The higher the grain size, the higher the strength and hardness, which can avoid excessive loss of the wire rod strength caused by the coarsening or increased proportion of ferrite. In a preferred embodiment, the grain size of the ferrite is ≥12 grades.

[0020] An increase in the proportion of pearlite will cause an increase in the tensile strength of the cold-heading wire rod and a decrease in the reduction of area, but too little proportion of pearlite will lead to excessive loss of the tensile strength of the cold-heading wire rod. In a preferred embodiment, the volume proportion of pearlite is 41% - 45%.

[0021] On the longitudinal section of the cold-heading wire rod, the width of the strip formed by pearlite, that is, the longitudinal pearlite strip width, is relatively narrow, which can further avoid the adverse effect of the banded structure caused by carbon and alloy element segregation on the mechanical properties of the material. In a preferred embodiment, the longitudinal pearlite strip width in the microstructure of the cold-heading wire rod is ≤30 μm.

[0022] On the longitudinal section of the cold-heading wire rod, the longitudinal pearlite lamellar spacing is relatively small, which can make the pearlite have better mechanical properties. In a preferred embodiment, the longitudinal pearlite lamellar spacing in the microstructure of the cold-heading wire rod is 80 - 120 nm.

[0023] The production method of the above-mentioned cold-heading wire rod for 10.9-grade non-quenched and tempered fasteners includes the processes of electric furnace smelting, LF refining, VD vacuum refining, bloom continuous casting, steel billet flaw detection and grinding, heating, rolling, wire laying and controlled cooling in sequence. During the wire laying process, the wire laying temperature is controlled at 780 - 820 °C, and during the controlled cooling process, the cooling rate of the wire rod on the roller table is controlled at 0.5 - 1.2 °C / s.

[0024] Based on the low and medium carbon design + MnVNb microalloying composition design, the above-mentioned 10.9-grade non-quenched and tempered fasteners adopt the controlled rolling and controlled cooling process of low-temperature wire laying and slow cooling. Compared with water cooling or strong air cooling after wire laying, it can avoid the growth of austenite grains, reduce the influence of segregation, extend the phase transformation time, and then make the low and medium carbon, MnVNb microalloying design cooperate with phase transformation deformation control, promote the full transformation of austenite into ferrite and pearlite, reduce the risk of abnormal bainite structure precipitation, promote the refinement of ferrite grains and longitudinal pearlite lamellar spacing, form a more stable structure, make the produced non-quenched and tempered cold-heading steel wire rod obtain better ductility and have good strength and plasticity indexes, and facilitate subsequent drawing and reduce the risk of cold heading cracking.

[0025] During the electric furnace smelting process, selecting an appropriate scrap ratio can further reduce the material cost, control the tapping temperature, tapping carbon content, and phosphorus content, maintain the fluidity of the molten steel, stably control the carbon content, reduce the influence of segregation, and further improve the production continuity and stability. In a preferred embodiment, during the electric furnace smelting process, the scrap ratio is controlled ≥20%, the tapping temperature is ≥1580°C, the tapping carbon content is controlled at 0.12% - 0.20%, and the tapping phosphorus content is controlled at P≤0.015%.

[0026] During the LF refining process, selecting an appropriate deoxidation method can further improve the purity of the molten steel and reduce the influence of inclusions. In a preferred embodiment, during the LF refining process, aluminum pellets are used for diffusion deoxidation, and the white slag holding time is ≥15 min.

[0027] During the VD vacuum refining process, selecting an appropriate vacuum degree and holding time can further improve the quality of the molten steel and control the smelting purity. In a preferred embodiment, during the VD vacuum refining process, the vacuum degree is controlled <67 Pa and the holding time is >10 min.

[0028] In a preferred embodiment, during the VD vacuum refining process, calcium treatment is carried out before tapping the VD station, and the soft blowing time is controlled ≥15 min, which can further reduce the inclusions in the molten steel, improve the fluidity of the molten steel, prevent the clogging of the continuous casting nozzle, and shorten the smelting cycle.

[0029] Selecting a lower superheat degree during the bloom continuous casting can improve the solidification structure, reduce casting defects, and improve production efficiency. In a preferred embodiment, during the bloom continuous casting process, the superheat degree is controlled at 20 - 35°C.

[0030] Selecting a combined electric stirring and appropriate secondary cooling water ratio control during the bloom continuous casting process can further improve the general porosity, center porosity, and ingot shape segregation of the bloom billet, providing favorable conditions for improving the density and tissue uniformity of the cold heading wire rod. In a preferred embodiment, during the bloom continuous casting process, the combined method of mold electric stirring and end electric stirring is used for stirring, and the secondary cooling water ratio is controlled at 0.3 - 0.8 L / kg to obtain a bloom billet with a length of 9 - 16 m.

[0031] During the rolling process, selecting an appropriate starting rolling temperature can reduce the deformation resistance of the billet and avoid the growth of austenite grains and the decrease in plasticity caused by too high a starting rolling temperature. Selecting appropriate finishing rolling temperatures and sizing temperatures, through rolling deformation control, can cooperate with the effects of Nb and V to promote grain refinement, avoid the coarsening of austenite grains caused by too high a temperature and thus reduce mechanical properties, and avoid too low a temperature from increasing the deformation resistance of the steel and reducing plasticity, causing dimensional over-tolerance or surface defects. At the same time, controlling the coiling temperature provides favorable conditions for phase transformation deformation control. In a preferred embodiment, during the rolling process, the starting rolling temperature is controlled at 1000 - 1080 °C, the finishing rolling temperature is 840 - 860 °C, and the sizing temperature is 810 - 830 °C.

[0032] In a preferred embodiment, in the controlled cooling process, a heat preservation cover opening and closing control is used to control the cooling rate of the wire rod.

[0033] Compared with the prior art, the beneficial effects of the present invention are at least as follows:

[0034] The MnVNb series non-quenched cold heading steel wire rod of the present invention adopts the design of "low-medium carbon design + MnVNb microalloying + phase transformation deformation synergistic control". Through the reasonable design and distribution of carbon and alloying elements, combined with low-temperature coiling and slow cooling for phase transformation control, the microstructure is reasonably regulated, so that the proportion of ferrite F + pearlite P ≥ 95%, and the ferrite grain size ≥ 12 grades. It can take into account the material cost, and the produced non-quenched cold heading steel wire rod has good strength and plasticity indexes, reaching a hot-rolled tensile strength ≥ 680 MPa and an area reduction ≥ 55%. It can replace alloy cold heading steels such as 10B35, SCM435, and SCM440, and solve the problems of existing medium-high carbon alloy steels with C ≥ 0.32% having insufficient plasticity and being prone to cracking when producing large-deformation flange bolts, or existing low-carbon alloy steels with C ≤ 0.15% having high alloying element content, high cost, and poor tissue uniformity. The obtained cold heading wire rod can be used to produce 10.9-grade fasteners, eliminating the spheroidizing annealing and quenching and tempering processes, reducing processing costs and shortening the production cycle. Especially when producing parts with a large length-to-diameter ratio, good straightness can be guaranteed. It has the characteristics of high strength and plasticity, carbon reduction, cost reduction, shortening the production cycle, and precise control of product straightness. After testing, it does not crack at 1 / 5 in the hot-rolled state, and after 5% - 50% cold drawing deformation, it can meet the requirements of users for strength and plasticity in non-quenched cold heading forming, and at the same time can meet the requirements of 10.9-grade high-strength bolts for fatigue and anti-delayed fracture performance, and has good market application prospects. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] The above and / or additional aspects and advantages of the present invention will become obvious and easy to understand from the description of the embodiments in conjunction with the following drawings, where:

[0036] Figure 1It is the metallographic structure diagram of the cold heading wire rod obtained in Example 1 of the present invention;

[0037] Figure 2 It is the longitudinal ferrite and pearlite band diagram of the cold heading wire rod obtained in Example 1 of the present invention. Specific embodiments

[0038] The present invention will be further described below in conjunction with specific embodiments, but the protection scope of the present invention is not limited thereto.

[0039] Example 1:

[0040] A preferred embodiment of the cold heading wire rod for 10.9 grade non-quenched and tempered fasteners described in the present invention. The composition of the cold heading wire rod includes, by mass percentage: C: 0.2%, Si: 0.35%, Mn: 1.5%, P: 0.02%, S: 0.004%, V: 0.05%, Nb: 0.04%, Cr: 0.06%, Ni: 0.04%, Al: 0.023%, with the balance being Fe and inevitable impurities, where V + Nb = 0.09%, and C / 4 + Si + Mn / 2 + Cr / 2 + 2Ni = 1.26%; the production method of the cold heading wire rod is produced according to the technological process of electric furnace smelting → LF + VD double refining → continuous casting of square billets → inspection and grinding of steel billets → heating → rolling → wire laying → controlled cooling. Specifically:

[0041] During the electric furnace smelting process, the molten iron and scrap steel raw materials are put into the converter for smelting into molten steel. The scrap steel ratio is controlled at 23%, the tapping temperature is 1583°C, the tapping carbon content is controlled at 0.15%, and the tapping phosphorus content is controlled at P = 0.012%.

[0042] The molten steel obtained from the electric furnace smelting is transported from the converter to the LF furnace refining station with the ladle for LF refining. The elements such as C, Mn, V, Nb, and Al are adjusted to the target composition. During the LF refining process, aluminum pellets are used for diffusion deoxidation, and the white slag holding time ≥ 15 min.

[0043] The molten steel after LF refining is lifted into the VD station with the ladle, and VD vacuum refining treatment is carried out with a high vacuum degree. During the VD vacuum refining process, the smelting purity is controlled, the vacuum degree is controlled < 67 Pa and the holding time > 10 min. Calcium treatment is carried out before leaving the VD station, and the soft blowing time is controlled at 16 min.

[0044] The molten steel refined by LF + VD is poured into a continuous caster. After the molten steel solidifies into a slab, it is continuously drawn out from the mold. The slab with a liquid core passes through the secondary cooling zone, and the specific water consumption in the secondary cooling is controlled to enable the slab to continue to solidify until it is completely solidified, and then it is cut into square billets with a length of 9 - 16 m. During the continuous casting process of the square billets, the superheat is controlled at 21°C, and a combined method of mold electric stirring and end electric stirring is adopted for stirring, and the specific water consumption in the secondary cooling is controlled at 0.5 L / kg.

[0045] The square billets are subjected to flaw detection and grinding to remove surface defects, and then heated and sent to the rolling line for controlled rolling deformation to produce wire rods. During the rolling process, the starting rolling temperature is controlled at 1020°C, the temperature entering the finishing mill is 845°C, and the temperature entering the sizing mill is 825°C.

[0046] The wire rods obtained by rolling are made into coils by a wire laying machine, and the coils are conveyed by roller tables into the controlled cooling line. During the wire laying process, the wire laying temperature is controlled at 785°C.

[0047] In the controlled cooling process, a heat preservation cover is used to control the cooling rate of the coils, and the cooling rate of the coils on the roller tables is controlled at 0.95°C / s. After being taken off the line, the cold heading coils are obtained. It can be seen from the appendix Figure 1 the metallographic structure and grain size of the cold heading coils. The volume ratio of ferrite + pearlite in the microstructure is ≥95%. The grain size of the ferrite is grade 13, and the volume ratio of the pearlite is 41%. It can be seen from the appendix Figure 2 longitudinal ferrite and pearlite bands. The average bandwidth of the longitudinal pearlite bands is 12 μm, and the lamellar spacing of the longitudinal pearlite is 115 nm.

[0048] Control examples are manufactured using ingredient ratios different from those in Example 1, and cold heading coils are manufactured using ingredient ratios and process parameters different from those in Example 1. The compositions of the cold heading coils in each example and control example are shown in Table 1 below by mass percentage:

[0049] Table 1. Composition table of different cold heading coils

[0050]

[0051]

[0052] As can be seen from Table 1, for the cold heading coils in each example, V + Nb ≥ 0.09%, 1.1% ≤ C / 4 + Si + Mn / 2 + Cr / 2 + 2Ni ≤ 1.3%. For the cold heading coils in Control Example 1, V + Nb = 0.08%, which is relatively small, and C / 4 + Si + Mn / 2 + Cr / 2 + 2Ni = 1.38%, which is relatively large. For the cold heading coils in Control Example 2, V + Nb = 0.14%, which is relatively large, and C / 4 + Si + Mn / 2 + Cr / 2 + 2Ni = 1.08%, which is relatively small.

[0053] The production process parameters of the cold-heading wire rod for each embodiment are shown in Table 2 below:

[0054] Table 2. Production process parameters of different cold-heading wire rods

[0055]

[0056] Sampling and specimen preparation for mechanical property tests were carried out in accordance with the GB / 2975 standard, the tensile test of metallic materials was carried out in accordance with the GB / T228.1 standard, the microscopic structure inspection of metals was carried out in accordance with the GB / T13298 standard, and the upsetting test of metallic materials was carried out in accordance with YB / T5293. The properties of the cold-heading steel wire rods obtained from each embodiment and the control examples were detected, and the results are shown in Table 3 below:

[0057] Table 3. Properties and microscopic structures of different cold-heading wire rods

[0058]

[0059]

[0060] From the comparison results of Examples 1-6 and Control Example 1 and Control Example 2, it can be seen that the present invention adopts "low-medium carbon design + microalloying of Mn, V, Nb + synergistic control of phase transformation deformation". Through the reasonable design and distribution of carbon and alloying elements, and further controlling the V+Nb content and the C / 4+Si+Mn / 2+Cr / 2+2Ni content, the volume fraction of ferrite + pearlite in the microscopic structure of the cold-heading wire rod is ≥95%. In Control Example 1, in addition to ferrite + pearlite, there is 10% bainite. In Control Example 2, the volume fraction of ferrite + pearlite is 98%, but the ferrite grain size level is low, so the strength is low. From the results of Examples 1 to 6, it can be seen that in the microscopic structure of the cold-heading steel wire rod of the present invention, the ferrite grain size ≥12 grades, the volume fraction of pearlite is 41% - 45%, the longitudinal pearlite strip width ≤30μm, and the longitudinal pearlite lamellar spacing is 80 - 120nm. It has the characteristics of high strength and plasticity, carbon reduction, and cost reduction, etc. It can reach a hot-rolled tensile strength ≥680MPa, a reduction of area ≥55%, and cold heading of 1 / 5 without cracking in the hot-rolled state, and can take into account the material cost and make the produced non-quenched and tempered cold-heading steel wire rod have good strength and plasticity indexes.

[0061] Application verification was carried out on the cold-heading wire rod for 10.9 grade non-quenched and tempered fasteners: under the condition of omitting the spheroidizing annealing and tempering processes, the cold-heading steel wire rods obtained from each embodiment and the control examples were used to manufacture bolts through non-quenched and tempered treatments such as drawing and cold heading, and the cold drawing area reduction rate and cold heading rod shrinking deformation rate during cold drawing and cold heading were controlled. The mechanical properties of the obtained bolt finished products were tested in accordance with the GB / T3098.1 standard, and the results are shown in Table 4 below:

[0062] Table 4. Parameters during cold drawing and cold upsetting deformation of different cold heading wire rods, and the tensile strength of the finished bolts

[0063] Serial number Cold drawing area reduction rate / % Cold upsetting rod deformation rate / % Finished product tensile strength / MPa Remarks Example 1 5 41 1063 None Example 2 42 19 1110 None Example 3 50 0 1080 None Example 4 35 22 1060 None Example 5 33 31 1099 None Example 6 28 27 1072 None Control example 1 35 12 1136 High cracking rate Control example 2 40 16 1024 Insufficient strength

[0064] From the comparison results of Example 1 with Comparative Example 1 and Comparative Example 2, it can be seen that in the present invention, the low and medium carbon design + microalloying of Mn, V, and Nb + synergistic control of phase transformation deformation are adopted. Based on the functions of V and Nb and the material cost, V + Nb ≥ 0.09% is controlled. At the same time, C / 4 + Si + Mn / 2 + Cr / 2 + 2Ni is controlled. When this value is relatively low, it will lead to insufficient work hardening rate of the cold heading wire rod and it is difficult to reach the strength grade required for the finished fastener. When this value is relatively high, the processing performance of the cold heading wire rod will become poor and cold upsetting is prone to cracking. It is preferably controlled that V + Nb ≥ 0.09% and 1.1% ≤ C / 4 + Si + Mn / 2 + Cr / 2 + 2Ni ≤ 1.3%. From the results of Examples 1 to 6, it can be seen that the cold heading wire rods of the present invention can balance the material cost after 5% - 50% cold drawing deformation, meet the requirements of non-quenched and tempered cold upsetting forming for the strength and plasticity of the wire rod. The tensile strength of the finished 10.9-grade bolts can reach ≥ 1040 MPa. At the same time, it can meet the requirements of 10.9-grade high-strength bolts for fatigue performance, can omit the spheroidizing annealing and tempering processes, reduce the processing cost and shorten the production cycle. Especially when producing parts with a large length-diameter ratio, good straightness can be guaranteed, and it has the characteristics of carbon reduction, cost reduction, production cycle shortening, and precise control of product straightness. The application verification results show that the MnVNb series non-quenched and tempered cold heading steel wire rods of the present invention can replace alloy cold heading steels such as 10B35, SCM435, and SCM440, and solve the problems existing in medium and high carbon alloy steels with C ≥ 0.32%, such as insufficient plasticity and easy cracking when producing large-deformation flange bolts, or low carbon alloy steels with C ≤ 0.15%, such as high alloy element content, high cost, and poor tissue uniformity, and have good market application prospects.

[0065] The series of detailed descriptions listed above are only specific descriptions of the feasible embodiments of the present invention, and they are not intended to limit the protection scope of the present invention. Any equivalent embodiments or changes made without departing from the technical spirit of the present invention should be included in the protection scope of the present invention.

Claims

1. A 10.9 grade cold heading wire rod for non-quenched and tempered fasteners, characterized in that: The composition of the cold heading wire rod includes, by mass percentage, C: 0.20% to 0.26%, Si: ≤0.35%, Mn: 1.2% to 1.6%, P: ≤0.02%, S: ≤0.005%, V: 0.04% to 0.14%, Nb: 0.02% to 0.10%, Cr: ≤0.2%, Ni: ≤0.1%, Al: 0.02% to 0.06%, and the remainder is Fe and unavoidable impurities, among which V+Nb≥0.09%, 1.1%≤C / 4+Si+Mn / 2+Cr / 2+2Ni≤1.3%; the volume proportion of ferrite+pearlite in the microstructure of the cold heading wire rod is ≥95%.

2. The 10.9 grade non-quenched and tempered cold heading wire rod for fasteners according to claim 1, characterized in that: The grain size of the ferrite is ≥ grade 12, and the volume proportion of the pearlite is 41% to 45%.

3. The 10.9 grade non-quenched and tempered cold heading wire rod for fasteners according to claim 1, characterized in that: The longitudinal pearlite strip width in the microstructure of the cold heading wire rod is ≤30 μm, and the longitudinal pearlite lamella spacing is 80-120 nm.

4. The method for producing a 10.9 grade non-quenched and tempered fastener cold heading wire rod according to any one of claims 1 to 3, characterized in that: The production method comprises the following steps: electric furnace smelting, LF refining, VD vacuum refining, billet continuous casting, billet flaw detection and grinding, heating, rolling, wire-spinning and controlled cooling. In the wire-spinning process, the wire-spinning temperature is controlled to be 780-820°C. In the controlled cooling process, the cooling speed of the wire rod on the roller is controlled to be 0.5-1.2°C / s.

5. The method for producing the 10.9 grade non-quenched and tempered fastener cold heading wire rod according to claim 4, characterized in that: During the electric furnace smelting process, the scrap steel ratio is controlled to be ≥20%, the tapping temperature is ≥1580°C, the tapping carbon content is controlled to be 0.12%-0.20%, and the tapping phosphorus content is controlled to be P≤0.015%.

6. The method for producing the 10.9 grade non-quenched and tempered fastener cold heading wire rod according to claim 4, characterized in that: During the LF refining process, aluminum particles are used for diffusion deoxidation, and the white slag retention time is ≥15 minutes.

7. The method for producing the 10.9 grade non-quenched and tempered fastener cold heading wire rod according to claim 4, characterized in that: During the VD vacuum refining process, the vacuum degree is controlled to be less than 67 Pa and the holding time is greater than 10 min. Calcium treatment is performed before leaving the VD station, and the soft blowing time is controlled to be greater than or equal to 15 min.

8. The method for producing the 10.9 grade non-quenched and tempered fastener cold heading wire rod according to claim 4, characterized in that: During the continuous casting of the square billet, the superheat is controlled at 20-35° C., a crystallizer electric stirring and a terminal electric stirring combination are used for stirring, and the secondary cooling water volume is controlled at 0.3-0.8 L / kg.

9. The method for producing the 10.9 grade non-quenched and tempered fastener cold heading wire rod according to claim 4, characterized in that: During the rolling process, the starting rolling temperature is controlled to be 1000-1080°C, the finishing rolling temperature is controlled to be 840-860°C, and the sizing temperature is controlled to be 810-830°C.

10. The method for producing the 10.9 grade non-quenched and tempered fastener cold heading wire rod according to claim 4, characterized in that: In the cooling control process, a heat preservation cover is opened and closed to control the cooling speed of the wire rod.

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