Annealing-free alloy cold heading steel wire rod for 12.9-grade bolt and production method of annealing-free alloy cold heading steel wire rod

Abstract

The invention relates to an annealing-free alloy cold heading steel wire rod for a 12.9-grade bolt and a production method of the annealing-free alloy cold heading steel wire rod. The production process comprises the steps of molten iron pre-removal of P / S, electric furnace or converter smelting, external refining, bloom continuous casting, rolling into an intermediate billet and steel billet finishing, intermediate billet heating, high-pressure water descaling, rough rolling, intermediate rolling, pre-finish rolling, finish rolling, spinning, coil collecting and cooling. The annealing-free alloy cold forging steel wire for the 12.9-grade bolt comprises the following components in percentage by mass: 0.30%-0.35% of C, 0.10%-0.30% of Si, 0.70%-1.20% of Mn, 1.0%-1.5% of Cr, 0.15%-0.30% of Mo, 0.015%-0.030% of Al, 0.0030%-0.0080% of N, 0.01%-0.04% of Nb, less than or equal to 0.30% of Ni, less than or equal to 0.10% of Cu, less than or equal to 0.015% of P, less than or equal to 0.010% of S, less than or equal to 0.012% of (As + Sb + Bi), less than or equal to 0.0010% of O and the balance of Fe and inevitable impurity elements, the niobium-nitrogen product (Nb * N) is greater than On the premise that main alloy components are not reduced, a fine grain structure can be obtained under a certain condition by adding grain refining elements, and the delayed phase change effect the same as that of a low-temperature finish rolling process is achieved by utilizing the delayed phase change characteristic of the fine grain structure.

Description

Technical Field

[0001] The invention relates to an annealing-free alloy cold heading steel wire for 12.9-grade bolts and a production method thereof, belonging to the technical field of metal materials. Background Art

[0002] The tensile strength of alloy cold heading steel SCM435 and SCM440 hot rolled wire rods is generally above 1000MPa, which means that the wire rod products cannot be directly drawn during the subsequent wire rod reforming process of downstream users. They must be annealed once before the next processing, which affects production efficiency. The annealing process costs about 500 yuan per ton, which increases energy loss and also causes carbon dioxide emissions. In order to save costs, most steel mills are developing alloy steel hot rolled wire rods that do not require annealing. However, most annealing-free products have low alloy composition, and the hardenability cannot be guaranteed, so high-strength bolts cannot be produced. Only a very small number of alloy steel wire rods for high-strength bolts have achieved annealing-free technology abroad. The process is unstable and the product quality cannot be ensured. There are no mature batches of annealing-free CrMo steel hot rolled products in China.

[0003] The production process of cold heading steel hot rolled wire rod includes: electric furnace or converter smelting → refining outside the furnace → continuous casting → billet finishing and pre-rolling → wire rolling and other processes. Through electric furnace or converter smelting and refining outside the furnace, molten iron with qualified composition is obtained, and the smelted molten iron is continuously cast into billets, and the billets after continuous casting are rolled into wire rods. The wire rolling process is specifically divided into billet heating → high-pressure water descaling → rough rolling → intermediate rolling → pre-finishing rolling → finishing rolling → wire laying → cooling → coiling and other processes. Some continuous casting billets are large and need to be opened into intermediate billets before wire rolling.

[0004] Design features of annealing-free products: There are roughly two types of annealing-free processes at present: the first is to design annealing-free steel by reducing the content of alloy elements or martensite-forming elements, such as reducing Mo and Mn elements, adding Cr and B elements instead of Mo elements, etc. This design can achieve the purpose of annealing-free, but the upper limit of bolt strength is too low, and large-sized or high-strength bolts cannot be produced. The second is to use low-temperature finishing rolling for alloy steel to avoid the generation of hard phases such as bainite and martensite. Low-temperature finishing rolling (720℃~800℃) accounts for a large proportion of the currently disclosed patent documents for annealing-free steel. The second method is actually difficult to promote, because under the current rolling equipment conditions, low-temperature finishing rolling is difficult to achieve in actual production.

[0005] For example, some of the following patent disclosures use one of the methods, while others use a combination of the two methods.

[0006] The patent application with publication number CN 103060676 A discloses an annealing-free wire rod for grade 10 high-strength nuts and a production method thereof. The alloy composition is controlled at a relatively low level, the final rolling temperature is controlled between 720°C and 800°C, the spinning temperature is controlled between 730°C and 810°C, and the hot-rolled wire rod is cooled at a speed of 3°C / s to 10°C / s to undergo a phase change in the range of 720°C to 660°C.

[0007] The patent application with patent application number CN202110031191.8 discloses an annealing-free high-strength cold heading steel and its manufacturing method. Chemical elements: C: 0.32%~0.35%, Si: 0.1%~0.3%, Mn: 0.60%~0.7%, Cr: 0.15%~0.19%, Ti: 0.01%~0.03%, B: 0.0015%~0.003%, N≤0.006%. In the hot rolling step: control the finishing temperature to 780~830℃, the sizing temperature to 780~810℃, and the high-wire spinning temperature to 760~800℃; the wire rod after spinning is cooled in the insulation cover, and the cooling rate is less than 1.5℃ / s.

[0008] The patent application with the publication number CN1799717A discloses a production method of hot-rolled anneal-free wire rod for 8.8-grade high-strength cold heading steel. The finishing rolling stage adopts a hot machine rolling process, which is characterized in that after pre-finishing rolling, the rolled piece is restored by water cooling and long channel, and the temperature is reduced to a low temperature zone of 730-830°C before entering the finishing mill, and the steel is deformed by a large amount in the two-phase (γ+α) zone; the cumulative reduction rate is 35%-82%, and then the rolled piece is quickly water-cooled to 800-620°C for phase transformation, and the holding time is 600-2400 seconds, and then naturally cooled to room temperature.

[0009] The above patent literature technology mentions the use of low-temperature finishing rolling process for alloy cold heading steel. This low-temperature finishing rolling process and finishing rolling large deformation process are difficult to achieve in actual production. The temperature of the steel before finishing rolling can generally reach more than 1000°C. The current rolling mill speed is very fast, and the water cooling time before finishing rolling is only about 1s. It is impossible to cool the rolled piece to 720°C to 800°C through one or more groups of water tanks in a short time. Therefore, the low-temperature finishing rolling process is difficult to achieve in conventional wire rolling unless a large amount of funds and manpower are invested to redesign and transform the production line. In addition, for the low-temperature finishing rolling process, because the temperature of the steel piece is low and the strength is increased, the finishing mill requires a larger equipment power to roll normally, and the rolling mill capacity needs to be improved. It is generally believed that steel has poor plasticity at low temperatures and better plasticity at high temperature rolling. When rolling at low temperatures, the finished product is prone to hairline or crack defects, which affects the subsequent cold heading performance. Summary of the invention

[0010] In order to solve the above-mentioned problems, the present invention discloses a 12.9 grade bolt annealing-free alloy cold heading steel wire and its production method. Under the premise of not reducing the main alloy components, by adding grain refining elements, a fine grain structure can be obtained under certain conditions, and the characteristic of delayed phase transformation of the fine grain structure is utilized to achieve the same delayed phase transformation effect as the low temperature finishing rolling process. The present invention can be realized without major technical changes to the existing production line. The specific technical scheme is as follows:

[0011] A production method for 12.9 grade annealing-free alloy cold heading steel wire for bolts, comprising: pre-desulfurization of molten iron → smelting in an electric furnace or converter → refining outside the furnace → continuous casting of large square billets → rolling into intermediate billets → heating of intermediate billets → descaling with high-pressure water → rough rolling → intermediate rolling → pre-finishing rolling → finishing rolling → spinning → coiling → cooling. The specific production process is:

[0012] Step 1: Pre-desulfurization of hot metal: Pre-desulfurization is carried out in a hot metal ladle. CaO is added by spraying powder in an amount of 6-10 kg / t. The mass contents of P and S in the hot metal after pretreatment are respectively less than 0.080% and 0.010%.

[0013] Step 2: smelting in an electric furnace or converter: adding the pre-P / S-removed molten iron into a converter or electric furnace for smelting. When a converter is used for smelting, 15% to 25% of high-quality scrap steel is added to the converter. When an electric furnace is used for smelting, 50% to 60% of high-quality scrap steel is added to the electric furnace. The ladle capacity of the converter is 100t to 200t. Ferromolybdenum and scrap steel are added to the converter together. The amount of ferromolybdenum added is calculated based on the final Mo content requirement and the proportion of Mo in ferromolybdenum. The converter uses a top gun to blow oxygen for smelting. The oxygen blows to oxidize the carbon in the molten iron until the end carbon mass content is 0.08% to 0.20%. The scrap steel is completely melted, and the target basicity of the converter final slag is 3.5. The smelting is completed. After the smelting is completed, aluminum blocks, ferrosilicon manganese, and ferrochrome are added in sequence for deoxidation and alloying. The steel-discharging time is 3 to 5 minutes.

[0014] Step 3: refining outside the furnace: add appropriate amount of aluminum particles according to the oxygen content and slag color. If the oxygen content is higher than 50ppm or the slag is black, gray or brown, add aluminum particles or aluminum powder for deoxidation; add refined synthetic slag according to the slag basicity, the final slag color is gray or white, and the slag basicity is 4-6. The amount of alloy added is adjusted according to the steel grade composition range. The alloy has been added before, and the composition will be tested during the refining process outside the furnace. According to the test results and target composition, the alloy is added. According to the Ni / Cu ratio, Ni alloy is added, and niobium iron is added for microalloying. The amount of niobium iron added is also calculated according to the final Nb content and the Nb content of niobium iron. After the alloy composition is adjusted, feed pure calcium wire for calcium treatment. When the Ca content in the molten iron is 20-30ppm, stop feeding the calcium wire and start soft blowing of argon at the same time. The soft blowing time is 15-25min.

[0015] Step 4: Continuous casting: The continuous casting process adopts an immersed nozzle, a tundish covering agent, and argon blowing at the nozzle to protect the casting. A special alloy steel crystallizer protective slag is used. The superheat range of molten steel is 20-30°C. The crystallizer electromagnetic stirring method with positive and negative cycle changes is adopted. The electromagnetic stirring current intensity is 500-550A, the positive and negative switching time is 20s, the cross-sectional size of the crystallizer is 300×350mm-350×420mm, the billet pulling speed is 0.60-0.65m / min, and the billet is cooled by water for secondary cooling after the crystallizer is discharged. After the billet is straightened, it is cooled for the third time. The surface temperature of the billet before the third cooling is 850-900°C, and the surface temperature of the billet is reduced to 400±25°C by water mist cooling. Due to the rapid surface cooling, there is still residual heat in the core to transfer heat outward, and the surface of the billet is heated up to 600-700°C again;

[0016] Step 5: Bloom rolling: The steel billet after three cooling in step 4 is sent to the billet opening workshop through a conveyor roller for heating and rolling, and rolled into intermediate billets of different specifications; the hot delivery temperature of the steel billet is 600-650°C, the heating temperature is 1150-1200°C, and after 7-9 rolling passes, the cross-section of the intermediate billet is 140×140mm-160×160mm;

[0017] Step 6: Wire rolling: During the wire rolling process, the intermediate billet is heated to 1050-1100°C, the start rolling temperature after high-pressure water descaling is 950-1000°C, the inlet temperature before finish rolling is 890-910°C, and the wire laying temperature is 820-840°C;

[0018] Step 7: Coiling: The rolled piece in step 6 is directly coiled after spinning, and after coiling, it passes through a section of rollers and enters the insulation corridor for slow cooling. The temperature entering the insulation corridor is 730-750°C. The insulation corridor is closed. The cooling rate of the coil in the insulation corridor is 0.05-0.1°C / s. The coil temperature after leaving the insulation corridor is 400-450°C.

[0019] Furthermore, in step 2, the capacity of the converter ladle is 130-160t.

[0020] Furthermore, the refined synthetic slag in step 3 includes: CaO, MgO, CaF2, SiO2, wherein the mass content of CaO is 55% to 70%, and the mass content of CaF2 is 5% to 10%.

[0021] Furthermore, in step 4, the cross-sectional dimensions of the crystallizer are 300×390 mm.

[0022] Furthermore, the specific process of water mist cooling in step 4 is: spraying water mist onto the surface of the steel billet through a nozzle to cool the steel billet, with a water volume of 15 to 25 m3 / h, the billet surface cooling rate is 10~15℃ / s.

[0023] Furthermore, the components of the special alloy steel crystallizer protection slag in step 4 include, by mass percentage, CaO 25% to 30%, SiO2 25% to 30%, Al2O3 10% to 15%, C 5% to 8%, Na2O 5% to 9%, MgO 0 to 5%, and MnO 0 to 5%.

[0024] Furthermore, in step 5, before rolling the large square billet, the steel billet is finished (using a grinding wheel to clean off excess cutting slag at both ends of the steel billet and grinding off obvious defects on the steel billet). After the large square billet is rolled, the intermediate billet is shot blasted and flaw detected to determine the depth and location of the defects, and the defects are cleaned using a grinding wheel.

[0025] Furthermore, the specific process of high-pressure water descaling in step 6 is: high-pressure water nozzles spray water on the surface of the steel billet, and there are several nozzles on the four sides of the steel billet. The water pressure is 18-25MPa, and the high-speed water flow is used to clean the iron oxide scale formed during the heating process of the steel billet.

[0026] A 12.9 grade annealing-free alloy cold heading steel wire for bolts prepared by the above production method, wherein the composition and the mass ratio of each component are as follows: C: 0.30% to 0.35%, Si: 0.10% to 0.30%, Mn: 0.70% to 1.20%, Cr: 1.0% to 1.5%, Mo: 0.15% to 0.30%, Al: 0.015% to 0.030%, N: 0.0030% to 0.0080%, Nb: 0.01% to 0.04%, Ni≤0.30%, Cu≤0.10%, P≤0.015%, S≤0.010%, (As+Sb+Bi)≤0.012%, O≤0.0010%, the rest is Fe and unavoidable impurity elements, niobium nitrogen product (Nb×N)≥6×10 -9 , Ni / Cu≥2.0.

[0027] Since copper has a low melting point, adding Cu to steel will cause copper brittleness, destroy the strength of the steel grain boundary, and cause surface crack defects. Ni can reduce the impact of copper brittleness. Controlling the Ni / Cu ratio above 2.0 can effectively improve the thermoplasticity of steel. The niobium nitrogen product is used to evaluate the precipitation temperature and precipitation ratio of Nb (C, N) precipitation phase. In order to obtain more Nb (C, N) precipitation phase, the niobium nitrogen product is controlled at 6×10 -9As, Sb, and Bi are all elements of the same family as P, and have similar properties to P. They will deteriorate the plasticity of steel and increase the strength of steel. Moreover, the atomic radius of As, Sb, and Bi is larger than that of P, and the harm is more serious, so they need to be controlled at a lower level. Comprehensively considered, (As+Sb+Bi)≤0.012%.

[0028] This alloy steel can form a fine-grained structure under certain rolling conditions, which affects the characteristics of the structure transformation through fine grains and delays the transformation of bainite and martensite.

[0029] The beneficial effects of the present invention are:

[0030] Conventional alloy cold heading steel wire has high strength, which is not conducive to downstream customers' processing and use. Currently, most patents use low-temperature rolling process to obtain low-strength products, but this process has too high requirements on equipment. The present invention does not require major changes to the production line.

[0031] During low temperature rolling, the surface plasticity of the steel billet is poor, which is prone to surface wrinkles, which will lead to subsequent cold heading cracking. In addition, improper control of alloy composition will produce a large amount of harmful elements in the steel billet, which will also lead to surface wrinkles.

[0032] The present invention adjusts the composition range, controls the content of residual elements and harmful elements, and adds grain-refining elements, so that the grains can be refined during the rolling process. At the same time, the rolling process is reasonably adjusted, and the rolling is performed within the high plasticity range, so that the surface quality of the finished product is excellent.

[0033] The present invention adopts a heat-insulating corridor to control the cooling speed, effectively utilizes the waste heat of hot rolling, and avoids abnormal structure caused by component fluctuation.

[0034] The alloy cold heading steel wire produced by the invention has low strength, reduces one annealing step, reduces downstream processing costs, and reduces energy consumption and environmental pollution.

[0035] After the rolling is completed, the present invention adopts the method of first coiling and then cooling to achieve the purpose of slow cooling. At the same time, a slow cooling insulation corridor device is adopted to greatly reduce the cooling speed and ensure that the hot-rolled wire rod structure after phase transformation is completely transformed into ferrite + pearlite structure. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Figure 1 It is the cooling transformation curve of the steel of the present invention and the existing common alloy steel.

[0037] Figure 1 In: Fs: Ferrite start transformation temperature; Pf: Pearlite complete transformation problem;

[0038] B: bainite transformation region; M: martensite transformation region.

[0039] Figure 2 This is the hot-rolled alloy steel structure produced by the present invention. DETAILED DESCRIPTION

[0040] The present invention is further described below in conjunction with the accompanying drawings and specific embodiments, as well as Examples 1 to 6 and Comparative Examples 1 to 2 (process parameters are shown in Tables 1 and 2). It should be understood that the following specific embodiments are only used to illustrate the present invention and are not intended to limit the scope of the present invention.

[0041] The production method of the 12.9 grade annealing-free alloy cold heading steel wire for bolts of the present invention comprises: pre-P / S removal of molten iron → smelting in an electric furnace or a converter → refining outside the furnace → continuous casting of large square billets → rolling into intermediate billets and finishing of steel billets → heating of intermediate billets → descaling with high-pressure water → rough rolling → intermediate rolling → pre-finishing rolling → finishing rolling → wire laying → coiling → cooling, etc.

[0042] The composition and mass content of annealing-free alloy cold heading steel wire for 12.9 grade bolts are as follows: C: 0.30%~0.35%, Si: 0.10%~0.30%, Mn: 0.70%~1.20%, Cr: 1.0%~1.5%, Mo: 0.15%~0.30%, Al: 0.015%~0.030%, N: 0.0030%~0.0080%, Nb: 0.01%~0.04%, Ni≤0.30%, Cu≤0.10%, P≤0.015%, S≤0.010%, (As+Sb+Bi)≤0.012%, O≤0.0010%, and the rest are Fe and unavoidable impurity elements. Among them, the mass fraction is recorded, and the niobium nitrogen product (Nb×N) ≥6×10 -9 , Ni / Cu≥2.0.

[0043] The specific process of the production method of the annealing-free alloy cold heading steel wire for 12.9 grade bolts of the present invention is as follows:

[0044] The step of pre-de-P / S of molten iron, the components of the molten iron before pre-P / S removal include, by mass content: C 2% to 4.5%, Mn ≤ 0.35%, Si content 0.25% to 0.65%, P content 0.08% to 0.12%, S 0.025% to 0.045%, (As+Sb+Bi) ≤ 0.012%, and the rest is Fe and a small amount of other impurities. Pre-P / S removal is carried out in a molten iron tank, and CaO is added by spraying powder, the addition amount is about 6 to 10 kg / t, and the P and S contents of the molten iron after pretreatment are respectively less than 0.080% and 0.010%.

[0045] Electric furnace or converter smelting, the pre-de-P / S molten iron is added to the converter or electric furnace for smelting, wherein the converter adds 15% to 25% of high-quality scrap steel, and the electric furnace adds 50% to 60% of high-quality scrap steel. Preferably, the converter is used for smelting. The capacity of the converter ladle is 100t to 200t, preferably 130 to 160t ladle. Ferromolybdenum and scrap steel are added to the converter together, and the amount of ferromolybdenum added is calculated according to the final Mo content requirement and the proportion of Mo in ferromolybdenum. The converter uses a top gun to blow oxygen for smelting, and the oxygen blows to oxidize the carbon in the molten iron until the final carbon mass content is 0.08% to 0.20%, the scrap steel is completely melted, and the target basicity of the converter final slag is 3.5. After smelting, aluminum blocks (added in an amount of 1.4 to 2.6 kg / t for deoxidation), ferrosilicomanganese (embodiments 1 to 6 and comparative examples 1 to 2 are added according to the Si and Mn components of the finished products shown in Table 1, and the Si and Mn contents in ferrosilicomanganese) and ferrochrome (embodiments 1 to 6 and comparative examples 1 to 2 are added according to the Cr components of the finished products shown in Table 1, and the Cr content in the ferrochrome) are added in sequence for alloying. Considering that there are also a small amount of alloying elements in the scrap steel, if more scrap steel is added, less alloy should be added. The steel tapping time is 3 to 5 minutes.

[0046] In the refining process outside the furnace, according to the oxygen content, slag color and basicity, an appropriate amount of refined synthetic slag or aluminum particles are added. If the oxygen content is higher than 50ppm or the slag is black, gray or brown, aluminum particles or aluminum powder are added for deoxidation; according to the basicity of the slag, refined synthetic slag is added. If the slag basicity is low, synthetic slag is added, which is composed of CaO (55% to 70%), MgO, CaF2 (5% to 10%), and SiO2. The final slag color is grayish white or white, and the slag basicity is 4 to 6. The amount of alloy added is adjusted according to the composition range of the steel grade, and niobium iron is added for microalloying. Finally, the compositions of Examples 1 to 6 after Nb alloying and Comparative Examples 1 to 2 without Nb microalloying are shown in Table 1. In Examples 1 to 6, attention should be paid to controlling the impurity elements, (As+Sb+Bi)≤0.012%, and ensuring that the niobium nitrogen product (Nb×N)≥6×10 -9 , Ni / Cu≥2.0; Comparative Cases 1-2 use conventional smelting process, which cannot guarantee Ni / Cu ratio and niobium nitrogen product (Nb×N). Feed pure calcium wire for calcium treatment. When the Ca content of molten iron is 20-30ppm, stop feeding calcium wire. At the same time, start soft blowing of argon gas, and the soft blowing time is 15-25min.

[0047] The continuous casting process adopts the methods of immersion nozzle, tundish covering agent, nozzle argon blowing, etc. to protect the casting, and adopts a special alloy steel crystallizer protective slag (CaO 25%-30%, SiO2 25%-30%, Al2O3 10%-15%, C 5%-8%, Na2O 5%-9%, a small amount of MgO, MnO, etc.). The superheat range of molten steel is 20-30°C, and the crystallizer electromagnetic stirring mode with positive and negative periodic changes is adopted (by changing the direction of the crystallizer electromagnetic stirring current, the molten steel flows clockwise or counterclockwise, the molten steel flows clockwise for positive rotation, and counterclockwise for reverse rotation), the electromagnetic stirring current intensity is 500-550A, and the positive and negative switching time is 20s. The cross-sectional dimensions and pulling speed of the square billets of Examples 1-6 and Comparative Examples 1-2 are shown in Table 2. The billet is cooled by water cooling after exiting the crystallizer, and cooled three times after the billet is straightened. The surface temperature of the steel billet before the third cooling is about 850-900℃. The water mist cooling is used to reduce the surface temperature of the steel billet to 400±25℃. Due to the rapid surface cooling, there is still residual heat in the core to transfer outwards, and the surface temperature of the steel billet returns to 600-700℃ again.

[0048] The large square billet is rolled into the intermediate billet and the billet finishing stage. The billet after the three coolings is sent to the billet opening workshop through the conveyor roller for heating and rolling, and rolled into intermediate billets of different specifications. The hot delivery temperature of the billet is about 600-650°C, the heating temperature is 1150-1200°C, and after 7-9 rolling passes, the cross-sections of the intermediate billets of Examples 1-6 and Comparative Examples 1-2 are shown in Table 2. The intermediate billet is shot blasted and flaw detected to determine the depth and location of the defects, and the defects are cleaned with a grinding wheel.

[0049] During the wire rolling process, the intermediate billet is heated to 1050-1100°C, the start rolling temperature after high-pressure water descaling is 950-1000°C, the finishing inlet temperatures of Examples 1-6 and Comparative Examples 1-2 are shown in Table 2, and the wire laying temperature is 820-840°C. The high-pressure water pressure is 18-25 MPa.

[0050] The rolled pieces are directly coiled after spinning, and after coiling, they pass through a section of rollers and then enter the insulation corridor for slow cooling. The temperature entering the insulation corridor, the cooling rate, and the coiling temperature after leaving the insulation corridor of Examples 1 to 6 are shown in Table 2. Comparative Examples 1 to 2 do not pass through the insulation corridor but adopt ordinary air cooling line cooling. The cooling process is shown in Table 2.

[0051] The finished product performance and surface wrinkles of Examples 1 to 6 and Comparative Examples 1 to 2 are shown in Table 3. Examples 1 to 6 can meet the production process requirements of direct drawing of downstream customers, saving an annealing process. The strength of the bolts made after quenching and tempering can meet the requirements of 12.9 grade products, with a large through-hardening diameter and excellent comprehensive mechanical properties. Examples 1 to 6 control the Ni / Cu ratio and the content of impurity elements such as As+Sb+Bi, and the wrinkles on the surface of the finished products basically disappear.

[0052] In comparative examples 1 to 2, low temperature rolling is performed. Since the plasticity of the steel billet is poor at low temperature and the impurity elements are not controlled, the surface of the rolled product has too many wrinkles, which will increase the proportion of cold heading cracking in the downstream.

[0053] When conventionally produced alloy steel wire is cooled, Figure 1 As shown by the dotted line, 0.5℃ / s is a relatively slow cooling rate in conventional production. When the cooling rate is higher than this cooling rate, bainite and martensite structures will appear in the alloy steel. The strength of bainite and martensite structures is much higher than that of ferrite F and pearlite P, so the hardness of hot-rolled wire rods of general alloy steel is relatively high.

[0054] The structural transformation curve of the product produced by the present invention when cooled is as follows: Figure 1 As shown by the solid line in the middle, compared with conventional steel grades, the bainite B phase transformation is delayed, and at a cooling rate of 0.5℃ / s, no bainite B and martensite M structures are formed. In addition, a slow cooling heat preservation corridor device is used, and the cooling rate is below 0.1℃ / s to ensure that the finished products can be completely transformed into ferrite F+pearlite P structures without generating bainite B and martensite M structures.

[0055] Table 1 Finished product composition 1 of the embodiments and comparative examples, unit: %

[0056] C Si Mn Cr Mo Nb P S Example 1 0.30 0.30 1.20 1.30 0.25 0.020 0.015 0.005 Example 2 0.31 0.25 1.10 1.00 0.20 0.015 0.013 0.004 Example 3 0.32 0.20 1.00 1.10 0.15 0.015 0.011 0.003 Example 4 0.35 0.10 0.70 1.50 0.20 0.010 0.009 0.008 Example 5 0.34 0.15 0.80 1.20 0.30 0.030 0.007 0.010 Example 6 0.33 0.20 0.90 1.20 0.18 0.040 0.011 0.002 Comparative Example 1 0.30 0.25 1.00 1.30 0.20 0.0001 0.018 0.012 Comparative Example 2 0.35 0.15 0.90 1.20 0.25 0.0001 0.020 0.013

[0057] Table 1: Composition 2 of finished products of the embodiments and comparative examples, unit: %

[0058] Ni Cu Al N O As+Sb+Bi Ni / Cu Nb×N Example 1 0.08 0.04 0.015 0.004 0.001 0.012 2 <![CDATA[8×10 -9 ]]> Example 2 0.05 0.01 0.020 0.004 0.008 0.010 5 <![CDATA[6×10 -9 ]]> Example 3 0.06 0.02 0.020 0.006 0.006 0.008 3 <![CDATA[9×10 -9 ]]> Example 4 0.12 0.03 0.030 0.008 0.005 0.006 4 <![CDATA[8×10 -9 ]]> Example 5 0.30 0.10 0.030 0.003 0.006 0.005 3 <![CDATA[9×10 -9 ]]> Example 6 0.04 0.01 0.018 0.004 0.010 0.009 4 <![CDATA[16×10 -9 ]]> Comparative Example 1 0.02 0.04 0.030 0.009 0.020 0.018 0.5 <![CDATA[0.09×10 -9 ]]> Comparative Example 2 0.03 0.05 0.040 0.008 0.025 0.015 0.6 <![CDATA[0.08×10 -9 ]]>

[0059] Table 2 Process of Examples and Comparative Examples

[0060]

[0061] Table 3 Finished product properties of the embodiments and comparative examples

[0062]

[0063] Based on the above ideal embodiments of the present invention, the relevant staff can make various changes and modifications without departing from the technical concept of the present invention through the above description. The technical scope of the present invention is not limited to the contents of the specification, and its technical scope must be determined according to the scope of the claims.

Claims

1. A method for producing annealing-free alloy cold heading steel wire for 12.9 grade bolts, characterized in that: include: Pre-desulfurization of hot metal → smelting in electric furnace or converter → refining outside the furnace → continuous casting of large square billets → rolling into intermediate billets → heating of intermediate billets → descaling with high-pressure water → rough rolling → intermediate rolling → pre-finishing rolling → finishing rolling → wire laying → coiling → cooling. The specific production process is as follows: Step 1: Pre-desulfurization of hot metal: Pre-desulfurization is carried out in a hot metal ladle. CaO is added by spraying powder in an amount of 6-10 kg / t. The mass contents of P and S in the hot metal after pretreatment are respectively less than 0.080% and 0.010%. Step 2: smelting in an electric furnace or converter: adding the pre-P / S-removed molten iron into a converter or electric furnace for smelting. When a converter is used for smelting, 15% to 25% of high-quality scrap steel is added to the converter. When an electric furnace is used for smelting, 50% to 60% of high-quality scrap steel is added to the electric furnace. The ladle capacity of the converter is 100t to 200t. Ferromolybdenum and scrap steel are added to the converter together. The amount of ferromolybdenum added is calculated based on the final Mo content requirement and the proportion of Mo in ferromolybdenum. The converter uses a top gun to blow oxygen for smelting. The oxygen blows to oxidize the carbon in the molten iron until the end carbon mass content is 0.08% to 0.20%. The scrap steel is completely melted, and the target basicity of the converter final slag is 3.

5. The smelting is completed. After the smelting is completed, aluminum blocks, ferrosilicon manganese, and ferrochrome are added in sequence for deoxidation and alloying. The steel-discharging time is 3 to 5 minutes. Step 3: Refining outside the furnace: add appropriate amount of aluminum particles according to the oxygen content and slag color. If the oxygen content is higher than 50ppm or the slag is black, gray or brown, add aluminum particles or aluminum powder for deoxidation; add refined synthetic slag according to the slag basicity, the final slag color is gray or white, and the slag basicity is 4-6. Adjust the amount of alloy added according to the steel grade composition range. The alloy has been added before, and the composition will be tested during the refining outside the furnace. According to the test results and target composition, add alloy. According to the Ni / Cu ratio, add Ni alloy and add niobium iron for microalloying. The amount of niobium iron added is also calculated according to the final Nb content and the Nb content of niobium iron. After the alloy composition is adjusted, feed pure calcium wire for calcium treatment. When the Ca content in the molten iron is 20-30ppm, stop feeding calcium wire and start soft blowing of argon at the same time. The soft blowing time is 15-25min. Step 4: Continuous casting: The continuous casting process adopts an immersed nozzle, a tundish covering agent, and argon blowing at the nozzle to protect the casting. A special alloy steel crystallizer protective slag is used. The superheat range of molten steel is 20-30°C. The crystallizer electromagnetic stirring method with positive and negative cycle changes is adopted. The electromagnetic stirring current intensity is 500-550A, the positive and negative switching time is 20s, the cross-sectional size of the crystallizer is 300×350mm-350×420mm, the billet pulling speed is 0.60-0.65m / min, and the billet is cooled by water for secondary cooling after the crystallizer is discharged. After the billet is straightened, it is cooled for the third time. The surface temperature of the billet before the third cooling is 850-900°C, and the surface temperature of the billet is reduced to 400±25°C by water mist cooling. Due to the rapid surface cooling, there is still residual heat in the core to transfer heat outward, and the surface of the billet is heated up to 600-700°C again; Step 5: Bloom rolling: The steel billet after three cooling in step 4 is sent to the billet opening workshop through a conveyor roller for heating and rolling, and rolled into intermediate billets of different specifications; the hot delivery temperature of the steel billet is 600-650°C, the heating temperature is 1150-1200°C, and after 7-9 rolling passes, the cross-section of the intermediate billet is 140×140mm-160×160mm; Step 6: Wire rolling: During the wire rolling process, the intermediate billet is heated to 1050-1100°C, the start rolling temperature after high-pressure water descaling is 950-1000°C, the inlet temperature before finish rolling is 890-910°C, and the wire laying temperature is 820-840°C; Step 7: Coiling: The rolled piece in step 6 is directly coiled after spinning, and after coiling, it passes through a section of rollers and enters the insulation corridor for slow cooling. The temperature entering the insulation corridor is 730-750°C. The insulation corridor is closed. The cooling rate of the coil in the insulation corridor is 0.05-0.1°C / s. The coil temperature after leaving the insulation corridor is 400-450°C.

2. The method for producing annealing-free alloy cold heading steel wire for 12.9 grade bolts according to claim 1, characterized in that: The capacity of the converter ladle in step 2 is 130-160t.

3. The method for producing annealing-free alloy cold heading steel wire for 12.9 grade bolts according to claim 1, characterized in that: The refined synthetic slag in step 3 includes: CaO, MgO, CaF2, and SiO2, wherein the mass content of CaO is 55% to 70%, and the mass content of CaF2 is 5% to 10%.

4. The method for producing annealing-free alloy cold heading steel wire for 12.9 grade bolts according to claim 1, characterized in that: The cross-sectional dimensions of the crystallizer in step 4 are 300×390 mm.

5. The method for producing annealing-free alloy cold heading steel wire for 12.9 grade bolts according to claim 1, characterized in that: The specific process of water mist cooling in step 4 is: spraying water mist onto the surface of the steel billet through a nozzle to cool the steel billet, with a water volume of 15-25m 3 / h, the billet surface cooling rate is 10~15℃ / s.

6. The method for producing annealing-free alloy cold heading steel wire for 12.9 grade bolts according to claim 1, characterized in that: The components of the special alloy steel crystallizer protection slag in step 4 include CaO 25% to 30%, SiO2 25% to 30%, Al2O3 10% to 15%, C 5% to 8%, Na2O 5% to 9%, MgO 0 to 5%, and MnO 0 to 5% by mass percentage.

7. The method for producing annealing-free alloy cold heading steel wire for 12.9 grade bolts according to claim 1, characterized in that: In step 5, before rolling the large square billet, the steel billet is finished. After rolling the large square billet, the intermediate billet is shot blasted and flaw detected to determine the depth and location of the defects, and the defects are cleaned with a grinding wheel.

8. The method for producing annealing-free alloy cold heading steel wire for 12.9 grade bolts according to claim 1, characterized in that: The specific process of high-pressure water descaling in step 6 is: high-pressure water nozzles spray water on the surface of the steel billet, and there are several nozzles on the four sides of the steel billet. The water pressure is 18-25MPa, and the high-speed water flow is used to clean the iron oxide scale formed during the heating process of the steel billet.

9. An annealing-free alloy cold heading steel wire for 12.9 grade bolts, characterized in that: The invention is produced by the production method of the annealing-free alloy cold heading steel wire for 12.9 grade bolts as described in any one of claims 1-8.

10. The annealing-free alloy cold heading steel wire for 12.9 grade bolts according to claim 9, characterized in that: The composition and mass ratio of each component are as follows: C: 0.30% to 0.35%, Si: 0.10% to 0.30%, Mn: 0.70% to 1.20%, Cr: 1.0% to 1.5%, Mo: 0.15% to 0.30%, Al: 0.015% to 0.030%, N: 0.0030% to 0.0080%, Nb: 0.01% to 0.04%, Ni≤0.30%, Cu≤0.10%, P≤0.015%, S≤0.010%, (As+Sb+Bi)≤0.012%, O≤0.0010%, the rest is Fe and unavoidable impurity elements, niobium nitrogen product (Nb×N)≥6×10 -9 , Ni / Cu≥2.0.

Images