10.9-grade non-quenched and tempered cold heading steel wire rod and production method thereof

Abstract

The invention provides a 10.9-grade non-quenched and tempered cold heading steel wire rod and a production method thereof. The 10.9-grade non-quenched and tempered cold heading steel wire rod comprises the following chemical components: 0.23-0.27% of C, 0.30-0.50% of Si, 1.45-1.55% of Mn, less than or equal to 0.025% of P, less than or equal to 0.025% of S, 0.40-0.50% of Cr, 0.025-0.050% of Ti, 0.0025-0.0050% of B, less than or equal to 0.0050% of N and more than or equal to 0.015% of [Ti]-3.5 * [N]. According to the invention, on the basis of component design, a controlled rolling process deformation induced phase change, an online salt bath isothermal treatment cooling process and an artificial aging process are matched, so that rapid cooling and isothermal phase change of the wire rod are realized, and the prepared wire rod is uniform in through ring mechanical property, high in yield ratio, good in plasticity and long in fatigue life. The steel wire rod can be used for directly cold-drawing the steel wire without annealing, and the cold-drawn steel wire can meet the performance requirement of a 10.9-grade fastener without quenching and tempering after being formed. According to the production method of the wire rod, the production process can be simplified, the fastener production period is shortened, the cost is reduced, meanwhile, the energy consumption is reduced, and the method conforms to the current green and low-carbon development route.

Description

Technical Field

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

[0002] Grade 10.9 high-strength fasteners are primarily used in automotive and high-end engineering machinery. Conventional 10.9-grade fasteners are typically produced from alloy steels such as SCM435, which are doped with precious elements like Mo and V, after annealing and tempering. The manufacturing process generally involves annealing and heat-treating the hot-rolled alloy steel wire rod, drawing it to the appropriate specifications. The resulting wire is then formed into the desired shape using equipment such as a cold heading machine and subjected to a tempering heat treatment to achieve the desired mechanical property grade. Energy conservation and emission reduction are high on the agenda for major OEMs. Driven by the dual demands of green energy conservation and vehicle lightweighting, the use of heat-treatment-free processes to produce 10.9-grade fasteners without annealing or tempering has become an inevitable trend.

[0003] Annealing-free and quenching-and-tempering-free Grade 10.9 fasteners eliminate the annealing heat treatment before drawing and the tempering heat treatment after forming. This improves the mechanical properties of hot-rolled wire rod, combined with the processing strengthening after wire rod drawing due to area reduction. The resulting steel wire approaches or exceeds the mechanical properties of quenched and tempered fasteners, and the finished fasteners meet the mechanical property requirements of the corresponding grade. The manufacturing process for annealing-free and quenching-and-tempering-free Grade 10.9 fasteners is generally as follows: hot-rolled wire rod - cold-drawn steel wire - forming - surface treatment.

[0004] At present, the non-quenched and tempered steel produced by traditional domestic technology has a ferrite and pearlescent structure with low strength level. The strength of the wire rod produced is between 630-700MPa, and the area reduction is about 50%. If its strength reaches the 10.9 grade fastener strength level, it needs to be drawn with a large deformation of more than 40%, but its plasticity is reduced and it cannot meet the requirements of cold heading. In addition, a large amount of precious alloy elements such as Nb are added, which is costly. Stelmor air cooling is used, and the structure of the overlap and non-overlap points is uneven, with the fluctuation of the through-ring exceeding 100MPa. The fatigue performance of the fasteners produced is reduced, which cannot meet the performance requirements of 10.9 grade high-strength fasteners. Summary of the Invention

[0005] The present invention addresses the technical problem that non-tempered steel produced by traditional processes cannot meet the performance requirements of high-strength, large-deformation 10.9-grade non-quenched and tempered fasteners. It proposes a wire rod that can be directly cold-drawn without annealing. The cold-drawn steel wire can meet the performance requirements of 10.9-grade fasteners without quenching and tempering treatment after forming. The production method can simplify the production process, improve production efficiency, reduce energy consumption, save energy and reduce emissions, and is in line with the current green and low-carbon development route.

[0006] In order to achieve the above object, the technical solution adopted by the present invention is:

[0007] A 10.9-grade non-quenched and tempered cold heading steel wire rod has the following chemical compositions by weight: C 0.23-0.27%, Si 0.30-0.50%, Mn 1.45-1.55%, P≤0.025%, S≤0.025%, Cr 0.40-0.50%, Ti 0.025-0.050%, B 0.0025-0.0050%, N≤0.0050%, [Ti]-3.5*[N]≥0.015%, and the balance being Fe and unavoidable impurity elements.

[0008] Preferably, its chemical composition by weight percentage is: C 0.25%, Si 0.35%, Mn 1.50%, P≤0.015%, S≤0.015%, Cr 0.45%, Ti 0.030%, B 0.0030%, N 0.0038%.

[0009] Preferably, the lower bainite content in the wire rod structure is ≥95%.

[0010] In the composition design process, by studying the influence of elements such as Mn, Si, Cr, and B on the structure and performance, the rapid cooling + isothermal phase transformation of the online salt bath isothermal treatment is used to fully transform the fine austenite structure into a bainite structure with high strength and plasticity, without adding precious alloying elements such as Mo, V, and Nb, which has good economic efficiency. The content of impurity elements such as P and S is strictly controlled to improve the purity of the grain boundaries of the steel and increase the grain boundary strength. In the present invention, the Ti content of the steel is more than 0.025%, and [Ti]-3.5*[N]≥0.015%, mainly to play the role of fixing nitrogen and preserving boron, to exert the effect of B in improving hardenability to inhibit ferrite phase transformation, and to improve toughness and delayed fracture performance through TiC. The specific chemical composition design is as follows:

[0011] C: Carbon (C) is the most critical strengthening element in steel. It improves the comprehensive mechanical properties of the material mainly through solid solution strengthening and carbide dispersion strengthening mechanisms. It has a decisive influence on the morphology and performance of bainite: a higher carbon content promotes the formation of lower bainite, and the precipitation of fine carbides can significantly improve the hardness and strength, but at the expense of the toughness of the material; while under low carbon conditions, lamellar upper bainite is easily formed, which has better toughness but relatively insufficient strength. In addition, appropriately increasing the carbon content can reduce the bainite transformation temperature, refine the microstructure, and achieve an optimized match of strength and toughness by delaying the decomposition of austenite. Therefore, the present invention controls the carbon content in the steel to 0.23-0.27%.

[0012] Si: Silicon (Si) is an important alloying element in steel, and it functions as both a reducing agent and a deoxidizer. It can significantly improve the elastic limit, yield strength, and tensile strength of steel through solid solution strengthening. In terms of microstructure regulation, silicon tends to inhibit cementite precipitation and promote ferrite formation. Excessive silicon content will lead to increased hardness of the material and significantly deteriorate its cold forming processing performance. Therefore, the present invention controls the silicon content in the steel to 0.30-0.50%.

[0013] Mn: Manganese (Mn), as an austenite-stabilizing element, significantly lowers the bainite transformation temperature, inhibits carbon diffusion, and delays carbide precipitation, thereby refining the size of the bainitic ferrite lamellae and carbide grains. This microstructural manipulation not only enhances the material's toughness and work-hardening ability but also optimizes the strength-toughness balance. Excessive Mn content prolongs the phase transformation time, adversely affecting actual production schedules. Therefore, the present invention controls the manganese content in the steel to 1.45-1.55%.

[0014] Cr: Chromium (Cr), as a key alloying element, significantly lowers the lower bainite transformation temperature and refines the ferrite lamellar structure. By inhibiting carbon atomic diffusion, it effectively delays the precipitation of coarse cementite and promotes the dispersion of fine alloy carbides, thereby improving the overall material performance. However, excessive chromium content can easily form brittle phases or coarse carbides, reducing the material's toughness. Therefore, the present invention controls the chromium content in the steel to 0.40-0.50%.

[0015] P, S: In principle, the phosphorus and sulfur contents should be reduced as much as possible, but too low a content will significantly increase smelting costs. Therefore, the present invention controls the P and S contents to below 0.025%.

[0016] B: The addition of trace amounts of boron (B) can significantly improve the hardenability of steel, inhibit the precipitation of proeutectoid ferrite, and promote the transformation to lower bainite. Its mechanism of action is primarily manifested in grain boundary segregation, which delays austenite decomposition, reduces the bainite transformation temperature, and simultaneously refines the size of ferrite lamellae and carbides, thereby improving the material's strength and toughness matching. However, excessive boron content can easily lead to grain boundary embrittlement, requiring precise control of its addition to achieve structural homogenization and optimize performance. Therefore, the present invention controls the B content in the steel to 0.0025-0.0050%.

[0017] Ti: Titanium can fully utilize the role of boron (B) in improving hardenability and inhibiting ferrite phase transformation by fixing nitrogen and retaining boron. Further increasing the titanium content will form a TiC phase, which not only refines the austenite grains, but also acts as a hydrogen trap, thereby improving the toughness and delayed fracture performance of the steel. However, too high a titanium content will cause the titanium nitride (TiN) particles to coarsen, increasing the difficulty of controlling the casting process. Therefore, the Ti content in the present invention is controlled in the range of 0.025 to 0.0050%, and at the same time, [Ti]-3.5*[N]≥0.015% is satisfied to ensure sufficient TiC precipitation.

[0018] Nitrogen: Free nitrogen can produce solid solution strengthening, but it can also cause aging embrittlement. As nitrogen content increases, the titanium ratio required for stable nitrogen fixation increases accordingly, which leads to coarsening of the titanium nitride precipitates. If the precipitated particles are too large or form macroscopic inclusions directly in the liquid phase, the fatigue properties of the material can be significantly impaired. Therefore, the nitrogen content in the steel of this invention is controlled to ≤ 0.0050%.

[0019] The present invention also provides a method for producing a 10.9 grade non-quenched and tempered cold heading steel wire rod, comprising the following steps:

[0020] Smelting: The raw materials are sequentially smelted in a converter, refined in LF, and degassed in RH to produce molten steel. The converter smelting process controls the tapping P≤0.015%, S≤0.015%. Slag and part of the alloy are added along the steel flow during tapping.

[0021] Continuous Casting: The molten steel superheat is controlled between 10°C and 30°C. 180mm x 240mm billets are cast in the continuous casting tundish under argon protection throughout the entire process. The billets are then fire-cut and pile-cooled. Precise composition control is implemented throughout the entire process to ensure metallurgical quality. The cooled billets are peeled to remove surface defects.

[0022] Heating in heating furnace: adopt walking beam heating furnace for heating, residual oxygen content in heating furnace is ≤3%, temperature in heating equalization section of heating furnace is controlled at 1050-1150℃, total heating time is controlled at 100-150min to ensure that wire rod obtains low decarburization layer.

[0023] Rolling: includes rough rolling, intermediate rolling, pre-finishing rolling, finishing rolling, and sizing reduction. The rough rolling temperature is controlled at 950-1000℃, the intermediate rolling temperature and pre-finishing rolling temperature are controlled at 850-900℃, and the finishing rolling / sizing reduction temperature is controlled at 780-820℃. Grain refinement is achieved by suppressing recrystallization. The spinning temperature is controlled at 800-850℃ to ensure that the steel billet remains in a fully austenitized state before entering the salt bath equipment.

[0024] Salt bath cooling: After rolling and spinning, the wire rod directly enters the online salt bath isothermal treatment equipment for cooling. The salt bath temperature is between 460-480℃ (fluctuation range ±1℃) to achieve rapid cooling and constant temperature phase transformation: when the salt temperature is lower than the lower limit, it will lead to an increase in the martensite (M) content. Although it will increase the strength of the wire rod, it will cause the fluctuation of the tensile strength of the through-loop to increase (>50MPa) and plasticity deterioration, affecting the subsequent drawing and cold heading processing performance; while the salt temperature is too high, it will cause the ferrite (F) content to exceed the standard, making the mechanical properties substandard; the total cooling time of the wire rod is greater than 600 seconds to ensure sufficient structural transformation and make the proportion of lower bainite more than 95%; a drop section and gas internal circulation stirring are set on the salt bath roller to improve the temperature field and the uniformity of the wire rod through-loop. In the Stelmor cooling process: the roller speed is 0.2-0.6m / s, and the insulation cover is adjusted according to the specifications. This process reduces phase transformation stress and thermal stress, reduces crack sensitivity, and improves the ductility and fatigue properties of the material.

[0025] Preferably, Ti and B microalloying elements are precisely added after the reducing slag is formed, and the molten steel is allowed to stand and soft-poured with argon for 15 minutes after the LF refining is completed.

[0026] Preferably, the steel billet is descaled by high-pressure water after being heated in a heating furnace, and the dephosphorization pressure is ≥18Mpa, which effectively prevents the iron oxide scale from being pressed into the matrix.

[0027] Preferably, the coils are collected after cooling in the salt bath, and the collected wire rods are placed in an aging warehouse for aging. The aging time is 3-5 days and the aging temperature is 60-100°C. This process effectively releases the residual stress inside the material, reduces the local stress concentration factor, and achieves plasticity improvement.

[0028] Compared with the prior art, the advantages and positive effects of the present invention are:

[0029] The present invention improves hardenability by designing a composition of lower bainite structure without precious alloy elements and low C containing Si, Cr, Mn, Ti, and B. It cooperates with the controlled rolling process to induce phase transformation, the online salt bath isothermal treatment cooling process, and the artificial aging process to achieve rapid cooling and isothermal phase transformation of the wire rod. The prepared wire rod has uniform mechanical properties, high yield strength ratio, good plasticity, and high fatigue life. The wire rod can be directly cold-drawn into steel wire without annealing. After the cold-drawn steel wire is formed, it can meet the performance requirements of 10.9-grade fasteners without tempering treatment. The production method of the 10.9-grade non-tempered cold heading steel wire rod of the present invention can simplify the production process, improve production efficiency, shorten the fastener production cycle, reduce costs, and at the same time reduce energy consumption, save energy, and reduce emissions, which is in line with the current green and low-carbon development route.

[0030] The cold-heading steel wire rod described in the present invention has the following characteristics: a hot-rolled diameter range of 5.5-20 mm, a microstructure of high-purity lower bainite (content ≥95%), and mechanical properties including a yield strength of 800-860 MPa, a tensile strength of 940-997 MPa, an elongation of 20% or more, and a reduction of area of ​​75% or more. It also exhibits excellent processability (no cracking during 1 / 8 cold forging) and surface quality (decarburization depth ≤0.7%D). After drawing with a 15%-25% area reduction, the wire's mechanical properties are further improved to a yield strength of 975-1038 MPa and a tensile strength of 1058-1127 MPa, while maintaining good plasticity (elongation ≥15%, reduction of area ≥70%) and formability (no cracking during 1 / 7 cold forging). This product can be directly used for cold processing and forming of fasteners. After stabilization treatment, it can meet the performance requirements of 10.9 grade fasteners, eliminating the traditional quenching and tempering heat treatment process. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 This is a metallographic structure diagram of a wire rod 500X according to an embodiment of the present invention;

[0032] Figure 2 This is a 5KX electron microscope organization diagram of the wire rod according to an embodiment of the present invention;

[0033] Figure 3 This is the metallographic structure diagram of wire rod 500X of comparative example 1 of the present invention;

[0034] Figure 4 This is a 5KX electron microscope organization diagram of the wire rod of comparative example 1 of the present invention;

[0035] Figure 5 This is the metallographic structure diagram of wire rod 500X of comparative example 2 of the present invention;

[0036] Figure 6 This is a 5KX electron microscope organization diagram of the wire rod of comparative example 2 of the present invention;

[0037] Figure 7 This is a real picture of 1 / 8 of the wire rod sample after cold forging according to an embodiment of the present invention. DETAILED DESCRIPTION

[0038] In order to better understand the present invention, the following is a detailed description with reference to the accompanying drawings and embodiments.

[0039] Example

[0040] A 10.9-grade non-quenched and tempered cold-heading steel wire rod has the following chemical composition by weight: C: 0.25%, Si: 0.35%, Mn: 1.50%, P: ≤ 0.015%, S: ≤ 0.015%, Cr: 0.45%, Ti: 0.030%, B: 0.0030%, N: 0.0038%, [Ti]-3.5*[N]: 0.017%. The wire rod has a diameter of 10 mm.

[0041] The production method of the wire rod comprises the following steps:

[0042] (1) Smelting: The main raw materials are sequentially subjected to converter smelting, LF refining, and RH degassing to produce molten steel. The converter smelting process controls the tapping P≤0.015%, S≤0.015%. During tapping, slag and part of the alloy are added along the steel flow to obtain the above-mentioned composition of the molten steel. After the RH refining is completed, the molten steel is allowed to stand and soft-purged with argon for 15 minutes.

[0043] (2) Continuous Casting: The superheat of the molten steel is controlled between 10°C and 30°C. The continuous casting tundish is cast into 180mm×240mm billets under argon protection throughout the entire process. The billets are then fire-cut and pile-cooled. The composition is precisely controlled throughout the entire process to ensure metallurgical quality. The cooled billets are peeled to remove surface defects.

[0044] (3) Heating in a heating furnace: After finishing treatment, the billet is heated to 1100°C in a walking beam heating furnace with the residual oxygen content of the heating furnace ≤3%. The billet is kept warm for 120 minutes before being taken out of the furnace to ensure that the wire rod obtains a low decarburization layer.

[0045] (4) Rolling: The starting rolling temperature is 980℃, the rough rolling temperature is controlled at 980℃, the intermediate rolling temperature and pre-finishing rolling temperature are controlled at 880℃, and the finishing rolling / sizing temperature is controlled at 800℃ to achieve grain refinement by inhibiting recrystallization; the spinning temperature is controlled at 830℃ to ensure that the steel billet remains in a fully austenitized state before entering the salt bath equipment.

[0046] (5) Salt bath cooling: After rolling and spinning, the wire rod is directly cooled in the online salt bath isothermal treatment equipment. The salt bath temperature is between 475℃ (fluctuation range ±1℃). The total cooling time of the wire rod is 700 seconds to ensure sufficient microstructure transformation and make the proportion of lower bainite above 95%. The gas internal circulation stirring is turned on, and after exiting the online salt bath isothermal treatment equipment, it is slowly cooled in Stelmor and then coiled.

[0047] (6) Aging treatment: The coiled wire rods are placed in an aging warehouse at 80°C for 4 days.

[0048] The wire rod prepared according to the above steps was subjected to a tensile test according to GB / T 228.1 "Metallic Materials Tensile Test Part 1: Room Temperature Test Method". The yield strength of the hot-rolled wire rod was 800-860MPa, the tensile strength was 940-997MPa, the through-ring tensile strength fluctuation was 20-45MPa, the elongation was 22-24%, and the cross-sectional reduction was 77-79%. The sample was subjected to a 1 / 8 cold upsetting test according to YB / T5293 "Metallic Materials Upsetting Test Method". Figure 7 As shown in Figure 2, the sample has no cracks. The metallographic structure and electron microscope structure of the sample are as follows: Figure 1 and Figure 2 As shown in the figure, it can be seen that the wire rod structure is a uniform lower bainite structure.

[0049] The wire rod prepared in this embodiment was drawn with a 20% area reduction rate and tested according to the same test standards as above, and the steel wire yield strength was 975-1038MPa, tensile strength was 1058-1127MPa, elongation was 18-20%, cross-sectional shrinkage was 72-73%, and 1 / 7 cold forging did not crack.

[0050] Comparative Example 1

[0051] This comparative example produces a non-tempered cold heading steel wire rod, whose composition is: C: 0.25%, Si: 0.35%, Mn: 1.50%, P: ≤0.015%, S: ≤0.015%, Cr: 0.45%, Ti: 0.030%: B: 0.0030%, N: 0.0038%, [Ti]-3.5*[N]: 0.017%, and the wire rod diameter is 10 mm.

[0052] The process flow is as follows: the main raw materials undergo converter smelting and LF+RH furnace refining to achieve the molten steel composition described above. After RH refining, the molten steel is allowed to stand for 15 minutes and then soft-purged with argon before being sent to a continuous caster to cast 180*240mm² billets. The molten steel is cast into continuous-cast billets using argon protection at a low superheat of 10-30°C throughout the casting process. The billets are then slowly cooled and then subjected to finishing treatment. After treatment, the billets are heated to 1100°C in a heating furnace, held at that temperature for 120 minutes, and then removed from the furnace. The rolling temperature starts at 980°C, with the roughing rolling temperature controlled at 980°C, the intermediate rolling temperature and pre-finishing rolling temperature at 880°C, the finishing rolling / reducing temperature at 800°C, and the spinning temperature at 830°C. After spinning, the billets are cooled in an online salt bath isothermal treatment facility at a salt temperature of 445°C for 700 seconds. Internal gas circulation and stirring are activated. After exiting the online salt bath isothermal treatment facility, they are slowly cooled in Stelmor before being coiled. The coiled wire rods are placed in an aging warehouse and aged at 80°C for 4 days.

[0053] The prepared wire rod was subjected to tensile testing in accordance with GB / T 228.1 "Metallic Materials Tensile Test Part 1: Room Temperature Test Methods". The yield strength of the hot-rolled wire rod was 890-990MPa, the tensile strength was 950-1100MPa, the through-ring tensile strength fluctuation was 35-80MPa, the elongation was 18-21%, and the cross-sectional shrinkage was 65-72%. According to YB / T 5293 "Metallic Materials Upsetting Test Methods", a 1 / 8 cold upsetting test was carried out, and cracks appeared on the sample. Its metallographic structure and electron microscope structure are shown as follows: Figure 3 and Figure 4 As shown in the figure, it can be seen that there is a lot of martensite in the wire rod structure.

[0054] Comparative Example 2

[0055] This comparative example relates to a non-quenched and tempered cold-headed steel wire rod having the following composition: C: 0.25%, Si: 0.35%, Mn: 1.50%, P: ≤ 0.015%, S: ≤ 0.015%, Cr: 0.45%, Ti: 0.030%, B: 0.0030%, N: 0.0038%, [Ti]-3.5*[N]: 0.017%. The wire rod has a diameter of 10 mm.

[0056] The process flow is as follows: the main raw materials undergo converter smelting and LF+RH furnace refining, resulting in molten steel having the composition of Example 1. After RH refining, the molten steel is allowed to stand for 15 minutes and then soft-purged with argon before being sent to a continuous casting machine for casting 180*240 mm². The molten steel is cast into continuous casting billets using argon protection at a low superheat of 10-30°C throughout the casting process. The billets are then slowly cooled and subjected to finishing treatment. After treatment, the billets are heated to 1100°C in a heating furnace, held at that temperature for 120 minutes, and then removed from the furnace. The rolling temperature is initially 980°C, the rough rolling temperature is controlled at 980°C, the intermediate rolling temperature and pre-finishing rolling temperature are controlled at 880°C, the finishing rolling / reducing temperature is controlled at 800°C, and the spinning temperature is 830°C. After spinning, the billets are first cooled in an online salt bath isothermal treatment device with a salt temperature of 495°C and a salt bath time of 700 seconds. Gas internal circulation and stirring are activated. After exiting the online salt bath isothermal treatment device, they are slowly cooled in Stelmor and then coiled. The coiled wire rods are placed in an aging warehouse and aged at 80°C for 4 days.

[0057] The prepared wire rod was subjected to tensile testing according to GB / T 228.1 "Metallic Materials Tensile Test Part 1: Room Temperature Test Methods". The test results showed that the yield strength of the hot-rolled wire rod was 740-890MPa, the tensile strength was 790-920MPa, the through-ring tensile strength fluctuation was 45-120MPa, the elongation was 20-25%, and the cross-sectional shrinkage was 62-71%. According to YB / T 5293 "Metallic Materials Upsetting Test Methods", a 1 / 8 cold upsetting test was carried out and cracks appeared on the sample. Its metallographic structure and electron microscope structure are shown as follows: Figure 5 and Figure 6As shown in the figure, it can be seen that there is more ferrite in the wire rod structure.

[0058] The above description is merely a preferred embodiment of the present invention and does not constitute any other form of limitation to the present invention. Any person skilled in the art may utilize the technical contents disclosed above to change or modify them into equivalent embodiments with equivalent changes for application in other fields. However, any simple modification, equivalent change, and modification of the above embodiments made in accordance with the technical essence of the present invention without departing from the technical solution of the present invention shall still fall within the scope of protection of the technical solution of the present invention.

Claims

1. A 10.9 grade non-quenched and tempered cold heading steel wire rod, characterized in that: Its chemical composition by weight percentage is: C 0.23-0.27%, Si 0.30-0.50%, Mn 1.45-1.55%, P≤0.025%, S≤0.025%, Cr 0.40-0.50%, Ti 0.025-0.050%, B0.0025%-0.0050%, N≤0.0050%, [Ti]-3.5*[N]≥0.015%, and the balance is Fe and unavoidable impurity elements.

2. The 10.9 grade non-quenched and tempered cold heading steel wire rod according to claim 1, characterized in that: Its chemical composition by weight percentage is: C 0.25%, Si 0.35%, Mn 1.50%, P≤0.015%, S≤0.015%, Cr 0.45%, Ti0.030%, B 0.0030%, N 0.0038%.

3. The 10.9 grade non-quenched and tempered cold heading steel wire rod according to claim 1, characterized in that: The lower bainite content in the wire rod structure is ≥95%.

4. A method for producing 10.9 grade non-quenched and tempered cold heading steel wire rod, characterized in that: The following steps are involved: Smelting: The raw materials are sequentially smelted in a converter, refined in LF, and degassed in RH to produce molten steel. The converter smelting process controls the tapping P≤0.015%, S≤0.015%. Slag and some alloys are added along the steel flow during tapping. Continuous casting: The superheat of molten steel is controlled between 10℃ and 30℃. The continuous casting tundish is cast into 180mm×240mm billets under argon protection throughout the entire process. The continuous casting billets are then heat-cut and pile-cooled. Heating furnace: Use walking beam heating furnace for heating, the residual oxygen content of the heating furnace is ≤3%, the temperature of the heating furnace heating section is controlled at 1050-1150℃, and the total heating time is controlled at 100-150min; Rolling: including rough rolling, intermediate rolling, pre-finishing rolling, finishing rolling, and sizing reduction. The rough rolling temperature is controlled at 950-1000℃, the intermediate rolling temperature and pre-finishing rolling temperature are controlled at 850-900℃, the finishing rolling / sizing reduction temperature is controlled at 780-820℃, and the spinning temperature is controlled at 800-850℃. Salt bath cooling: After rolling and spinning, the wire rod directly enters the online salt bath isothermal treatment equipment for cooling. The salt bath temperature is between 460-480℃, and the total cooling time of the wire rod is more than 600 seconds, so that the proportion of lower bainite is more than 95%. A drop section and gas internal circulation stirring are set on the salt bath roller to improve the temperature field and the uniformity of the wire rod.

5. The method for producing 10.9 grade non-quenched and tempered cold heading steel wire rod according to claim 1, characterized in that: After LF refining, the molten steel was allowed to stand and soft-blown with argon for 15 minutes.

6. The method for producing 10.9 grade non-quenched and tempered cold heading steel wire rod according to claim 1, characterized in that: After being heated in a heating furnace, the steel billet is descaled by high-pressure water, and the dephosphorization pressure is ≥18MPa.

7. The method for producing 10.9 grade non-quenched and tempered cold heading steel wire rod according to claim 1, characterized in that: After cooling in the salt bath, the coils are collected and sent to the aging warehouse for aging. The aging time is 3-5 days and the aging temperature is 60-100℃.

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