Steel wire rod for 10.9-grade high-Si non-quenched and tempered fastener and manufacturing method of steel wire rod

Abstract

The invention relates to a 10.9-grade high-Si non-quenched and tempered steel wire rod for a fastener and a manufacturing method of the 10.9-grade high-Si non-quenched and tempered steel wire rod. The 10.9-grade high-Si non-quenched and tempered steel wire rod for the fastener comprises the following chemical components in percentage by weight: 0.08 to 0.12 percent of C, 1.45 to 1.90 percent of Si, 1.50 to 1.80 percent of Mn, less than or equal to 0.020 percent of P, less than or equal to 0.010 percent of S, 0.0005 to 0.0030 percent of B, 0.01 to 0.03 percent of Ti, 0.010 to 0.050 percent of Als, 0.02 to 0.04 percent of Nb and the balance of iron and trace impurities. The manufacturing process of the hot-rolled wire rod comprises the following steps: preparing raw materials according to chemical components, and sequentially carrying out converter / electric furnace-LF refining-vacuum degassing-casting-wire rod controlled rolling-wire rod controlled cooling. The steel wire rod produced by the method can obtain a uniform bainite structure at a relatively large range of cooling speed, and has the characteristics of uniform structure, good plasticity and excellent cleaning performance, and a fastener produced by drawing with relatively small area reduction rate, relatively small cold heading deformation and aging subsequently has the characteristics of stable mechanical property and excellent delayed fracture resistance; and the requirement of a 10.9-level fastener is met.

Description

Technical Field

[0001] The invention belongs to the technical field of special steel smelting, and specifically relates to a wire rod for a 10.9-grade high-Si non-quenched and tempered fastener and a manufacturing method thereof. Background Art

[0002] At present, high-strength fasteners above grade 8.8 are mainly made of traditional medium carbon steel or medium carbon alloy steel. During the manufacturing process, at least one annealing treatment and quenching and tempering treatment after cold heading are usually carried out, which not only takes a long time but also consumes a lot of energy. Compared with quenched and tempered fasteners, the production process of non-quenched and tempered fasteners can omit annealing before cold heading, reduce the number of drawing times, and omit quenching and tempering heat treatment after cold heading, which can simplify the production process, shorten the production cycle, and reduce energy consumption. In the strong atmosphere of green manufacturing, it has important economic and social benefits.

[0003] For the cold-worked hardened non-quenched and tempered steel for 10.9 grade high-strength bolts, such as the patents with publication numbers CN101619414B and CN101935806B, the structure is bainite. To obtain the bainite structure, Mn is controlled to be above 1.8%, but Mn in the steel promotes grain growth, thus increasing the overheating sensitivity and crack tendency of the steel, which is not conducive to the delayed fracture resistance of the steel. The Si content basically follows the design of traditional cold-headed quenched and tempered fasteners, and controls the Si content below 0.35%. Although the cold working performance can be guaranteed, the role of Si in low-carbon bainite rolling and steel is ignored. In order to ensure the mechanical properties of 10.9 grade non-quenched and tempered fasteners, it is necessary to introduce large cold deformation in the process of drawing and cold heading to produce a large number of defects such as dislocations. Although the density and distribution of dislocations can be changed after aging treatment, it still affects the delayed fracture resistance. For 10.9 grade high-strength bolts, delayed fracture resistance is an important indicator. Improper control will lead to early failure of the bolts. Summary of the invention

[0004] The technical problem to be solved by the present invention is to provide a method for manufacturing a 10.9 grade high Si non-quenched and tempered fastener steel and its hot-rolled wire rod in response to the above-mentioned prior art, which has the characteristics of meeting the strength requirements of 10.9 grade fasteners under relatively small cold deformation and having excellent delayed fracture resistance.

[0005] The technical solution adopted by the present invention to solve the above problem is: a 10.9 grade high Si non-quenched and tempered wire rod for fasteners, the chemical composition of the wire rod is as follows by mass percentage: C: 0.08-0.12%, Si: 1.45-1.90%, Mn:

[0006] 1.50~1.80%, P≤0.020%, S≤0.010%, B: 0.0005~0.0030%, Ti: 0.01~0.03%, Als: 0.010~0.050%, Nb: 0.02~0.04%, and the rest are Fe and unavoidable impurities.

[0007] The main functions and design basis of each chemical element of the steel of the present invention are:

[0008] 1) Determination of C content

[0009] C is the most basic strengthening element in steel, but too high a C content will have an adverse effect on the toughness and plasticity of the steel. The C content of the present invention is determined to be in the range of 0.08-0.12%, and the steel material involved in the present invention belongs to the category of low-carbon bainite steel.

[0010] 2) Determination of Si content

[0011] Si is a solid solution strengthening element. As the Si content in steel increases, bainite transformation occurs at a lower temperature, and the cooling rate range of bainite transformation increases, which can ensure that bainite steel will not cause a large fluctuation range in mechanical properties due to different seasons and cooling rates. However, Si increases the overheating sensitivity, cracking and decarburization tendency in steel. At the same time, an increase in Si content will promote the formation of proeutectoid ferrite and reduce the strength of steel. The range of Si content in the present invention is determined to be 1.45-1.90%.

[0012] 3) Determination of Mn content

[0013] As a deoxidizing element in the steelmaking process, Mn can improve the hardenability of steel. Mn can also fix the form of sulfur in steel and form MnS and (Fe, Mn)S, which are less harmful to the performance of steel, reduce or inhibit the production of FeS, and prevent the effect of thermal embrittlement. Manganese is an important element to increase the strength of steel, and it is also one of the most effective low-cost alloying elements for forming bainite structure. However, Mn is an element that is easy to segregate, especially the center segregation of wire, which has an adverse effect on drawing processing. In addition, Mn has the effect of promoting grain growth, which increases the overheating sensitivity and crack tendency of steel, which is not conducive to the delayed fracture resistance of steel. In order to obtain excellent microstructure, mechanical properties and delayed fracture resistance, the range of Mn content in the present invention is determined to be 1.50-1.80%.

[0014] 4) Determination of P and S content

[0015] P causes serious segregation during solidification in steel. P dissolves in ferrite to cause grain distortion and coarsening, and increases cold brittleness, which is not conducive to the delayed fracture resistance of steel. The range of P content in the present invention is determined to be ≤0.020%. S causes hot brittleness in steel, reduces the ductility and toughness of steel, and is not conducive to the delayed fracture resistance of steel. The range of S content in the present invention is determined to be ≤0.010%.

[0016] 5) Determination of B content

[0017] B is the most significant element for improving the hardenability of steel. However, if the B content exceeds 0.003%, a brittle phase of BN will easily appear, which will deteriorate the delayed fracture resistance of the steel. Therefore, the B content of the present invention is controlled to be 0.0005-0.0030%.

[0018] 6) Determination of Ti content

[0019] Ti mainly plays the role of nitrogen fixation. It can combine with free nitrogen in steel in the early stage of continuous casting solidification to form TiN particles in the steel, thereby reducing the combination of free N and B and giving full play to the role of B in improving hardenability. Therefore, the present invention controls the Ti content to 0.01-0.03%.

[0020] 7) Determination of Al content

[0021] Al is added as a deoxidizing element in steel. In addition to reducing the dissolved oxygen in molten steel, Al and N form dispersed and fine aluminum nitride, which can refine the grains and increase the temperature of steel grain coarsening. However, when the Al content is high, brittle inclusions such as Al2O3 are easily formed during the molten steel smelting process, which reduces the purity of the molten steel. When the Al content is high, the grain size of the steel is more likely to grow and coarsen. The range of the Al content in the present invention is determined to be 0.010-0.050%.

[0022] 8) Determination of Nb content

[0023] Nb is a typical microalloying element, which has a strong affinity with C and N, and can form stable and fine carbonitrides. The precipitation strengthening and grain refinement of its carbonitrides can significantly improve the strength of steel. Fine carbonitrides can also serve as hydrogen traps, inhibiting the diffusion of hydrogen and making hydrogen uniformly distributed, thereby improving the delayed fracture resistance. At the same time, the composite addition of Nb+B elements can significantly improve the hardenability of steel and promote the formation of bainite during air cooling. The range of Nb content in the present invention is determined to be 0.02-0.04%.

[0024] The above-mentioned method for manufacturing a 10.9 grade high Si non-quenched and tempered hot-rolled wire rod for fasteners comprises the following process steps:

[0025] Step 1: Smelting and casting

[0026] The raw materials for smelting are successively smelted in converter / electric furnace, refined, vacuum degassed and molded or continuously cast to produce billets with accurate chemical composition, good uniformity and high purity. High-quality molten iron from blast furnace, scrap steel and raw and auxiliary materials are used for smelting to reduce the content of harmful elements in molten steel. Molten steel smelting involves primary smelting in converter / electric furnace, LF refining, vacuum degassing, KR molten iron pretreatment and converter / electric furnace slag making to achieve C and P removal. Slag should be avoided when tapping steel. One of the purposes of vacuum treatment is to control the hydrogen content below 0.5ppm, effectively reducing the hazards of hydrogen embrittlement of high-strength fasteners. Molten steel is cast into billets by continuous casting or mold casting. If continuous casting is used, the pouring superheat is 20-35℃ during continuous casting. Soft pressure and electromagnetic stirring are used to improve the segregation of the billet during continuous casting. Protective casting is used in the whole process of continuous casting to isolate the billet from air and prevent secondary oxidation.

[0027] Step 2: Controlled rolling and controlled cooling of wire rod: the billet is heated in sections in the heating furnace, the temperature of the preheating section is controlled to be ≤720℃, the temperature of the high temperature section is controlled at 1150~1250℃, the insulation time of the high temperature section is not less than 60min, and the residual oxygen content in the furnace is ≤2.0%. After the insulation is completed, the intermediate billet is descaled to remove the surface oxide layer after being taken out of the furnace, and then rolled. Rough rolling stage: the starting rolling temperature is 850~950℃, and the total compression ratio is ≥60%; secondary descaling is performed before medium rolling; medium finishing rolling stage: the medium finishing rolling temperature is 820~900℃, and the total compression ratio is ≥90%; the temperature of the sizing mill is 750~820℃, and the compression ratio of the last two passes is ≥30%; the wire laying temperature is 780~830℃, and EDC water cooling is used after rolling, and the EDC water temperature is above 97℃. The temperature of the wire rod out of EDC is controlled at 450-530℃, and the temperature difference between the overlap point and the middle point of the wire rod does not exceed 30℃. After leaving EDC, the wire rod is put into the heat preservation cover for slow cooling, and the cooling rate is controlled within 10℃ / S.

[0028] The wire rod produced by the above method has a specification of Φ5.5~Φ16mm; the metallographic structure is bainite, the tensile strength is 850~950MPa, and the cross-sectional shrinkage is ≥55%. The fasteners produced by the wire rod after subsequent drawing with a small area reduction rate, small cold heading deformation and aging have the characteristics of stable mechanical properties and excellent delayed fracture resistance, meeting the requirements of 10.9 grade fasteners.

[0029] Compared with the prior art, the advantages of the present invention are:

[0030] 1) Composition design: Based on low-carbon steel, this patent adopts the composite addition of Si and Mn elements, which can make the steel obtain bainite structure at a lower temperature and a larger cooling rate range during the rolling process. Since the bainite structure has no carbide precipitation on the original austenite grain boundary and the effective grain size is small, it has good delayed fracture resistance. At the same time, compared with conventional low-carbon bainite, the Mn content in the steel is reduced, the crack tendency is reduced, which is beneficial to the delayed fracture resistance of the steel. The Al+Nb+B microalloying technology is adopted, and the precipitation strengthening and grain refinement of its carbonitrides significantly improve the strength of the steel. The fine precipitates of carbonitrides can also act as hydrogen traps, inhibit the diffusion of hydrogen and make hydrogen evenly distributed, and improve the delayed fracture resistance together with the refined grains. At the same time, the addition of Nb+B microalloy improves the hardenability of the steel.

[0031] 2) During molten steel smelting, converters are used to produce molten steel with low S and P content. LF refining + vacuum degassing helps to produce molten steel with uniform chemical composition and purity. The vacuum treatment should control the hydrogen content to less than 0.5ppm, effectively reducing the harm of hydrogen embrittlement of high-strength fasteners and improving the delayed fracture resistance.

[0032] 3) Controlled rolling: The rough rolling and medium finishing rolling of wire rod adopt a large compression ratio, which forms a large number of dislocations and deformation bands inside the steel, and accumulates a large amount of storage energy, preparing for the subsequent sizing rolling. When the temperature of the sizing mill is controlled at 750-820℃, combined with the last two passes of rolling with a compression ratio of ≥30%, the austenite is rolled in the non-recrystallized area, the grains are refined, and the delayed fracture resistance is improved.

[0033] 4) Controlled cooling: After spinning, the EDC water bath cooling process is used to avoid high-temperature phase change. After leaving the EDC, the wire rod is slowly cooled in the insulation cover. The cooling rate is controlled within 10℃ / S, and a uniform bainite structure can be obtained. The obtained bainite steel will not cause a large fluctuation range in mechanical properties due to different seasons and cooling rates.

[0034] 5) The fasteners produced by the wire rod through drawing with a small area reduction rate, small cold heading deformation and aging have a small deformation, small density and distribution of dislocations, and the impact on delayed fracture resistance is reduced. The fasteners produced have the characteristics of stable mechanical properties and excellent delayed fracture resistance, meeting the requirements of 10.9 grade fasteners. DETAILED DESCRIPTION

[0035] The technical solution of the present invention is described in more detail in conjunction with the preferred embodiments of the present invention. However, these embodiments are only descriptions of the preferred implementation methods of the present invention and cannot impose any limitation on the scope of the present invention.

[0036] Example 1

[0037] The specification of the 10.9 grade Si-containing non-quenched and tempered fastener steel involved in this embodiment is Φ9mm, and the components and their mass percentages included are: C: 0.09%, Si: 1.60%, Mn: 1.71%, P: 0.012%, S≤0.002%, B: 0.0021%, Ti: 0.0253%, Als: 0.030%, Nb: 0.025%, and the rest are Fe and unavoidable impurities.

[0038] The raw materials are configured according to the chemical composition, and the processes are carried out in sequence: converter / electric furnace—LF refining—vacuum degassing—casting—wire controlled rolling—wire controlled cooling.

[0039] During the continuous casting process, the pouring superheat is 25°C. Soft pressure reduction and electromagnetic stirring technology are used to improve the segregation of the ingot. The entire continuous casting process adopts protective pouring to isolate the ingot from air to prevent secondary oxidation.

[0040] Controlled rolling and controlled cooling of wire rod: the billet is heated in sections in the heating furnace, the temperature of the preheating section is controlled at 700℃, the temperature of the high temperature section is controlled at 1180℃, the holding time of the high temperature section is 70min, and the residual oxygen content in the furnace is 1.0%. After the insulation is completed, the intermediate billet is descaled to remove the surface oxide layer after being taken out of the furnace, and then rolled. Rough rolling stage: the starting rolling temperature is 930℃, and the total compression ratio is 68%; secondary descaling is performed before medium rolling; medium finishing rolling stage: the medium finishing rolling temperature is 860℃, and the total compression ratio is 98%; the temperature of the sizing mill is 801℃, and the compression ratio of the last two passes is 36%; the wire laying temperature is 810℃, and EDC water cooling is used after rolling, and the EDC water temperature is above 97℃. The temperature of the wire rod out of EDC is controlled at 510℃, and the temperature difference between the overlap point and the middle point of the wire rod is 10℃. After leaving EDC, the wire rod enters the insulation cover for slow cooling, and the cooling rate is controlled at 3℃ / S.

[0041] The metallographic structure of the wire rod produced by the manufacturing method is bainite structure, the tensile strength of the wire rod is 875-900MPa, and the cross-sectional shrinkage is 60-62%.

[0042] Example 2

[0043] The specification of the 10.9 grade Si-containing non-quenched and tempered fastener steel involved in this embodiment is Φ14mm, and the components and their mass percentages included are: C: 0.10%, Si: 1.52%, Mn: 1.60%, P: 0.011%, S≤0.002%, B: 0.0019%, Ti: 0.024%, Als: 0.027%, Nb: 0.024%, and the rest are Fe and unavoidable impurities.

[0044] The raw materials are configured according to the chemical composition, and the processes are carried out in sequence: converter / electric furnace—LF refining—vacuum degassing—casting—wire controlled rolling—wire controlled cooling.

[0045] The pouring superheat during the continuous casting process is 23°C. Soft pressure reduction and electromagnetic stirring technology are used in the continuous casting process to improve the segregation of the ingot. Protective pouring is used in the entire continuous casting process to isolate the ingot from air and prevent secondary oxidation.

[0046] Controlled rolling and controlled cooling of wire rod: the billet is heated in sections in the heating furnace, the temperature of the preheating section is controlled at 690℃, the temperature of the high temperature section is controlled at 1210℃, the holding time of the high temperature section is 65min, and the residual oxygen content in the furnace is 1.0%. After the insulation is completed, the intermediate billet is descaled to remove the surface oxide layer after being taken out of the furnace, and then rolled. Rough rolling stage: the starting rolling temperature is 955℃, and the total compression ratio is 68%; secondary descaling is performed before medium rolling; medium finishing rolling stage: the medium finishing rolling temperature is 850℃, and the total compression ratio is 96.5%; the temperature of the sizing mill is 790℃, and the compression ratio of the last two passes is 34%; the wire laying temperature is 805℃, and EDC water cooling is used after rolling, and the EDC water temperature is above 97℃. The temperature of the wire rod out of EDC is controlled at 490℃, and the temperature difference between the overlap point and the middle point of the wire rod is 10℃. After leaving EDC, the wire rod enters the insulation cover for slow cooling, and the cooling rate is controlled at 8℃ / S.

[0047] The metallographic structure of the wire rod produced by the manufacturing method is bainite structure, the tensile strength of the wire rod is 880-915MPa, and the cross-sectional shrinkage is 61-63%.

[0048] Although the preferred embodiments of the present invention are described in detail above, it should be clearly understood that various modifications and variations are possible for those skilled in the art. Any modification, equivalent substitution, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A 10.9 grade high Si non-quenched and tempered wire rod for fasteners, characterized in that: The chemical composition of the wire rod is calculated by mass percentage as follows: C: 0.08-0.12%, Si: 1.45-1.90%, Mn: 1.50-1.80%, P≤0.020%, S≤0.010%, B: 0.0005-0.0030%, Ti: 0.01-0.03%, Als: 0.010-0.050%, Nb: 0.02-0.04%, and the rest is Fe and unavoidable impurities.

2. The 10.9 grade high Si non-quenched and tempered wire rod for fasteners according to claim 1, characterized in that: The specification of the wire rod is Φ5.5~Φ16mm.

3. The 10.9 grade high Si non-quenched and tempered wire rod for fasteners according to claim 2, characterized in that: The metallographic structure of the wire rod is bainite structure, the tensile strength is 850-950MPa, and the cross-sectional shrinkage rate is ≥55%.

4. A method for manufacturing a 10.9 grade high Si non-quenched and tempered fastener wire rod as claimed in claim 1, characterized in that: The method mainly comprises the following steps: (1) The raw materials are smelted in converter / electric furnace, refined, vacuum degassed and mold cast or continuous cast in sequence to produce steel billets with precise chemical composition, good uniformity and high purity; (2) The billet is heated in stages in the heating furnace, the temperature of the preheating stage is controlled to be ≤720°C, the temperature of the high temperature stage is controlled to be between 1150°C and 1250°C, the holding time of the high temperature stage is not less than 60 minutes, the residual oxygen content in the furnace is ≤2.0%, and after the holding is completed, the intermediate billet is taken out of the furnace to remove the scale and surface oxide layer; (3) Rolling in stages: Rough rolling stage: the starting rolling temperature is 850-950℃, the total compression ratio is ≥60%; secondary descaling is performed before intermediate rolling; intermediate finishing rolling stage: the intermediate finishing rolling temperature is 820-900℃, the total compression ratio is ≥90%; the temperature of the sizing mill is 750-820℃, and the compression ratio of the last two passes is ≥30%; the wire laying temperature is 780-830℃; (4) After rolling, EDC water cooling is adopted. The water temperature of EDC is above 97°C. After exiting EDC, the wire rod is slowly cooled in the insulation cover. The cooling rate is controlled within 10°C / S.

5. The method for manufacturing a 10.9 grade high Si non-quenched and tempered wire rod for fasteners according to claim 4, characterized in that: The hydrogen content was controlled to be less than 0.5 ppm by vacuum degassing.

6. The method for manufacturing a 10.9 grade high Si non-quenched and tempered wire rod for fasteners according to claim 4, characterized in that: When the continuous casting process is adopted, the pouring superheat is 20-35°C during the continuous casting process. The continuous casting process adopts light pressure reduction and electromagnetic stirring technology to improve the segregation of the ingot. The whole continuous casting process adopts protective pouring to isolate the ingot from air.

7. The method for manufacturing a 10.9 grade high Si non-quenched and tempered wire rod for fasteners according to claim 4, characterized in that: The temperature of the wire rod out of EDC is controlled at 450-530℃, and the temperature difference between the overlap point and the middle point of the wire rod does not exceed 30℃.