Cold heading steel wire rod and preparation method and application thereof

Abstract

The present application relates to the field of cold heading steel, and particularly relates to a cold heading steel wire rod, a preparation method and application thereof.The cross section of the wire rod is circular, and the ratio of the sum of the carbon element and the chromium element content at the center to the sum of the carbon element and the chromium element content at 1 / 2 radius is 1.1-1.2:1.The present application overcomes the technical prejudice about segregation, reduces the critical cooling rate of the formation of martensite in the core by controlling the sum of the carbon element and the chromium element in the wire rod to meet a specific segregation ratio, improves the hardenability of the core, so that the wire rod has excellent hardenability in the subsequent quenching process, and the treated wire rod has excellent mechanical properties.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of cold heading steel, in particular to a cold heading steel wire rod and a preparation method and application thereof. BACKGROUND

[0002] Cold heading steel is mainly used for producing fasteners such as bolts and nuts, and is widely used in the automobile, shipbuilding, engineering machinery and other industries. In recent years, with the development of the industry and the progress of science and technology, 12.9 grade high-strength fasteners are increasingly widely used.

[0003] Traditional 12.9 grade high-strength fastener steels are mostly medium-carbon alloy steels, among which medium-carbon CrMo cold heading steel is the representative, and the grades include SCM435, SCM440, ML42CrMo, etc. In such steels, the carbon and alloy content is high, and the cold heading forming performance is poor. Therefore, complete spheroidizing annealing treatment must be carried out before cold heading. The spheroidizing annealing process is time-consuming and consumes a lot of fuel, which is not conducive to low-carbon environmental protection and energy saving requirements.

[0004] Compared with alloy cold heading steel, boron-containing cold heading steel has the advantages of good processing performance and low cost. By using boron-containing cold heading steel, the carbon content in the steel and the content of Cr, Mo and other alloy elements added to improve the hardenability can be reduced, and the same strength can be obtained by replacing them with trace boron and a small amount of alloy, which has good plasticity and cold heading performance. The annealing process of the downstream customer can be omitted or simplified, the manufacturing cost of the fastener can be saved, and it has good development prospects. It is the development trend of future energy-saving cold heading steel. Currently, some manufacturers can produce annealing-free or simplified annealing boron-containing steel wire rods for the production and manufacture of 12.9 grade fasteners.

[0005] With the economic development, the demand for large-size 12.9 grade fastener steel for large-scale engineering, bridges, mechanical equipment and the like is increasing. For large-size fastener steel, hardenability has become a crucial factor for measuring the performance, which directly affects the final organization and performance of the fastener. Since the size of the large-size wire rod is large, its hardenability is often difficult to meet the use requirements.

[0006] The currently disclosed cold heading steel preparation process cannot guarantee the hardenability of the large-size wire rod, and the mechanical properties of the wire rod after quenching and tempering treatment cannot meet the actual application. SUMMARY

[0007] Therefore, the technical problem to be solved by the present application is to overcome the defects in the prior art that the hardenability of the large-size wire rod is poor, and the mechanical properties of the wire rod after treatment are poor, which cannot meet the actual application, so as to provide a cold heading steel wire rod and a preparation method and application thereof.

[0008] The first aspect of the present application protects a cold heading steel wire rod, wherein, based on the mass of the wire rod, C: 0.25-0.30wt%, Si: 0.15-0.30wt%, Mn: 0.35-0.60wt%, P≤0.020wt%, S≤0.015wt%, Cr: 0.70-1.00wt%, Ni: 0.21-0.30wt%, Cu≤0.10wt%, Al: 0.015-0.050wt%, N≤0.007wt%, B: 0.0018-0.0030wt%, Ti: 0.020-0.035wt%, Re: 0.01-0.10wt%, and the balance being Fe and unavoidable impurities.

[0009] The cross section of the wire rod is circular, and the ratio (segregation ratio) of the total content of carbon and chromium elements at the center of the circle to the total content of carbon and chromium elements at 1 / 2 radius is 1.1-1.2:1.

[0010] In the present application, the content of the elements of the wire rod as a whole is designed by a pre-designed composition method, and the specific method is to fine-tune the content of the elements by adding alloys according to the pre-designed results; the content of the elements at different positions is measured by a carbon-sulfur analyzer, an ICP analyzer or an electron probe analyzer.

[0011] In the present application, when the cross section of the wire rod is circular, as shown in the figure, Figure 1 The segregation ratio is the ratio of the content of the elements at position a (the center of the circle) to the content of the elements at position b (1 / 2 radius) (or the same ring as position b).

[0012] In the present application, each element in the wire rod:

[0013] C: plays a solid solution strengthening role to improve the hardenability and strength of the wire rod after quenching and tempering, but too high C content will cause the plasticity and toughness of the wire rod after quenching and tempering to decrease, resulting in poor forming performance of the fastener and increasing the risk of delayed fracture. After comprehensive consideration, the C content is controlled to be 0.25-0.30wt%.

[0014] Si: is a deoxidizing element in steel, which improves the strength of the wire rod after quenching and tempering through solid solution strengthening, but too high Si content will cause the plasticity and toughness of the wire rod after quenching and tempering to decrease, which is not conducive to the forming of the fastener, and will accelerate the decarburization of the steel during the heating process. After comprehensive consideration, the Si content is controlled to be 0.15-0.30wt%.

[0015] Mn: in the present application, it forms a gap solid solution in combination with Cr to constitute cluster strengthening, thereby improving the strength and hardenability of the wire rod after quenching and tempering, but too high Mn content will cause the temper brittleness to rise, thereby reducing the toughness and delayed fracture resistance of the wire rod after quenching and tempering. After comprehensive consideration, the Mn content is controlled to be 0.35-0.60wt%.

[0016] P, S: both are impurity elements, easy to segregate in grain boundary to form low-strength inclusion phase, resulting in the decrease of plasticity and toughness of the wire rod after quenching and tempering, and increase of delayed fracture sensitivity. In comprehensive consideration, the content of P is controlled to be ≤0.020wt%, and the content of S is controlled to be ≤0.015wt%.

[0017] Cr: in the present application, Cr combines with Mn to form interstitial solid solution, constituting cluster strengthening, improving the strength and hardenability of the wire rod after quenching and tempering, and meanwhile, Cr can form chromium carbide with C, improving the anti-tempering softening capacity and delayed fracture resistance of the wire rod after quenching and tempering. However, too high content of Cr will result in high strength of the wire rod after quenching and tempering and deterioration of the performance of fastener forming, and decrease of the plasticity of the wire rod after quenching and tempering. In comprehensive consideration, the content of Cr is controlled to be 0.70-1.00wt%.

[0018] Ni: through solid solution strengthening, the strength of the wire rod after quenching and tempering is improved, the hardenability is enhanced, and the ductile-brittle transition temperature is decreased. However, too high content of Ni will increase the cost of the wire rod after quenching and tempering. In comprehensive consideration, the content of Ni is controlled to be 0.21-0.30wt%.

[0019] Cu: in the present application, Cu is a residual element, which will cause grain boundary embrittlement, increase crack sensitivity, and decrease delayed fracture resistance. The content of Cu is controlled to be ≤0.10wt%.

[0020] Al: as a strong deoxidizing element, Al can improve the oxidation resistance of the wire rod after quenching and tempering, and meanwhile, Al can refine grains and improve the strength and toughness of the wire rod after quenching and tempering. However, excessive Al will cause the number of coarse carbonitride inclusions to increase, and the delayed fracture resistance of the wire rod after quenching and tempering to decrease. In comprehensive consideration, the content of Al is controlled to be 0.015-0.050wt%.

[0021] N: has solid solution strengthening effect, but excessive N will cause the steel to produce aging brittleness, and decrease the plasticity and bolt forming performance of the wire rod after quenching and tempering. The content of N is controlled to be ≤0.007wt%.

[0022] B: can improve the hardenability and delayed fracture resistance of the wire rod after quenching and tempering, but excessive B will cause the toughness of the wire rod after quenching and tempering to decrease. In comprehensive consideration, the content of B is controlled to be 0.0018-0.0030wt%.

[0023] Ti: mainly plays a role of fixing nitrogen, to ensure the hardenability of boron. However, too high Ti will increase the number of inclusions in the steel, form fatigue crack source, and cause the deterioration of bolt forming performance. In combination with the content of N in the steel and the effect of Al and Si in fixing nitrogen, in comprehensive consideration, the content of Ti is controlled to be 0.020-0.035wt%.

[0024] Re: in order to reduce harmful elements such as P, S and impurity elements in grain boundary, change the composition, morphology, distribution and the like of inclusions in steel, improve the low-temperature toughness and fatigue performance of the wire rod after quenching and tempering, reduce the temper brittleness, and refine the grain size. Comprehensive consideration, the content of Re is controlled to be 0.01-0.10wt%.

[0025] According to the application, the wire rod is Fe and inevitable impurities except the above elements.

[0026] According to the application, the diameter of the wire rod is 20-40mm.

[0027] In the application, the Cr element and the Mn element form an interstitial solid solution in the steel, and the formation of clusters can further improve the tensile strength and hardenability of the boron-containing cold upsetting wire rod after quenching and tempering. According to related experimental research, the content of the Cr element and the content of the Mn element satisfy the formula Cr / Mn=1.5-2.0:1.

[0028] In the application, the Ni element, the Ti element and the Al element combine to form a nano precipitate phase, which further improves the strength of the wire rod and forms a strong hydrogen trap, thereby improving the resistance to delayed fracture of the wire rod after quenching and tempering. According to related experimental research, the content of the Ni element, the content of the Ti element and the content of the Al element satisfy the formula Ni / (Ti+Al)=3.5-5:1.

[0029] In the application, according to the contribution of each alloying element to the hardenability of the boron-containing cold upsetting steel, comprehensive consideration is given to meet the hardenability of the large-size wire rod, and the content of the C element, the content of the Cr element, the content of the Mn element, the content of the Ni element and the content of the B element satisfy the formula (35xC+13xCr+10xMn+3xNi+8000xB) x100≥43.

[0030] The second aspect of the application protects a preparation method of the foregoing cold upsetting steel wire rod, wherein the preparation method comprises the following steps:

[0031] The molten iron is smelted, refined, continuously cast and rolled to obtain the wire rod.

[0032] In the continuous casting, the overheat degree of the tundish molten steel is 30-40℃, and the drawing speed of the drawing and straightening is 2.5-2.7m / min.

[0033] According to the application, the smelting is converter smelting, and the refining is LF refining.

[0034] Converter smelting: the hot metal enters the converter for oxygen blowing smelting, the molten steel at the end of converter smelting has C 0.08-0.20wt%, P≤0.015wt%, the tapping temperature at the end of converter smelting is 1620-1660℃, after 25-30% of the molten steel is tapped, aluminum ingot, ferrosilicon, low-carbon ferromanganese, high-carbon ferrochrome, ferro-nickel and lime are sequentially added for deoxidation and alloying, argon is bottom-blown in the ladle throughout the tapping process, the argon flow rate is controlled to be 650-800L / min in the middle of the tapping before the tapping, and the argon flow rate is controlled to be 300-450L / min after 70-80% of the molten steel is tapped.

[0035] LF refining: after the converter molten steel enters the LF station, lime and fluorite are sequentially added for white slag adjustment, after the white slag is refined for 5-10min, ferrotitanium, ferroboron and rare earth iron alloy are sequentially added for alloying, the final slag basicity in the refining is controlled to be 3.5-5, the argon flow rate in the ladle bottom blowing is controlled to be 300-450L / min during the alloying; then the temperature is raised to 1550-1565℃ by electric heating, and a sample is taken for chemical composition detection, according to the detection result, the chemical composition is finely adjusted by adding alloy, the argon flow rate in the ladle bottom blowing is controlled to be 150-300L / min during the fine adjustment of the chemical composition; before soft stirring, calcium wire is fed into the molten steel, the calcium wire addition amount is 0.8-1.0m / t, the calcium wire feeding speed is 170-200m / min, the soft stirring time is 25-35min, and the argon flow rate in the bottom blowing is 30-80L / min during the soft stirring; the tapping temperature at the end of the LF refining is controlled to be 1570-1595℃.

[0036] Continuous casting: after the LF refining is completed, the continuous casting is performed, the long nozzle argon sealing of the large ladle, the alkali covering agent of the tundish and the submerged nozzle are adopted for full protection pouring, the low-carbon steel protection slag is adopted; the superheat degree of the molten steel in the tundish is controlled to be 30-40℃, the liquid level height of the crystallizer is controlled to be 80-85% of the height of the crystallizer, the liquid level fluctuation range is within ±3%, the electromagnetic stirring current of the crystallizer is 350-500A, the stirring frequency is 1.5-2Hz, the specific water consumption of the secondary cooling is controlled to be 0.8-1.0L / kg, and the casting speed of the straightening is 2.5-2.7m / min.

[0037] In the application, the casting blank is obtained after the continuous casting, and the cross-sectional size of the casting blank is mainly determined by the equipment, when the cross-section of the casting blank is circular, the diameter of the circle is 140-160mm; when the cross-section of the casting blank is square, the side length of the square is 140-160mm.

[0038] According to the application, the rolling includes heating, high-speed rolling and controlled cooling.

[0039] Specifically as follows:

[0040] Heating: the heating process includes a heating section and a soaking section, the temperature of the heating section is 1050-1090℃, the holding time is 30-40min; the temperature of the soaking section is 1080-1120℃, the holding time is 15-25min, and the air-fuel ratio of the heating furnace is controlled to be 0.60-0.65:1. Through the above-mentioned heating section temperature and heating section holding time setting, the alloy elements can be further fully dissolved in the austenite, the temperature stress in the rolling process is avoided to be too large to produce cracks, at the same time, the rolling production rhythm is improved, the uniformity of heating is ensured, the oxidation and decarburization are reduced as much as possible, and the defects such as overheating and overburning are prevented. Through the above-mentioned soaking section temperature and soaking section holding time setting, the temperature difference between the surface and the core of the wire rod can be further reduced, the steel temperature is homogenized, and the internal organization and performance of the billet can be improved.

[0041] High-speed wire rolling: the open rolling temperature is controlled to be 960-1000℃, the finish rolling inlet temperature is 880-910℃, three-section water tank cooling is carried out, and the three-section water tank flow ratio is (5±0.5):(3±0.5):(2±0.5). Through the above-mentioned open rolling temperature and finish rolling temperature setting, the brittle temperature range of the wire rod can be further avoided, the wire rod has good organization and performance, the water tank water amount is reasonably distributed, the temperature difference between the surface and the core of the wire rod is reduced, and the wire rod organization uniformity is improved.

[0042] Controlled cooling: the coiling temperature is controlled to be 840-870℃, the first 1-2 air blowers are opened, the air blower air volume is controlled to be 70-100%, the remaining air blowers are all closed, and the heat preservation cover is all closed. Through the above-mentioned controlled cooling process setting, the wire rod microstructure uniformity can be further controlled, and fine equiaxed ferrite and pearlite organization is formed.

[0043] The third aspect of the present application protects the application of the wire rod prepared by the above-mentioned preparation method or the above-mentioned wire rod after the quenching and tempering treatment in the 12.9 grade fastener.

[0044] In the present application, the quenching and tempering treatment is a conventional quenching and tempering treatment, first quenching, the quenching temperature is 850-880℃, the quenching holding time is 30-60min, then tempering treatment, the tempering temperature is 510-530℃, and the tempering holding time is 80-120min.

[0045] The technical scheme of the present application has the following advantages:

[0046] 1. The present application provides a kind of cold heading steel wire rod, wherein, based on the mass of wire rod, C: 0.25-0.30wt%, Si: 0.15-0.30wt%, Mn: 0.35-0.60wt%, P≤0.020wt%, S≤0.015wt%, Cr: 0.70-1.00wt%, Ni: 0.21-0.30wt%, Cu≤0.10wt%, Al: 0.015-0.050wt%, N≤0.007wt%, B: 0.0018-0.0030wt%, Ti: 0.020-0.035wt%, Re: 0.01-0.10wt%, the rest is Fe and inevitable impurities;The cross section of the wire rod is circular, and the ratio of the sum of carbon element and chromium element content at the center of the circle to the sum of carbon element and chromium element content at 1 / 2 radius is 1.1-1.2:1.The present application overcomes the technical prejudice about segregation, unlike the prior art which wants to reduce the segregation ratio, the present application instead controls the sum of carbon element and chromium element in the wire rod to meet a specific segregation ratio, reduces the critical cooling rate of martensite formation in the core of the wire rod, improves the hardenability of the core, so that the wire rod has excellent hardenability in the subsequent quenching process, and the treated wire rod has excellent mechanical properties.

[0047] 2, The present application limits the specific size of the wire rod, which proves that the present application can be applied to treat large-size wire rods, and although the size is large, it can still be quenched completely with good hardenability, thereby improving the reliability of the wire rod as a fastener, reducing the risk of failure, and further improving the quality performance of large-size cold heading steel wire rods.

[0048] 3, The preparation method of the present application comprises the following steps: molten iron is prepared into a casting blank after smelting, refining and continuous casting, and the casting blank is rolled into a wire rod;Among them, in continuous casting, the superheat of tundish molten steel is 30-40 DEG C, and the drawing speed of drawing straightening is 2.5-2.7 m / min.In the present application, by controlling the specific superheat, the specific drawing speed of drawing straightening, the wire rod center is intentionally made to have certain segregation, so that the critical cooling rate of martensite formation in the core is reduced, and the hardenability of the core is improved, so that the treated wire rod has excellent mechanical properties.

[0049] 4, The wire rod obtained by the present application has excellent hardness and tensile properties after quenching and tempering treatment, and can meet the application in 12.9 grade fastener steel. BRIEF DESCRIPTION OF DRAWINGS

[0050] In order to more clearly illustrate the technical solutions in the specific embodiments of the present application or the prior art, the following will briefly introduce the drawings needed to be used in the description of the specific embodiments or the prior art. Obviously, the drawings in the following description are some embodiments of the present application, and other drawings can be obtained by those skilled in the art without any creative effort on the basis of these drawings.

[0051] Figure 1 is a schematic diagram of the cross section of the wire rod of embodiment 1 of the present application.

[0052] Reference signs:

[0053] a - center of circle; b - 1 / 2 radius. DETAILED DESCRIPTION

[0054] The following examples are provided to better further understand the present application and are not limited to the best mode, and do not constitute a limitation on the content and scope of protection of the present application. Any person under the inspiration of the present application or the combination of the present application with other prior art features will fall within the scope of protection of the present application.

[0055] The specific experimental steps or conditions not mentioned in the examples can be carried out according to the conventional experimental steps described in the literature in the art or the operation or conditions. The reagents or instruments not mentioned by the manufacturer are all conventional reagent products that can be obtained by purchase.

[0056] Preparation example

[0057] The present preparation example provides a cold heading steel casting blank and a preparation method thereof, and the specific steps are as follows:

[0058] Embodiments 1-10 and comparative examples 1-10 are all operated according to this step, and the parameters with differences are shown in Table 1.

[0059] Converter smelting: the molten iron enters the converter for oxygen blowing smelting, and the C and P contents in the molten steel at the end of the converter smelting are shown in Table 1, and the tapping temperature at the end of the converter smelting is shown in Table 1. After 25% of the tapping, aluminum ingots, ferrosilicon, low-carbon manganese iron, high-carbon chromium iron, nickel iron and lime are sequentially added for deoxidation and alloying, and the dosages of various components are controlled. The argon is blown at the bottom of the ladle throughout the tapping process, the argon flow rate is controlled at 700 L / min in the middle of the tapping before the tapping, and the argon flow rate is controlled at 400 L / min after 75% of the tapping.

[0060] LF refining: after the converter liquid steel enters the LF station, lime and fluorite are added in sequence to adjust the white slag, and after white slag refining for 8 min, titanium iron, boron iron and rare earth iron alloy are added in sequence for alloying, the final slag basicity of refining is shown in Table 1, and the argon flow rate of the ladle bottom blowing is 400 L / min during alloying; then the temperature is raised to 1560℃ by electric heating, and the chemical composition is detected, and the chemical composition is adjusted according to the detection results, and the argon flow rate of the ladle bottom blowing is 250 L / min during the chemical composition adjustment; before soft stirring, calcium wire is fed into the molten steel, the calcium wire addition amount is 0.8 m / t, the calcium wire feeding speed is 180 m / min, the soft stirring time is 30 min, and the argon flow rate of the bottom blowing is 60 L / min during soft stirring; the tapping temperature of the LF refining endpoint is shown in Table 1.

[0061] Continuous casting: after the LF refining, the continuous casting is carried out, the argon sealing of the large ladle long nozzle, the alkaline tundish covering agent and the submerged nozzle are used for full protection pouring, and the low carbon steel protection slag is used; the molten steel superheat degree of the tundish is shown in Table 1, the height of the crystallizer liquid surface is 83% of the height of the crystallizer, the liquid surface fluctuation range is within ±3%, the electromagnetic stirring current of the crystallizer is 350 A, the stirring frequency is 1.8 Hz, the specific water quantity of the secondary cooling is 0.8 L / kg, and the casting speed of the straightening is shown in Table 1.

[0062] The cast blank is obtained by rolling, and the rolling process specifically includes heating, high line rolling and controlled cooling.

[0063] Heating: the heating process includes a heating section and a soaking section, the temperature of the heating section is 1065℃, the holding time is 35 min, the temperature of the soaking section is 1096℃, the holding time is 23 min, and the air-fuel ratio of the heating furnace is controlled to be 0.62:1. Through the above-mentioned heating section temperature and heating section holding time setting, the alloying elements can be further fully dissolved in the austenite, the temperature stress in the rolling process is avoided to be too large to produce cracks, the rolling production rhythm is improved, the uniformity of heating is ensured, the oxidation and decarburization are reduced as much as possible, and defects such as overheating and overburning are prevented. Through the above-mentioned soaking section temperature and soaking section holding time setting, the temperature difference between the surface and the core of the cast blank can be further reduced, the steel temperature is homogenized, and the internal organization and performance of the steel blank can be improved.

[0064] High line rolling: the open rolling temperature is controlled to be 984℃, the finish rolling inlet temperature is 891℃, three-section water tank cooling is carried out, the first section water tank flow rate is 2141 L / min, the second section water tank flow rate is 1264 L / min, the third section water tank flow rate is 857 L / min, and the water tank flow rate ratio is 4.9:2.9:2. Through the above-mentioned open rolling temperature and finish rolling temperature setting, the brittle temperature range of the wire rod of the present application can be further avoided, the wire rod has good organization and performance, the water tank water quantity is reasonably distributed, the temperature difference between the surface and the core of the wire rod is reduced, and the uniformity of the wire rod organization is improved.

[0065] Controlled cooling: the coiling temperature was controlled at 849 ℃, the first fan was opened, the air volume was controlled at 80%, the second fan was opened, the air volume was controlled at 80%, the rest of the fans were all closed, and the heat preservation cover was all closed, to obtain the rod.

[0066] Table 1

[0067] C content in molten steel / % P content in molten steel / % Tapping temperature at the end of converter smelting / °C Refining final slag basicity Tapping temperature at the end of LF refining / °C Superheat of molten steel in tundish / °C Drawing speed of straightening / (m / min) Example 1 0.12 0.012 1635 3.98 1583 32 2.5 Example 2 0.13 0.007 1642 4.22 1592 31 2.6 Example 3 0.18 0.008 1639 4.67 1577 39 2.7 Example 4 0.09 0.01 1651 4.34 1590 35 2.6 Example 5 0.15 0.01 1643 4.51 1582 33 2.5 Example 6 0.12 0.006 1638 4.76 1573 34 2.6 Example 7 0.11 0.01 1657 4.83 1586 35 2.6 Example 8 0.12 0.011 1634 3.62 1587 36 2.5 Example 9 0.14 0.007 1646 4.15 1582 33 2.6 Example 10 0.11 0.009 1641 4.37 1591 34 2.5 Comparative Example 1 0.08 0.01 1625 4.42 1574 37 2.7 Comparative Example 2 0.12 0.008 1646 4.23 1585 36 2.5 Comparative Example 3 0.13 0.01 1635 3.76 1579 35 2.6 Comparative Example 4 0.09 0.011 1648 4.19 1583 32 2.5 Comparative Example 5 0.12 0.01 1637 4.34 1581 35 2.6 Comparative Example 6 0.17 0.01 1628 4.05 1584 34 2.7 Comparative Example 7 0.15 0.011 1647 4.72 1576 35 2.5 Comparative Example 8 0.16 0.01 1639 3.95 1594 38 2.6 Comparative Example 9 0.18 0.011 1649 4.87 1588 32 2.5 Comparative Example 10 0.13 0.009 1634 4.64 1573 35 2.7

[0068] The segregation ratio of the sum of carbon element and chromium element in the rod is shown in Table 2.

[0069] Table 2

[0070] Segregation ratio Example 1 1.13 Example 2 1.15 Example 3 1.17 Example 4 1.16 Example 5 1.14 Example 6 1.11 Example 7 1.14 Example 8 1.12 Example 9 1.14 Example 10 1.15 Comparative Example 1 1.01 Comparative Example 2 0.99 Comparative Example 3 1.05 Comparative Example 4 1.02 Comparative Example 5 1.03 Comparative Example 6 0.98 Comparative Example 7 1.03 Comparative Example 8 0.97 Comparative Example 9 1.02 Comparative Example 10 1.04

[0071] The content of the whole element in the rod is designed by the method of pre-designed composition, and the specific method is that according to the pre-designed results and the test results in Table 3, the content of the element is fine-tuned by adding alloy, and the specific data of each element in the round rod is shown in Table 3.

[0072] Table 3

[0073]

[0074] Table 3 continued

[0075] Example 6 Example 7 Example 8 Example 9 Example 10 C 0.25 0.3 0.28 0.27 0.26 Si 0.2 0.22 0.23 0.17 0.26 Mn 0.37 0.59 0.57 0.58 0.53 P 0.008 0.012 0.013 0.009 0.011 S 0.004 0.008 0.009 0.01 0.006 Cr 0.72 0.93 0.98 0.92 0.97 Ni 0.28 0.27 0.23 0.21 0.23 Cu 0.03 0.03 0.02 0.04 0.03 Al 0.033 0.028 0.031 0.029 0.025 N 0.0046 0.0061 0.0058 0.0059 0.0063 B 0.0028 0.0019 0.0022 0.0027 0.002 Ti 0.024 0.032 0.031 0.028 0.035 Re 0.02 0.06 0.08 0.05 0.02 Cr / Mn 1.95 1.58 1.72 1.59 1.83 Ni / (Ti+Al) 4.91 4.50 3.71 3.68 3.83 (35xC+13xCr+10xMn+3xNi+8000xB)xlOO 45.05 44.50 46.53 49.44 43.70

[0076] Table 3 continued

[0077] Comparative Example 1 Comparative Example 2 Comparative Example 3 Comparative Example 4 Comparative Example 5 C 0.19 0.28 0.27 0.28 0.27 Si 0.25 0.23 0.18 0.25 0.27 Mn 0.5 0.54 0.72 0.46 0.58 P 0.012 0.01 0.012 0.013 0.012 S 0.007 0.005 0.006 0.005 0.007 Cr 0.98 0.53 0.96 1.31 0.74 Ni 0.25 0.22 0.21 0.23 0.24 Cu 0.02 0.03 0.02 0.02 0.02 Al 0.037 0.026 0.032 0.037 0.033 N 0.0054 0.0062 0.0051 0.006 0.0058 B 0.0023 0.0026 0.0021 0.0022 0.0026 Ti 0.027 0.032 0.026 0.023 0.028 Re 0.03 0.04 0.02 0.05 0.04 Cr / Mn 1.96 0.98 1.33 2.85 1.28 Ni / (Ti+Al) 3.91 3.79 3.62 3.83 3.93 (35xC+13xCr+10xMn+3xNi+8000xB)xlOO 43.54 43.55 46.56 49.72 46.39

[0078] Table 3 continued

[0079] Comparative Example 6 Comparative Example 7 Comparative Example 8 Comparative Example 9 Comparative Example 10 C 0.38 0.26 0.27 0.29 0.28 Si 0.24 0.21 0.26 0.25 0.22 Mn 0.53 0.4 0.49 0.51 0.56 P 0.014 0.013 0.012 0.014 0.011 S 0.005 0.004 0.009 0.006 0.005 Cr 0.97 0.76 0.93 0.94 0.96 Ni 0.25 0.23 0.28 0.25 0.28 Cu 0.03 0.02 0.02 0.02 0.03 Al 0.038 0.041 0.025 0.034 0.045 N 0.0064 0.0049 0.0055 0.0066 0.0052 B 0.0023 0.002 0.0024 0.0022 0.0025 Ti 0.024 0.023 0.052 0.032 0.022 Re 0.06 0.05 0.03 0.07 0.04 Cr / Mn 1.83 1.90 1.90 1.84 1.71 Ni / (Ti+Al) 4.03 3.59 3.64 3.79 4.18 (35xC+13xCr+10xMn+3xNi+8000xB)xlOO 50.36 39.67 46.48 45.82 48.72

[0080] The rods of Examples 1-10 and Comparative Examples 1-10 were further subjected to a quenching and tempering treatment.

[0081] The specific steps of the quenching and tempering treatment were as follows: the rod was subjected to quenching treatment, the quenching temperature was 870 ℃, the quenching holding time was 40 min, the tempering temperature was 530 ℃, and the tempering holding time was 80 min, to obtain the rod after quenching and tempering treatment.

[0082] According to GB / T 230.1-2018, the Rockwell hardness (HRC) of the cross section and surface of the rod after quenching and tempering treatment was detected, and the specific results are shown in Table 4.

[0083] Table 4

[0084]

[0085] After the quenching and tempering treatment, the Rockwell hardness of the cross section center of the example is greater than or equal to 37.5HRC, the Rockwell hardness of the cross section 1 / 2 radius is greater than or equal to 40.8HRC, the Rockwell hardness of the cross section near the surface 2mm is greater than or equal to 42HRC, and the Rockwell hardness of the surface of the rod is greater than or equal to 43HRC. As can be seen from the example data, the center also has high hardness, indicating that the depth of the hardness value is large, which can prove that the example has high hardenability; the cross section hardness difference of the example is less than or equal to 5HRC, and the cross section hardness difference of the comparison example is greater than 10HRC. The hardenability of the example is obviously improved compared with the comparison example, and the surface hardness of the comparison example 1 and the comparison example 2 is less than 39HRC, and the service life is not as good as the example.

[0086] The segregation ratio in the comparison example does not meet the range defined in the application, the composition difference between the core and the matrix of the rod is small, the hardenability is poor, and the hardness of the core obtained finally is low. The cross section hardness difference is large. Compared with the comparison example, the example has excellent hardenability, and at the same time has high strength and excellent delayed fracture resistance, and the comprehensive performance is excellent.

[0087] The tensile strength, yield strength and reduction of area of the rod after the quenching and tempering treatment are detected at 25 DEG C according to GB / T 228.1-2021;

[0088] The notched sample is processed according to GB / T 39039-2020; the sample is subjected to electrochemical hydrogen charging in 1000ml of 4g / L NaOH solution, the hydrogen charging time is 48h, and the current density is 15A / m 2 ; after the hydrogen charging sample is cleaned and dried, the 25 DEG C tensile test is carried out on a slow strain rate tensile testing machine, the tensile strain rate is 5x10 -6 / s, the tensile strength of the hydrogen charged notched sample is measured; the same parameters are used for the test of the unhydrogenated notched sample, and the tensile strength of the unhydrogenated notched sample is obtained; the delayed fracture strength ratio is calculated, and the delayed fracture strength ratio is the ratio of the tensile strength of the hydrogen charged notched sample to the tensile strength of the unhydrogenated notched sample;

[0089] The specific results are shown in Table 5.

[0090] Table 5

[0091] Tensile strength / MPa Yield strength / MPa Elongation / % Delayed fracture strength ratio Example 1 1289 1182 41 0.85 Example 2 1265 1164 42.3 0.84 Example 3 1278 1173 42.1 0.85 Example 4 1324 1215 40.2 0.83 Example 5 1267 1170 40.4 0.86 Example 6 1271 1172 41.8 0.85 Example 7 1264 1163 42.6 0.86 Example 8 1292 1189 41.5 0.84 Example 9 1343 1242 40.7 0.82 Example 10 1280 1175 41.8 0.83 Comparative Example 1 1127 1020 42.3 0.84 Comparative Example 2 1118 1029 42.6 0.85 Comparative Example 3 1271 1158 35.5 0.76 Comparative Example 4 1379 1268 34.2 0.75 Comparative Example 5 1136 1035 42.7 0.86 Comparative Example 6 1325 1196 33.5 0.73 Comparative Example 7 1179 1074 42.2 0.84 Comparative Example 8 1238 1131 36.4 0.75 Comparative Example 9 1183 1080 39 0.82 Comparative Example 10 1262 1165 37.7 0.79

[0092] In the application, when the tensile strength is greater than or equal to 1220MPa, it represents that the strength of the prepared fastener is greater than or equal to 12.9 grade, and the data in the example indicates that the rod prepared by the application can be applied in the 12.9 grade fastener steel after the quenching and tempering treatment.

[0093] The greater the delay fracture strength ratio is, the better the delay fracture resistance is. Generally, the delay fracture strength ratio is considered excellent when it is greater than or equal to 0.8, and the examples of the present application all meet this data.

[0094] The wire rod of the examples of the present application not only has good tensile properties and high strength after the quenching and tempering treatment, but also has excellent delay fracture resistance, which meets the strength requirement of 12.9-grade fasteners. The tensile properties of the wire rods of the comparative examples 1, 2, 5, 7 and 9 do not meet the strength requirement of 12.9-grade fasteners after the quenching and tempering treatment, and the delay fracture resistance of the wire rods of the comparative examples 3, 4, 6, 8 and 10 is poor after the quenching and tempering treatment.

[0095] Obviously, the above examples are only examples for clearly illustrating but not limitation to the embodiments. Other different forms of changes or variations can be made by those skilled in the art on the basis of the above description. All the embodiments do not need to be exhausted here. The obvious changes or variations derived therefrom are still within the protection scope of the present application.

Claims

1. A cold heading steel wire rod, characterized in that: Based on the mass of the wire rod, C: 0.25-0.30wt%, Si: 0.15-0.30wt%, Mn: 0.35-0.60wt%, P≤0.020wt%, S≤0.015wt%, Cr: 0.70-1.00wt%, Ni: 0.21-0.30wt%, Cu≤0.10wt%, Al: 0.015-0.050wt%, N≤0.007wt%, B: 0.0018-0.0030wt%, Ti: 0.020-0.035wt%, Re: 0.01-0.10wt%, and the rest are Fe and unavoidable impurities; The cross section of the wire rod is circular, and the ratio of the sum of the carbon and chromium contents at the center of the circle to the sum of the carbon and chromium contents at 1 / 2 radius is 1.1-1.2:1; The content of the Cr element and the content of the Mn element satisfy the formula Cr / Mn=1.5-2.0:1; The content of the Ni element, the content of the Ti element, and the content of the Al element satisfy the formula Ni / (Ti+Al)=3.5-5:1; The content of the C element, the content of the Cr element, the content of the Mn element, the content of the Ni element, and the content of the B element satisfy the formula (35×C+13×Cr+10×Mn+3×Ni+8000×B)×100≥43.

2. The wire rod according to claim 1, characterized in that The diameter of the wire rod is 20-40 mm.

3. A method for preparing cold heading steel wire rod according to claim 1 or 2, characterized in that: The preparation method comprises the following steps: The molten iron is smelted, refined, continuously cast and rolled to obtain wire rod; Among them, during continuous casting, the superheat of the molten steel in the tundish is 30-40°C, and the drawing speed for straightening is 2.5-2.7m / min.

4. The preparation method according to claim 3, characterized in that The smelting is converter smelting.

5. The preparation method according to claim 3 or 4, characterized in that The refining is LF refining.

6. The preparation method according to claim 3, characterized in that The steel rolling includes heating, high-speed wire rolling and controlled cooling.

7. Use of the wire rod according to claim 1 or 2 or the wire rod produced by the method according to any one of claims 3 to 6 after quenching and tempering in 12.9 grade fastener steel.

8. The use according to claim 7, characterized in that The quenching and tempering treatment is: quenching the wire rod and then tempering it.

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