Annealing-free alloy cold heading steel hot-rolled wire rod and manufacturing method thereof

Abstract

The application discloses a kind of annealing-free alloy cold heading steel hot rolling wire rod, it contains Fe and inevitable impurities, it also contains the following chemical elements with mass percentage as follows: C:0.31-0.39%, Si≤0.15%, Mn:0.60-0.70%, Cr:0.90-0.99%, Mo:0.16-0.19%, Al:0.010-0.025%;The microstructure of the annealing-free alloy cold heading steel hot rolling wire rod is ferrite+pearlite, and the total volume fraction of bainite and martensite is not more than 10%. Accordingly, the application also discloses a manufacturing method of the above-mentioned annealing-free alloy cold heading steel hot rolling wire rod, and the annealing-free alloy cold heading steel hot rolling wire rod prepared by the manufacturing method has the characteristics of low strength and low hardness, which can omit the annealing process and be directly drawn by downstream users, and has the advantages of energy saving and environmental protection.

Description

Technical Field

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

[0002] Cold heading steel is a general term for a type of steel wire products. This type of wire is formed by cold working at room temperature, and can be effectively made into parts such as hexagonal bolts, socket head cap bolts, bolts with flanges, special bolts, and ball head pins, for use in connecting or transmitting force in automobiles and machinery.

[0003] As is well known, according to the national standard GB / T 3098.1 "Mechanical Properties of Fasteners - Bolts, Screws and Studs", most commonly used fasteners are currently grade 4.6-12.9 (equivalent to tensile strength levels of 400MPa-1200MPa).

[0004] In current technology, according to the national standard GB / T 6478 "Steel for Cold Heading and Cold Extrusion", the steel grades for cold heading and cold extrusion specifically cover carbon steel, carbon boron steel, medium and low carbon low alloy steel, and medium and low carbon alloy steel. When manufacturing bolts of grade 6.8 and below, low carbon steel wire is typically used, requiring no heat treatment and being produced solely through cold deformation. However, when manufacturing bolts of grade 8.8-12.9, medium carbon, low alloy steel, and medium carbon alloy steel wire are generally used. The manufacturing process involves: annealing and softening → cold drawing to achieve high dimensional accuracy and surface quality → cutting, cold heading of bolt heads, and cold thread rolling to form threads → quenching and tempering heat treatment to improve strength and hardness → surface rust prevention treatment, etc., to produce finished fasteners of grade 8.8-12.9.

[0005] However, in response to the national call for energy conservation and emission reduction, many machinery manufacturers have imposed higher restrictions on the economic production of parts and the overall carbon emissions throughout the process. Among these, energy use and consumption are the main sources of carbon emissions in parts manufacturing, with heat treatment being a major energy-consuming process. Therefore, reducing heat treatment processes can significantly reduce energy consumption and carbon emissions, which is a goal pursued in the parts manufacturing industry.

[0006] Research has found that in the parts manufacturing process, the quenching and tempering heat treatment process can easily adjust the strength and other properties of the final parts and is usually indispensable; while the annealing heat treatment process before cold deformation is mainly to reduce the hardness of the hot-rolled wire delivered by the steel mill, so as to facilitate the smooth forming process.

[0007] Therefore, if the hardness of hot-rolled wire delivered by steel mills can be effectively reduced without significantly increasing manufacturing difficulty and cost, and without affecting the strength, hardness and other properties of other processing procedures and final parts, the annealing process can be omitted in the production process, thereby achieving the goal of reducing carbon emissions. At the same time, omitting the annealing process can also shorten the processing steps, which can effectively improve production efficiency.

[0008] For example, Chinese patent document CN101028680A, published on September 5, 2007, entitled "A Production Process Method for 8.8 Grade High-Strength Fasteners", discloses a production process method for 8.8 grade high-strength fasteners. It uses non-annealed cold-heading steel, which reduces tensile strength and hardness and increases plasticity. This cold-heading steel does not require annealing treatment and can be directly used to make hexagonal socket head cap screws, flange bolts and nuts, simplifying the process and saving processing costs. The strength grade of the bolts produced is limited to 8.8 grade.

[0009] For example, Chinese patent document CN101045238A, published on October 3, 2007, entitled "Online Softening Rolling Method for Medium Carbon Alloy Cold Heading Steel," discloses an online softening rolling method for medium carbon alloy cold heading steel. The chemical composition of the steel is: C: 0.03-0.38%, Si: 0.18-0.40%, Mn: 0.60-0.80%, Cr: 0.9-1.2%, Mo: 0.015-0.30%, S≤0.015%, P≤0.025%, with the remainder being Fe. The key features of this technical solution are: the steel billet is heated to 950-1180℃ for primary, intermediate, and pre-finish rolling; after pre-finish rolling, the rolled piece is cooled in a water tank, and then finished rolled at 700-820℃, with a cumulative reduction in surface area of ​​50-85% and a deformation rate of 1 / 5-1 / 30s. After finishing rolling, the rolled piece is water-cooled to 680-780℃ and exits the wire rod mill. After the wire is coiled, it is slowly cooled at a cooling rate of 0.01-0.5℃ / s for 600-2400 seconds, and then the wire is naturally cooled to room temperature. Through the above treatment, this medium-carbon alloy cold heading steel wire is softened directly online during the rolling process, with significantly reduced strength and hardness and increased reduction of area, making it suitable for cold working. This effectively simplifies the spheroidizing annealing process, saves energy, and reduces the spheroidizing annealing time.

[0010] For example, Chinese patent document CN104593679A, published on May 6, 2015, entitled "10.9 Grade High-Strength Screw Cold Heading Steel Wire Without Annealing and Its Production Process," discloses a 10.9 grade high-strength screw cold heading steel wire without annealing and its production process. This process eliminates the need for annealing while ensuring the mechanical properties of the steel bar. In this technical solution, the chemical composition of the designed cold heading steel wire is: C: 0.39–0.42%, Si: 0.15–0.25%, Mn: 0.70–0.85%, S: 0–0.020%, P: 0–0.020%, Alt: 0.015–0.05%, Cr: 1.00–1.15%, Mo: 0.17–0.22%, with the remainder being Fe. Accordingly, in the processing technology, the finishing rolling temperature is controlled at 780-820℃, the sizing temperature at 760-800℃, the wire forming temperature at 750-790℃, and the transmission rate during the cooling stage is ≤0.3m / s. Summary of the Invention

[0011] One of the objectives of this invention is to provide a non-annealing alloy cold heading steel hot-rolled wire rod, which has the characteristics of low strength and low hardness. It can be directly drawn by downstream users to prepare the required fasteners without annealing treatment. The fasteners prepared are of grade 10.9 and above, and can be effectively applied to automotive and mechanical parts.

[0012] To achieve the above objectives, the present invention provides a non-annealing alloy cold heading steel hot-rolled wire rod, which contains Fe and unavoidable impurities, and also contains the following chemical elements in the following mass percentages:

[0013] C: 0.31-0.39%, Si≤0.15%, Mn: 0.60-0.70%, Cr: 0.90-0.99%, Mo: 0.16-0.19%, Al: 0.010-0.025%;

[0014] The microstructure of the non-annealed alloy cold heading hot-rolled wire rod is ferrite + pearlite with a total volume fraction of not less than 90%, and an unavoidable small amount of bainite and / or martensite.

[0015] Furthermore, in the non-annealing alloy cold heading steel hot-rolled wire rod described in this invention, the mass percentage content of each chemical element is as follows:

[0016] C: 0.31-0.39%, Si≤0.15%, Mn: 0.60-0.70%, Cr: 0.90-0.99%, Mo: 0.16-0.19%, Al: 0.010-0.025%; balance is Fe and unavoidable impurities.

[0017] In this invention, the non-annealing alloy cold-heading steel hot-rolled wire designed herein can be used to produce threaded fasteners, ball joint connectors, and other parts of grade 10.9 and above. This non-annealing alloy cold-heading steel hot-rolled wire features low strength and low hardness, allowing for direct drawing without the annealing process when downstream users manufacture fasteners, thus offering advantages in energy saving and environmental protection.

[0018] In the non-annealing alloy cold heading hot-rolled wire rod described in this invention, the design principles of each chemical element are as follows:

[0019] C: In the non-annealing alloy cold heading steel hot-rolled wire rod described in this invention, C is an inexpensive element. Its addition to steel can improve the hardenability and hardenability of the steel. At the same time, C can form compounds with iron and various alloying elements during the tempering process and play a role in second-phase dispersion strengthening. Therefore, adding a certain amount of carbon can effectively improve the service strength of the material.

[0020] Studies have found that when the carbon (C) content in steel is below 0.31%, the strength of the steel after quenching and tempering is difficult to reach 1000 MPa or higher; while when the C content exceeds 0.39%, it leads to a decrease in the material's plasticity, making it difficult to achieve the requirement of anneal-free wire rod through process adjustments. Therefore, considering the influence of C on steel properties, in the anneal-free alloy cold heading hot-rolled wire rod described in this invention, the mass percentage of C is controlled between 0.31% and 0.39%.

[0021] Of course, in some preferred embodiments, in order to obtain better implementation results, the mass percentage content of element C can be further preferably controlled between 0.33-0.37%.

[0022] Si: In the non-annealed alloy cold-heading steel hot-rolled wire rod of the present invention, Si can substitute iron atoms in the steel through substitution, which can hinder dislocation movement and significantly improve the strength of the ferrite phase. However, it should be noted that the Si content in the steel should not be too high. A high Si content will increase the hardness of the material after annealing, which is not conducive to reducing the strength in the hot-rolled state through process adjustments, and will also reduce the life of the forming die. At the same time, a high Si content will reduce the low-temperature impact toughness of the steel. Based on this, in the non-annealed alloy cold-heading steel hot-rolled wire rod of the present invention, an appropriate amount of Si can be added to the steel, and specifically, the mass percentage of Si is controlled to Si≤0.15%.

[0023] Mn: In the non-annealing alloy cold heading hot-rolled wire rod described in this invention, Mn is an austenite stabilizing element. Adding an appropriate amount of Mn to the steel can effectively improve the hardenability and strength of the steel. Adding a certain amount of Mn has little effect on the plasticity of the steel, but Mn is prone to segregation during the solidification process of ferroalloys. Therefore, to ensure alloy strength and a certain hardenability, more than 0.60% Mn needs to be added to the steel. The Mn content in the steel should not be too high. To avoid the deterioration of material uniformity caused by manganese segregation, the Mn content in the steel should be controlled below 1.45%. When manganese acts in combination with alloying elements such as chromium and molybdenum, it will significantly increase the austenite stability of the material, thereby increasing hardenability. In order to achieve the transformation into a microstructure dominated by ferrite and pearlite during the controlled cooling process after rolling, the Mn content in the steel is specifically set below 0.70%. Based on this, under this combined effect, the mass percentage content of Mn element in the non-annealing alloy cold heading hot-rolled wire rod of the present invention is controlled between 0.60-0.70%.

[0024] Of course, in some preferred embodiments, in order to obtain better implementation results, the mass percentage content of Mn element can be further preferably controlled between 0.63-0.67%.

[0025] Cr: In the non-annealing alloy cold heading hot-rolled wire rod described in this invention, the addition of Cr element is beneficial to improving the hardenability of the ferroalloy. Simultaneously, Cr can precipitate fine, dispersed carbide particles during tempering, which can play a role in dispersion strengthening, thereby improving the material strength. Of course, Cr element also has the effect of refining the microstructure. Therefore, to leverage the solid solution strengthening and precipitation strengthening effects of chromium while improving the material microstructure, the mass percentage of Cr element in this invention is controlled between 0.90% and 0.99%.

[0026] Of course, in some preferred embodiments, in order to obtain better implementation results, the mass percentage content of Cr element can be further preferably controlled between 0.94-0.97%.

[0027] Mo: In the non-annealing alloy cold heading steel hot-rolled wire rod described in this invention, molybdenum has a large atomic weight and atomic radius, and is not easy to diffuse in ferroalloys. Therefore, when tempered at a lower temperature range, molybdenum mainly exists in the steel in solid solution form and plays a solid solution strengthening effect. When tempered at a higher temperature range, molybdenum will form fine carbides to improve the strength of the steel. Overall, it can improve the tempering resistance of the material.

[0028] It is important to note that when a higher tempering temperature is used, the amount of dislocations and subgrain boundaries in the material can be effectively reduced, thereby preventing delayed cracking caused by hydrogen accumulation at dislocations and grain boundaries. However, it is also important to note that the Mo content in the steel should not be too high, as excessive Mo will increase the material cost. Therefore, considering both cost factors and the beneficial effects of Mo, the mass percentage of Mo is controlled between 0.16% and 0.19% in this invention.

[0029] Al: In the non-annealing alloy cold heading hot-rolled wire rod of the present invention, Al element is added to the steel as a deoxidizer during the smelting and refining process, and a portion of aluminum element is retained in the steel as a grain refiner to prevent austenite grain growth during the heating process. Based on this, in the present invention, the mass percentage content of Al element is controlled between 0.010-0.025%.

[0030] Furthermore, in the non-annealing alloy cold heading steel hot-rolled wire rod described in this invention, among other unavoidable impurities: S≤0.010%.

[0031] In the above-described technical solution of the present invention, element S is an impurity element in the hot-rolled wire rod of the non-annealing alloy cold heading steel described in the present invention. Under the condition that technical conditions permit, in order to obtain steel with better performance and higher quality, the content of impurity element S should be reduced as much as possible.

[0032] Furthermore, in the non-annealing alloy cold heading hot-rolled wire rod of the present invention, the mass percentage content of each chemical element satisfies at least one of the following:

[0033] C: 0.33-0.37%,

[0034] Mn: 0.63-0.67%,

[0035] Cr: 0.94-0.97%.

[0036] Furthermore, in the non-annealing alloy cold heading hot-rolled wire rod of the present invention, when viewed from its cross-section, the ferrite grain size in the range from the surface to 0.5 mm below the surface differs from the ferrite grain size at other locations by no more than one grade.

[0037] Furthermore, in the non-annealing alloy cold heading steel hot-rolled wire rod described in this invention, its diameter is 8-20mm.

[0038] Furthermore, in the non-annealing alloy cold heading hot-rolled wire rod described in this invention, its tensile strength does not exceed 800 MPa and its reduction of area is not less than 40%.

[0039] Furthermore, in the non-annealing alloy cold heading hot-rolled wire rod described in this invention, samples are taken from different positions at intervals of not less than 400 mm along its length, and the tensile strength fluctuation does not exceed 100 MPa.

[0040] Accordingly, another objective of the present invention is to provide a method for manufacturing the above-mentioned non-annealing alloy cold heading steel hot-rolled wire rod, which is simple to operate and the non-annealing alloy cold heading steel hot-rolled wire rod obtained by the method has the characteristics of low strength and low hardness.

[0041] To achieve the above objectives, the present invention proposes a method for manufacturing the above-mentioned non-annealing alloy cold heading steel hot-rolled wire rod, which includes the steps of: smelting and casting, high-speed wire rod rolling and cooling.

[0042] In the high-speed wire rod rolling process: firstly, the billet is heated; then, after roughing, intermediate rolling, and pre-finishing rolling, the billet enters a water tank for water cooling. After water cooling, the billet enters the finishing mill after a temperature equalization time of not less than 5 seconds. The temperature when entering the finishing mill is 840-880℃. After finishing rolling, the billet enters the reducing and sizing mill for final rolling after water cooling and a temperature equalization section of not less than 2 seconds. The temperature when entering the reducing and sizing mill is 780-830℃. After the final rolling is completed, the billet enters the wire coiler for wire drawing after temperature control in a water tank.

[0043] In the cooling process: the cooling rate is controlled at 1.5-2.5℃ / s before the wire temperature reaches 750℃; after the wire temperature reaches 750℃, it is cooled to below 500℃ at a cooling rate not exceeding 0.6℃ / s, and then naturally cooled to room temperature.

[0044] In the manufacturing process designed in this invention, the specific diameter of the wire obtained by high-speed wire rolling is... The diameter can be controlled between 8-20mm. The high-speed wire rod rolling process mainly controls process parameters such as rolling speed and cooling rate. To ensure production efficiency, the smaller the specification, the higher the rolling speed, and the greater the temperature rise during the rolling process. At the same time, the cooling rate of the wire rod is difficult to control when the specification is small. Therefore, the lower limit of the rollable wire rod specification for this material is specified as [missing information]. When the wire rod dimensions are large, the water cooling efficiency during the wire rolling process is low, and it is not easy to cool evenly. Therefore, the upper limit of the wire rod dimensions that can be rolled from this material is specified.

[0045] It is important to note that before high-speed wire rod rolling, the billet needs to be heated. The billet requires a suitable temperature to ensure that the high-temperature hardness of the material is lower than that of a typical rolling mill. Similarly, if the temperature is too high, the rolled piece will not be able to cool evenly to the specified rolling temperature before water cooling temperature control. Therefore, in some preferred embodiments, the billet heating temperature can be preferably controlled at 900-1020℃.

[0046] Accordingly, in high-speed wire rod rolling, the steel billet needs to undergo roughing, intermediate rolling, and pre-finishing rolling before entering a water cooling tank. Furthermore, the water-cooled billet needs sufficient homogenization time to ensure uniform temperature throughout the rolled piece. Therefore, in this invention, it is specifically stipulated that the homogenization time before entering the finishing mill is no less than 5 seconds, and the homogenization time before entering the reducing mill is no less than 2 seconds.

[0047] Furthermore, the deformation temperature of the workpiece during finishing and reducing sizing mill rolling directly affects the post-rolling cooling phase transformation process. Excessively high temperatures lead to high-temperature austenite grain growth, inhibiting pearlite phase transformation; excessively low temperatures result in a significant increase in deformation resistance and increase rolling difficulty. Therefore, in this invention, the specific temperature entering the finishing mill is specified as 840-880℃, and the temperature entering the reducing sizing mill is specified as 780-830℃.

[0048] Additionally, it's important to note that after the wire is coiled, its microstructure is austenite before the temperature reaches 750°C. Therefore, a relatively fast cooling rate is required to suppress grain growth and decarburization defects. However, when the wire temperature cools to below 750°C up to 500°C, the austenite begins to undergo a phase transformation to ferrite and pearlite, requiring a slower cooling rate to ensure the transformation is fully completed. Therefore, in this invention, during the cooling process, the cooling rate should be controlled at 1.5-2.5°C / s when the wire temperature is above 750°C, and at a rate not exceeding 0.6°C / s when the temperature is below 750°C, cooling should proceed to below 500°C.

[0049] Furthermore, in the manufacturing method described in this invention, the billet heating temperature is 900-1020℃.

[0050] Furthermore, in the manufacturing method described in this invention, the spinning temperature of the spinning machine is controlled to be 800-850°C.

[0051] Compared with the prior art, the non-annealing alloy cold heading steel hot-rolled wire rod and its manufacturing method described in this invention have the following advantages and beneficial effects:

[0052] In this invention, the non-annealing alloy cold heading steel hot-rolled wire designed in this invention can be used to produce fasteners of grade 10.9 and above, and effectively reduces the hardness of the hot-rolled wire delivered by the steel mill without significantly increasing the manufacturing difficulty and cost, while not affecting the strength, hardness and other properties of other processing procedures and the final parts.

[0053] In practical applications, this non-annealing alloy cold heading steel hot-rolled wire has the characteristics of low strength and low hardness. When it is subsequently used by downstream users to manufacture fasteners, the annealing process can be omitted and it can be directly drawn, which has the advantages of energy saving and environmental protection.

[0054] Furthermore, since the fasteners prepared using the non-annealing alloy cold heading steel hot-rolled wire described in this invention can omit the annealing process, it can also effectively shorten the processing steps of the fasteners, thereby improving the production efficiency of enterprises. Detailed Implementation

[0055] The following will provide further explanation and description of the non-annealing alloy cold heading 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.

[0056] Examples 1-6 and Comparative Examples 1-5

[0057] In this invention, the anneal-free alloy cold heading steel hot-rolled wire rods of Examples 1-6 are prepared by the following steps:

[0058] (1) Smelting and casting according to the chemical composition shown in Table 1 to obtain the corresponding steel billet.

[0059] (2) High-speed wire rod rolling: In the high-speed wire rod rolling process: First, the billet is heated and the heating temperature of the billet is controlled at 900-1020℃; then the billet is rolled by roughing, intermediate rolling and pre-finishing rolling and then enters the water tank for water cooling. After the water cooling, the billet enters the finishing mill after a temperature equalization time of not less than 5s. The temperature when entering the finishing mill is 840-880℃. After finishing rolling, the billet enters the reducing sizing mill for final rolling after water cooling and a temperature equalization section of not less than 2s. The temperature of the billet entering the reducing sizing mill is 780-830℃. After the final rolling is completed, the billet is heated to 800-850℃ in the water tank and then enters the wire coiler to be coiled.

[0060] (3) Cooling: Cool the wire. Before the wire temperature reaches 750℃, control the cooling rate to 1.5-2.5℃ / s. After the wire temperature reaches 750℃, cool it to below 500℃ at a cooling rate not higher than 0.6℃ / s, and then let it cool naturally to room temperature.

[0061] It should be noted that, in this invention, the chemical composition design and related processes of the non-annealing alloy cold heading steel hot-rolled wire rods in Examples 1-6 all meet the design specifications of this invention.

[0062] Accordingly, in order to demonstrate the superiority of embodiments 1-6 of this application, the inventors have also designed three sets of comparative examples, namely comparative examples 1-5. The chemical composition design of the comparative wires of comparative examples 1-5 can be found in Table 1 below.

[0063] Although the comparative steel wires of Comparative Examples 1-5 were also produced using the above-mentioned process of smelting and casting, high-speed wire rolling and cooling, their chemical composition and related process parameters failed to meet the design requirements of this invention.

[0064] Table 1 lists the mass percentage of each chemical element for the non-annealed alloy cold heading steel hot-rolled wire rods of Examples 1-6 and the comparative wire rods of Comparative Examples 1-5.

[0065] Table 1. (wt%, balance Fe and other unavoidable impurities besides S)

[0066]

[0067] Table 2 lists the specific process parameters for the anneal-free alloy cold heading steel hot-rolled wire rods of Examples 1-6 and the comparative wire rods of Comparative Examples 1-5 in the above process steps.

[0068] Table 2.

[0069]

[0070] Samples were taken from the anneal-free alloy cold heading steel hot-rolled wire rods of Examples 1-6 and the comparative wire rods of Comparative Examples 1-5 obtained through the above process steps. The microstructure of the sample wire rods of Examples 1-6 and Comparative Examples 1-5 was observed and analyzed to obtain the microstructure of the sample wire rods of each example and comparative example. The specific observation and analysis results of the microstructure are listed in Table 3 below.

[0071] Accordingly, in addition to observing the microstructure, the inventors further observed the cross-sections of the wires in each embodiment and comparative example. The observation revealed that in the non-annealing alloy cold heading hot-rolled wires of Examples 1-6, the difference in ferrite grain size between the wire surface and 0.5 mm below the surface and other locations did not exceed level 1.

[0072] Table 3 lists the observation and analysis results of the non-annealed alloy cold heading hot-rolled wire rods of Examples 1-6 and the comparative wire rods of Comparative Examples 1-5.

[0073] Table 3.

[0074]

[0075]

[0076] Accordingly, after completing the microstructure observation of each of the above embodiments and comparative examples, the hot-rolled wire rods of the non-annealed alloy cold-heading steel of Examples 1-6 and the comparative wire rods of Comparative Examples 1-5 can be collected again, and the performance of the hot-rolled wire rods of the non-annealed alloy cold-heading steel of Examples 1-6 and the comparative wire rods of Comparative Examples 1-5 can be tested to obtain the performance of the finished wire rods of each embodiment and comparative example. The results obtained from the performance test are listed in Table 4 below.

[0077] The relevant performance testing methods are as follows:

[0078] Tensile test: Tensile tests were conducted according to the conditions specified in GB / T228.1, at room temperature (25±10℃), using Method A. This was to obtain the tensile strength and reduction of area of ​​the finished wires in each embodiment and comparative example at room temperature.

[0079] It should be noted that, in order to verify the strength fluctuation in this invention, for the non-annealed alloy cold heading steel hot-rolled wire rods of Examples 1-6 and the comparative wire rods of Comparative Examples 1-5, samples were taken at different positions along the length of the hot-rolled wire rods at intervals of not less than 400 mm, and the tensile strength was tested to obtain the corresponding tensile strength fluctuation.

[0080] Table 4 lists the relevant performance test results of the non-annealed alloy cold heading steel hot-rolled wire rods of Examples 1-6 and the comparative wire rods of Comparative Examples 1-5.

[0081] Table 4.

[0082]

[0083]

[0084] As can be seen from Table 4 above, the annealed alloy cold-heading steel hot-rolled wire rods of Examples 1-6 of the present invention exhibit superior performance compared to the comparative wire rods of Comparative Examples 1-5. In particular, in terms of strength, the tensile strength of the annealed alloy cold-heading steel hot-rolled wire rods of Examples 1-6 of the present invention is significantly lower than that of the comparative wire rods of Comparative Examples 1-5.

[0085] As shown in Table 4 above, in this invention, the highest tensile strength of the non-annealed alloy cold-heading steel hot-rolled wire rods of Examples 1-6 is between 700 and 765 MPa, and their tensile strength is no higher than 800 MPa, exhibiting low strength characteristics. Simultaneously, the non-annealed alloy cold-heading steel hot-rolled wire rods of Examples 1-6 have a high reduction of area, between 50% and 65%, which also means they possess low hardness. Furthermore, the non-annealed alloy cold-heading steel hot-rolled wire rods of Examples 1-6 exhibit uniform strength, with tensile strength fluctuations along the length direction all less than 100 MPa, specifically between 60 and 90 MPa.

[0086] The control wire of Comparative Example 1 achieved a maximum tensile strength that was far higher than the requirements set forth in this invention (tensile strength not greater than 800 MPa). It had high tensile strength and a very low reduction of area.

[0087] The highest tensile strength obtained by the comparative wire of Comparative Example 2 is far higher than the requirement specified in this invention (tensile strength not greater than 800 MPa).

[0088] Compared with the control wire of Comparative Example 3, the highest tensile strength obtained was much higher than the requirement of this invention (tensile strength not greater than 800MPa), and it had a very low cross-sectional reduction rate, and the tensile strength in the length direction fluctuated greatly.

[0089] Compared with the control wire of Comparative Example 4, the highest tensile strength obtained was much higher than the requirement specified in this invention (tensile strength not greater than 800MPa), and the tensile strength in the length direction fluctuated greatly.

[0090] The control wire of Comparative Example 5 achieved a maximum tensile strength that was far higher than the requirements set forth in this invention (tensile strength not greater than 800 MPa), and also had a very low reduction of area.

[0091] In summary, the non-annealing alloy cold heading steel hot-rolled wire designed in this invention has the characteristics of low strength and low hardness. When downstream users manufacture fasteners, the annealing process can be omitted and the wire can be directly drawn, which has the advantages of energy saving and environmental protection.

[0092] In this invention, the non-annealing alloy cold heading steel hot-rolled wire can be used to produce threaded fasteners, ball joint connecting rods and other parts of grade 10.9 and above, and has a very good prospect for promotion and application value.

[0093] 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.

[0094] 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 non-annealing alloy cold heading steel hot-rolled wire rod, characterized in that, Its mass percentage content of each chemical element is as follows: C: 0.31-0.39%, Si≤0.15%, Mn: 0.60-0.70%, Cr: 0.90-0.99%, Mo: 0.16-0.19%, Al: 0.010-0.025%; balance is Fe and unavoidable impurities; The microstructure of the non-annealing alloy cold heading hot-rolled wire is ferrite + pearlite, and bainite and martensite with a total volume fraction greater than 0 and not exceeding 10%; when viewed from its cross-section, the ferrite grain size in the range from the surface to 0.5 mm below the surface is no more than 1 grade different from the ferrite grain size in other locations. Its tensile strength shall not exceed 800 MPa, and its reduction of area shall not be less than 40%; Samples were taken from different locations at intervals of at least 400 mm along its length, and the tensile strength fluctuation did not exceed 100 MPa.

2. The non-annealing alloy cold heading hot-rolled wire rod as described in claim 1, characterized in that, In other unavoidable impurities: S≤0.010%.

3. The non-annealing alloy cold heading hot-rolled wire rod as described in claim 1, characterized in that, Its mass percentage content of each chemical element satisfies at least one of the following conditions: C：0.33-0.37%， Mn: 0.63-0.67%, Cr：0.94-0.97%。 4. The non-annealing alloy cold heading hot-rolled wire rod as described in claim 1, characterized in that, Its diameter is 8-20mm.

5. A method for manufacturing hot-rolled wire rod of non-annealed alloy cold heading steel as described in any one of claims 1-4, comprising the steps of: smelting and casting, high-speed wire rod rolling and cooling; characterized in that: In the high-speed wire rod rolling process: firstly, the billet is heated; then, after roughing, intermediate rolling, and pre-finishing rolling, the billet enters a water tank for water cooling. After water cooling, the billet enters the finishing mill after a temperature equalization time of not less than 5 seconds. The temperature when entering the finishing mill is 840-880℃. After finishing rolling, the billet enters the reducing and sizing mill for final rolling after water cooling and a temperature equalization section of not less than 2 seconds. The temperature when entering the reducing and sizing mill is 780-830℃. After the final rolling is completed, the billet enters the wire coiler for wire drawing after temperature control in a water tank. In the cooling process: the cooling rate is controlled at 1.5-2.5℃ / s before the wire temperature reaches 750℃; after the wire temperature reaches 750℃, it is cooled to below 500℃ at a cooling rate not exceeding 0.6℃ / s, and then naturally cooled to room temperature.

6. The manufacturing method as described in claim 5, characterized in that, The billet heating temperature is 900-1020℃.

7. The manufacturing method as described in claim 5, characterized in that, The spinning temperature of the spinning machine is 800-850℃.

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